MIC2085/MIC2086
Single Channel Hot Swap Controllers
InfiniBand is a trademark of InfiniBand Trade Association
PowerPAK is a trademark of Vishay Intertechnology, Inc.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (
408
) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
May 2006
M9999-050406
(408) 955-1690
General Description
The MIC2085 and MIC2086 are single channel positive
voltage hot swap controllers designed to allow the safe
insertion of boards into live system backplanes. The
MIC2085and MIC2086 are available in 16-pin and 20-pin
QSOP packages, respectively. Using a few external
components and by controlling the gate drive of an
external N-Channel MOSFET device, the MIC2085/86
provide inrush current limiting and output voltage slew rate
control in harsh, critical power supply environments.
Additionally, a circuit breaker function will latch the output
MOSFET off if the current limit threshold is exceeded for a
programmed period of time. The devices’ array of features
provide a simplified yet robust solution for many network
applications in meeting the power supply regulation
requirements and affords protection of critical downstream
devices and components.
Data sheets and support documentation can be found on
Micrel’s web site at www.micrel.com.
Features
• MIC2085: Pin for pin functional equivalent to the
LTC1642
• 2.3V to 16.5V supply voltage operation
• Surge voltage protection to 33V
• Operating temperature range –40°C to 85°C
• Active current regulation limits inrush current
independent of load capacitance
• Programmable inrush current limiting
• Analog foldback current limiting
• Electronic circuit breaker
• Dual-level overcurrent fault sensing
• Fast response to short circuit conditions (< 1µs)
• Programmable output undervoltage detection
• Undervoltage lockout protection
• Power-on reset (MIC2085/86) and power-good
(MIC2086) status outputs
• /FAULT status output
• Driver for SCR crowbar on overvoltage
Applications
• RAID systems
• Cellular base stations
• LAN servers
• WAN servers
• InfiniBand™ Systems
• Industrial high side switching
Typical Application
Micrel, Inc. MIC2085/2086
May 2006
2 M9999-050406
(408) 955-1690
Ordering Information
Part Number
Standard Pb-Free
Fast Circuit Breaker Threshold Discharge Outpu t Package
MIC2085-xBQS MIC2085-xYQS x = J, 95mV
x = K, 150mV*
x = L, 200mV*
x = M, Off
NA 16-Pin QSOP
MIC2086-xBQS MIC2086-xYQS x = J, 95mV
x = K, 150mV*
x = L, 200mV*
x = M, Off
Yes 20-Pin QSOP
* Contact factory for availability.
Pin Configur ation
MIC2085 MIC2086
16-Pin QSOP (QS) 20-Pin QSOP (QS)
Pin Description
Pin Number
MIC2086 Pin Number
MIC2085 Pin Name Pin Function
1 1 CRWBR
Overvoltage Timer and Crowbar Circuit Trigger: A capacitor connected to this
pin, sets the timer duration for which an overvoltage condition will trigger an
external crowbar circuit. This timer begins when the OV input rises above its
threshold as an internal 45µA current source charges the capacitor. Once the
voltage reaches 470mV, the current increases to 1.5mA.
2 2 CFILTER
Current Limit Response Timer: A capacitor connected to this pin defines the
period of time (t
OCSLOW
) in which an overcurrent event must last to signal a fault
condition and trip the circuit breaker. If no capacitor is connected, then t
OCSLOW
defaults to 5µs.
3 3 CPOR
Power-On Reset Timer: A capacitor connected between this pin and ground
sets the start-up delay (t
START
) and the power-on reset interval (t
POR
). When VCC
rises above the UVLO threshold, the capacitor connected to CPOR begins to
charge. When the voltage at CPOR crosses 1.24V, the start-up threshold
(V
START
), a start cycle is initiated if ON is asserted while capacitor CPOR is
immediately discharged to ground. When the voltage at FB rises above VFB,
capacitor CPOR begins to charge again. When the voltage at CPOR rises above
the power-on reset delay threshold (VTH), the timer resets by pulling CPOR to
ground, and /POR is deasserted. If CPOR = 0, then t
START
defaults to 20µs.
Micrel, Inc. MIC2085/2086
May 2006
3 M9999-050406
(408) 955-1690
Pin Description (Cont.)
Pin Number
MIC2086 Pin Number
MIC2085 Pin Name Pin Function
4 4 ON
ON Input: Active high. The ON pin, an input to a Schmitt-triggered comparator
used to enable/disable the controller, is compared to a V
TH
reference with
100mV of hysteresis. Once a logic high is applied to the ON pin (V
ON
> 1.24V), a
start-up sequence is initiated as the GATE pin starts ramping up towards its final
operating voltage. When the ON pin receives a low logic signal (V
ON
< 1.14V),
the GATE pin is grounded and /FAULT is high if VCC is above the UVLO
threshold. ON must be low for at least 20µs in order to initiate a start-up
sequence. Additionally, toggling the ON pin LOW to HIGH resets the circuit
breaker.
5 5 /POR
Power-On Reset Output: Open drain N-Channel device, active low. This pin
remains asserted during start-up until a time period t
POR
after the FB pin voltage
rises above the power-good threshold (VFB). The timing capacitor CPOR
determines t
POR
. When an output undervoltage condition is detected at the FB
pin, /POR is asserted for a minimum of one timing cycle, t
POR
. The /POR pin has
a weak pull-up to VCC
6 NA PWRGD
Power-Good Output: Open drain N-Channel device, active high. When the
voltage at the FB pin is lower than 1.24V, the PWRGD output is held low. When
the voltage at the FB pin is higher than 1.24V, then PWRGD is asserted. A pull-
up resistor connected to this pin and to VCC will pull the output up to VCC. The
PWRGD pin has a weak pull-up to VCC.
7 6 /FAULT
Circuit Breaker Fault Status Output: Open drain N-Channel device, active low.
The /FAULT pin is asserted when the circuit breaker trips due to an overcurrent
condition. Also, this pin indicates undervoltage lockout and overvoltage fault
conditions. The /FAULT pin has a weak pull-up to VCC.
8 7 FB
Power-Good Threshold Input: This input is internally compared to a 1.24V
reference with 3mV of hysteresis. An external resistive divider may be used to
set the voltage at this pin. If this input momentarily goes below 1.24V, then /POR
is activated for one timing cycle, t
POR
, indicating an output undervoltage
condition. The /POR signal de-asserts one timing cycle after the FB pin exceeds
the power-good threshold by 3mV. A 5µs filter on this pin prevents glitches from
inadvertently activating this signal.
9, 10 8 GND Ground Connection: Tie to analog ground
11 9 OV
OV Input: When the voltage on OV exceeds its trip threshold, the GATE pin is
pulled low and the CRWBR timer starts. If OV remains above its threshold long
enough for CRWBR to reach its trip threshold, the circuit breaker is tripped.
Otherwise, the GATE pin begins to ramp up one POR timing cycle after OV
drops below its trip threshold.
12 10 COMPOUT Uncommitted Comparator’s Open Drain Output.
13 11 COMP+ Comparator’s Non-Inverting Input.
14 12 COMP- Comparator’s Inverting Input.
15 Na DIS
Discharge Output: When the MIC2086 is turned off, a 550 internal resistor at
this output allows the discharging of any load capacitance to ground.
16 13 REF
Reference Output: 1.24V nominal. Tie a 0.1µF capacitor to ground to ensure
stability.
17 14 GATE
Gate Drive Output: Connects to the gate of an external N-Channel MOSFET. An
internal clamp ensures that no more than 13V is applied between the GATE pin
and the source of the external MOSFET. The GATE pin is immediately brought
low when either the circuit breaker trips or an undervoltage lockout condition
occurs.
Micrel, Inc. MIC2085/2086
May 2006
4 M9999-050406
(408) 955-1690
Pin Description (Cont.)
Pin Number
MIC2086 Pin Number
MIC2085 Pin Name Pin Function
18 15 SENSE
Circuit Breaker Sense Input: A resistor between this pin and VCC sets the
current limit threshold. Whenever the voltage across the sense resistor exceeds
the slow trip current limit threshold (V
TRIPSLOW
), the GATE voltage is adjusted to
ensure a constant load current. If V
TRIPSLOW
(48mV) is exceeded for longer than
time period t
OCSLOW
, then the circuit breaker is tripped and the GATE pin is
immediately pulled low. If the voltage across the sense resistor exceeds the fast
trip circuit breaker threshold, V
TRIPFAST
, at any point due to fast, high amplitude
power supply faults, then the GATE pin is immediately brought low without delay.
To disable the circuit breaker, the SENSE and VCC pins can be tied together.
The default V
TRIPFAST
for either device is 95mV. Other fast trip thresholds are
available: 150mV, 200mV, or OFF (V
TRIPFAST
disabled). Please contact factory for
availability of other options.
19, 20 16 VCC Positive Supply Input: 2.3V to 16.5V. The GATE pin is held low by an internal
undervoltage lockout circuit until VCC exceeds a threshold of 2.18V.If VCC
exceeds 16.5V, an internal shunt regulator protects the chip from VCC and
SENSE pin voltages up to 33V.
Micrel, Inc. MIC2085/2086
May 2006
5 M9999-050406
(408) 955-1690
Absolute Maximum Ratings(1)
(All voltages are referred to GND)
Supply Voltage (V
CC
)....................................... –0.3V to 33V
SENSE Pin ........................................... –0.3V to V
CC
+ 0.3V
GATE Pin ........................................................ –0.3V to 22V
ON, DIS, /POR, PWRGD, /FAULT,
COMP+, COMP-, COMPOUT......................... –0.3V to 20V
CRWBR, FB, OV, REF...................................... –0.3V to 6V
Maximum Currents
Digital Output Pins ......................................................10mA
(/POR, /FAUTL, PWRGD, COMPOUT)
DIS Pin ........................................................................30mA
EDS Rating
Human Body Model ................................................. 2kV
Machine Model ......................................................200V
Operating Ratings(2)
Supply voltage (V
CC
) .................................... 2.3V to +16.5V
Operating Temperature Range ................... –40°C to +85°C
Junction Temperature (T
J
) ......................................... 125°C
Package Thermal Resistance R
θ(JA)
16-pin QSOP ...................................................112°C/W
20-pin QSOP .....................................................91°C/W
Electrical Characteristics(3)
V
CC
= 5.0V; T
A
= 25°C, unless otherwise noted. Bold indicates specifications over the full operating temperature range of
–40°C to +85°C.
Symbol Parameter Condition Min Typ Max Units
V
CC
Supply Voltage 2.3 16.5 V
I
CC
Supply Current 1.6
2.5 mA
V
UV
Undervoltage Lockout
Threshold
V
CC
rising
V
CC
falling
2.05
1.85 2.18
2.0
2.28
2.10 V
V
V
UVHYST
UV Lockout Hysteresis 180 mV
V
FB
FB (Power-Good) Threshold
Voltage
FB rising 1.19 1.24 1.29 V
V
FBHYST
FB Hysteresis 3 mV
V
OV
OV Pin Threshold Voltage OV pin rising 1.19 1.24 1.29 mV
V
OV
OV Pin Threshold Voltage Line
Regulation
2.3V < V
CC
< 16.5V 5 15 mV
V
OVHYST
OV Pin Hysteresis 3 mV
I
OV
OV Pin Current 0.2 µA
V
TH
POR Delay and Overcurrent
(CFILTER) Timer Threshold
V
CPOR
, V
CFILTER
rising 1.19 1.24 1.29 V
I
CPOR
Power-On Reset Timer Current Timer on
Timer off
–2.5 –2.0
5
–1.5 µA
mA
I
TIMER
Current Limit /Overcurrent
Timer Current (CFILTER)
Timer on
Timer off
–30 –20
2.5
–15 µA
mA
V
CR
CRWBR Pin Threshold Voltage 2.3V < V
CC
< 16.5V 445 470 495 mV
V
CR
CRWBR Pin Threshold Voltage
Line Regulation
2.3V < V
CC
< 16.5V 4 15 µA
mA
I
CR
CRWBR Pin Current CRWBR On, V
CRWBR
= 0V
CRWBR On, V
CRWBR
= 2.1V
CRWBR Off, V
CRWBR
= 1.5V
–60 –45
–1.5
3.3
–30
–1.0
µA
mA
mA
V
TRIPSLOW
40 48 55 mV V
TRIP
Circuit Breaker Trip Voltage
(Current Limit Threshold)
V
TRIP
= V
CC
= V
SENSE
2.3V V
CC
16.5V V
TRIPFAST
x = J
x = K
x = L
80 95
150
200
110 mV
mV
mV
Micrel, Inc. MIC2085/2086
May 2006
6 M9999-050406
(408) 955-1690
Electrical Characteristics (Cont.)
Symbol Parameter Condition Min Typ Max Units
V
CC
< 3V
5V < V
CC
< 9V
V
GS
External Gate Drive V
GATE
– V
CC
9V < V
CC
<15.0V
4
11
4.5
8
12
21-V
CC
9
13
13
V
V
V
I
GATE
GATE Pin Pull-up Current Start cycle, V
GATE
> 0V
V
CC
= 16.5V
V
CC
= 2.3V
–22
–20
–16
–14
–8
–8
µA
µA
I
GATEOFF
GATE Pin Sink Current /FAULT = 0, V
GATE
> 1V
V
CC
= 16.5V
V
CC
= 2.3V
25
12
50
20
mA
mA
V
ON
ON Pin Threshold Voltage ON rising
ON falling
1.19
1.09 1.24
1.14
1.29
1.19
V
V
V
ONHYST
ON Pin Hysteresis 100 mV
I
ON
ON Pin Input Current V
ON
= V
CC
0.5 µA
V
START
Undervoltage Start-up Timer
Threshold
V
CPOR
rising 1.19 1.24 1.29 V
V
OL
/FAULT, /POR, PWRGD Output
Voltage
I
OUT
= 1.6mA
(PWRGD for MIC2086 only)
0.4 V
I
PULLUP
Output Signal Pull-up Current
/FAULT, /POR, PWRGD,
COMPOUT
/FAULT, /POR, PWRGD = GND
(PWRGD FOR MIC2086 only)
–20 µA
V
REF
Reference Output Voltage I
LOAD
= 0mA; C
REF
= 0.1 µF 1.21 1.24 1.27 V
V
LNR
Reference Line Regulation 2.3V < V
CC
< 16.5V 5 10 mV
V
LDR
Reference Load Regulation I
OUT
= 1mA 2.5 7.5 mV
I
RSC
Reference Short-Circuit Current V
REF
= 0V 3.5 mA
V
COS
Comparator Offset Voltage V
CM
= V
REF
–5 5 mV
V
CHYST
Comparator Hysteresis V
CM
= V
REF
3 mV
R
DIS
Discharge Pin Resistance ON pin toggles from HI to LOW 100 550 1000
AC Electrical Characteristics(3)
Symbol Parameter Condition Min Typ Max Units
t
OCFAST
Fast Overcurrent Sense to
GATE Low Trip Time
V
CC
= 5V
V
CC
– V
SENSE
= 100mV
C
GATE
= 10nF, See Figure 1
1 µs
t
OCSLOW
Slow Overcurrent Sense to
GATE Low Trip Time
V
CC
= 5V
V
CC
– V
SENSE
= 50mV
C
FILTER
= 0, See Figure 1
5 µs
t
ONDLY
ON Delay Filter 20 µs
t
FBDLY
FB Delay Filter 20 µs
Notes:
1. Exceeding the absolute maximum rating may damage the device.
2. The device is not guaranteed to function outside its operating rating.
3. Specification for packaged product only.
Micrel, Inc. MIC2085/2086
May 2006
7 M9999-050406
(408) 955-1690
Timing Diagrams
(V
CC
– V
SENSE
)
V
GATE
CFILTER
0
V
TRIPFAST
0
0
1.24V
1V
48mV
t
OCFAST
t
OCSLOW
1V
Figure 1. Current Limit Response
1.24V
1.24V
FB
0
0
0
CPOR
t
POR
/POR
Figure 2. Power-On Reset Response
Figure 3. Power-On Start-Up Delay Timing
0 200 400 600 800 1000
20
50
FB Voltage (mV)
Current Limit Threshold (mV)
Figure 4. Foldback Cu rrent Limit Response
Micrel, Inc. MIC2085/2086
May 2006
8 M9999-050406
(408) 955-1690
Typical Characteristics
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-40 -20 0 20 40 60 80 100
)Am(TNERRUCYLPPUS
TEMPERATURE (°C)
Supply Current
vs. Temperature
V
CC
= 2.3V
V
CC
= 5V
V
CC
= 16.5V
0
1
2
3
4
5
6
7
8
9
10
-40-200 20406080100
I
ROPC
)Am(
TEMPERATURE (°C)
Power-On Reset Timer (Off) Curre
n
vs. Temperature
V
CC
= 16.5V
V
CC
= 5V
V
CC
= 2.3V
10
14
18
22
26
30
34
-40 -20 0 20 40 60 80 100
I
REMIT
)Aµ(
TEMPERATURE (°C)
Overcurrent Timer Current
vs. Temperature
V
CC
= 16.5V
V
CC
= 5V V
CC
= 2.3V
0
2
4
6
8
10
12
14
16
-40 -20 0 20 40 60 80 100
V
SG
)V(
TEMPERATURE (°C)
External Gate Drive
vs. Temperature
V
CC
= 2.3V
V
CC
= 16.5V
V
CC
= 5V
1.20
1.21
1.22
1.23
1.24
1.25
-40 -20 0 20 40 60 80 100
V
HT
)Vm(
TEMPERATURE (°C)
POR Delay/Overcurrent
Timer Threshold
vs. Temperature
V
CC
= 2.3V
V
CC
= 16.5V
V
CC
= 5V
Micrel, Inc. MIC2085/2086
May 2006
9 M9999-050406
(408) 955-1690
Typical Characteristics (Cont.)
45
47
49
51
53
55
-40 -20 0 20 40 60 80 100
V
WOLSPIRT
)Vm(
TEMPERATURE (°C)
Current Limit Threshold
(Slow Trip)
vs. Temperature
V
CC
= 5V
V
CC
= 16.5V
V
CC
= 2.3V
1.15
1.20
1.25
1.30
-40 -20 0 20 40 60 80 100
)V(DLOHSERHTNO
TEMPERATURE (°C)
ON Pin Threshold (Rising)
vs. Temperature
V
CC
= 16.5V
V
CC
= 5V V
CC
= 2.3V
1.05
1.10
1.15
1.20
-40-200 20406080100
)V(DLOHSERHTNO
TEMPERATURE (°C)
ON Pin Threshold (Falling)
vs. Temperature
V
CC
= 16.5V V
CC
= 5V
V
CC
= 2.3V
0.0
0.1
0.2
0.3
0.4
0.5
-40 -20 0 20 40 60 80 100
)V(EGATLOVTESFFOROTARAPMOC
TEMPERATURE (°C)
Comparator Offset Voltage
vs. Temperature
V
CC
= 16.5V
V
CC
= 2.3V
V
CC
= 5V
Micrel, Inc. MIC2085/2086
May 2006
10 M9999-050406
(408) 955-1690
Test Circuit
TCR22-4
Q3
ZTX788A
Q2
R8
GND
Not all pins show for clarity.
CFILTERCPOR
DIS
/POR
FB
GATE
SENSEVCC
ON
MIC2086
0.033µF
C5
0.047µF
C3
0.047µF
C4
R6
39,10 2
15
5
8
17
1819,20
43
2
1
4
DIS
SW2
Signal
Downstream
R7
R5
1%
R4
1%
R1
0.022µF
C2
R
LOAD
C
LOAD
V
OUT
I
OUT
I
IN
Si7892DP
Q1
(PowerPAK™ SO-8)
R
SENSE
5%
0.47µF
C4
R2
1%
R3
1%
ON/OFF
SW1
V
IN
12V
Micrel, Inc. MIC2085/2086
May 2006
11 M9999-050406
(408) 955-1690
Functional Characteristics
Micrel, Inc. MIC2085/2086
May 2006
12 M9999-050406
(408) 955-1690
Functional Characteristics (Cont.)
Micrel, Inc. MIC2085/2086
May 2006
13 M9999-050406
(408) 955-1690
Functional Diagram
MIC2086 Block Diagram
Micrel, Inc. MIC2085/2086
May 2006
14 M9999-050406
(408) 955-1690
Functional Description
Hot Swap Insertion
When circuit boards are inserted into live system
backplanes and supply voltages, high inrush currents
can result due to the charging of bulk capacitance that
resides across the supply pins of the circuit board. This
inrush current, although transient in nature, may be high
enough to cause permanent damage to on-board
components or may cause the system’s supply voltages
to go out of regulation during the transient period which
may result in system failures. The MIC2085/86 acts as a
controller for external N-Channel MOSFET devices in
which the gate drive is controlled to provide inrush
current limiting and output voltage slew rate control
during hot plug insertions.
Power Supply
VCC is the supply input to the MIC2085/86 controller
with a voltage range of 2.3V to 16.5V. The VCC input
can with stand transient spikes up to 33V. In order to
help suppress transients and ensure stability of the
supply voltage, a capacitor of 1.0µF to 10µF from VCC
to ground is recommended. Alternatively, a low pass
filter, shown in the typical application circuit, can be used
to eliminate high frequency oscillations as well as help
suppress transient spikes.
Start-Up Cycle
When the voltage on the ON pin rises above its
threshold of 1.24V, the MIC2085/86 first checks that its
supply (V
CC
) is above the UVLO threshold. If it does
check above, the device is enabled and an internal 2µA
current source begins charging capacitor C
POR
to 1.24V
to initiate a start-up sequence (i.e., start-up delay times
out). Once the start-up delay (t
START
) elapses, CPOR is
pulled immediately to ground and a 15µA current source
begins charging the GATE output to drive the external
MOSFET that switches V
IN
to V
OUT
. The programmed
start-up delay is calculated using the following equation:
()
µFC0.62
I
V
Ct
POR
CPOR
TH
PORSTART
×≅×= (1)
where V
TH
, the POR delay threshold, is 1.24V, and I
CPOR
,
the POR timer current, is 2µA. As the GATE voltage
continues ramping toward its final value (VCC + VGS) at
a defined slew rate (See “Load Capacitance”/“Gate
Capacitance Dominated Start-Up” sections), a second
CPOR timing cycle begins if:1)/FAULT is high and
2)CFILTER is low (i.e., not an overvoltage, undervoltage
lockout, or overcurrent state).This second timing cycle,
t
POR
, starts when the voltage at the FB pin exceeds its
threshold (V
FB
) indicating that the output voltage is valid.
The time period t
POR
is equivalent to t
START
and sets the
interval for the /POR to go Low-to-High after “power is
good” (See Figure 2 of “Timing Diagrams”). Active
current regulation is employed to limit the inrush current
transient response during start-up by regulating the load
current at the programmed current limit value (See
“Current Limiting and Dual-Level Circuit Breaker”
section). The following equation is used to determine the
nominal current limit value:
SENSESENSE
TRIPSLOW
LIM
R
48mV
R
V
I== (2)
where V
TRIPSLOW
is the current limit slow trip threshold
found in the electrical table and R
SENSE
is the selected
value that will set the desired current limit. There are two
basic start-up modes for the MIC2085/86: 1)Start-up
dominated by load capacitance and 2)start-up
dominated by total gate capacitance. The magnitude of
the inrush current delivered to the load will determine the
dominant mode. If the inrush current is greater than the
programmed current limit (ILIM), then load capacitance
is dominant. Otherwise, gate capacitance is dominant.
The expected inrush current may be calculated using the
following equation:
GATE
LOAD
GATE
LOAD
GATE
C
C
15µ
C
C
IINRUSH ×Α≅×≅ (3)
where I
GATE
is the GATE pin pull-up current, C
LOAD
is the
load capacitance, and C
GATE
is the total GATE
capacitance (CISS of the external MOSFET and any
external capacitor connected from the MIC2085/86
GATE pin to ground).
Load Capacitance Dom i nated Start-Up
In this case, the load capacitance, C
LOAD
, is large
enough to cause the inrush current to exceed the
programmed current limit but is less than the fast-trip
threshold (or the fast-trip threshold is disabled, ‘M’
option). During start-up under this condition, the load
current is regulated at the programmed current limit
value (ILIM) and held constant until the output voltage
rises to its final value. The output slew rate and
equivalent GATE voltage slew rate is computed by the
following equation:
LOAD
LIM
OUT
C
I
/dtdV Rate,Slew VoltageOutput = (4)
where I
LIM
is the programmed current limit value.
Consequently, the value of C
FILTER
must be selected to
ensure that the overcurrent response time, t
OCSLOW
,
exceeds the time needed for the output to reach its final
value. For example, given a MOSFET with an input
capacitance C
ISS
= C
GATE
=4700pF, C
LOAD
is 2200µF,
and I
LIMIT
is set to 6A with a 12Vinput, then the load
capacitance dominates as determined by the calculated
INRUSH > I
LIM
. Therefore, the output voltage slew rate
determined from Equation 4 is:
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ms
V
2.73
2200µF
6A
/dtd Rate,Slew VoltageOutput
VOUT
==
and the resulting t
OCSLOW
needed to achieve a 12V
output is approximately 4.5ms. (See “Power-On Reset,
Start-Up, and Overcurrent Timer Delays” section to
calculate t
OCSLOW
.)
GATE Capacitance Domi nated Start-Up
In this case, the value of the load capacitance relative to
the GATE capacitance is small enough such that the
load current during start-up never exceeds the current
limit threshold as determined by Equation 3. The
minimum value of C
GATE
that will ensure that the current
limit is never exceeded is given by the equation below:
LOAD
LIM
GATE
GATE
C
I
I
(min)C ×= (5)
where C
GATE
is the summation of the MOSFET input
capacitance (C
ISS
) and the value of the external
capacitor connected to the GATE pin of the MOSFET.
Once CGATE is determined, use the following equation
to determine the output slew rate for gate capacitance
dominated start-up.
()
GATE
GATE
OUT
C
I
output/dtdV = (6)
Table 1 depicts the output slew rate for various values of
C
GATE
.
I
GATE
= 15µA
CGATE dVOUT/dt
0.001µF 15V/ms
0.01µF 1.5V/ms
0.1µF 0.150V/ms
1µF 0.015 µF/ms
Table 1. Output Slew Rate Selection for GATE
Capacitance Dominated Start-Up
Current Limiting and Dual-Level Circuit Breaker
Many applications will require that the inrush and steady
state supply current be limited at a specific value in order
to protect critical components within the system.
Connecting a sense resistor between the VCC and
SENSE pins sets the nominal current limit value of the
MIC2085/86 and the current limit is calculated using
Equation 2. However, the MIC2085/86 exhibits foldback
current limit response. The foldback feature allows the
nominal current limit threshold to vary from 24mVup to
48mV as the FB pin voltage increases or decreases.
When FB is at 0V, the current limit threshold is 24mV
and for FB 0.6V, the current limit threshold is the
nominal 48mV.(See Figure 4 for Foldback Current Limit
Response characteristic).The MIC2085/86 also features
a dual-level circuit breaker triggered via 48mV and 95mV
current limit thresholds sensed across the VCC and
SENSE pins. The first level of the circuit breaker
functions as follows. Once the voltage sensed across
these two pins exceeds 48mV, the overcurrent timer, its
duration set by capacitor CFILTER, starts to ramp the
voltage at CFILTER using a 2µA constant current
source. If the voltage at CFILTER reaches the
overcurrent timer threshold (VTH) of 1.24V, then
CFILTER immediately returns to ground as the circuit
breaker trips and the GATE output is immediately shut
down. For the second level, if the voltage sensed across
VCC and SENSE exceeds 95mV at any time, the circuit
breaker trips and the GATE shuts down immediately,
bypassing the overcurrent timer period. To disable
current limit and circuit breaker operation, tie the SENSE
and VCC pins together and the CFILTER pin to ground.
Output Undervoltage Detection
The MIC2085/86 employ output undervoltage detection
by monitoring the output voltage through a resistive
divider connected at the FB pin. During turn on, while the
voltage at the FB pin is below the threshold (V
FB
), the
/POR pin is asserted low. Once the FB pin voltage
crosses V
FB
, a 2µA current source charges capacitor
C
POR
. Once the CPOR pin voltage reaches 1.24V, the
time period t
POR
elapses as the CPOR pin is pulled to
ground and the /POR pin goes HIGH. If the voltage at
FB drops below V
FB
for more than 10µs, the/POR pin
resets for at least one timing cycle defined by t
POR
(see
Applications Information for an example).
Input Overvoltage Protection
The MIC2085/86 monitors and detects overvoltage
conditions in the event of excessive supply transients at
the input. Whenever the overvoltage threshold (V
OV
) is
exceeded at the OV pin, the GATE is pulled low and the
output is shut off. The GATE will begin ramping one
POR timing cycle after the OV pin voltage drops below
its threshold. An external CRWBR circuit, as shown in
the typical application diagram, provides a time period
that an overvoltage condition must exceed in order to trip
the circuit breaker. When the OV pin exceeds the
overvoltage threshold (V
OV
), the CRWBR timer begins
charging the CRWBR capacitor initially with a 45µA
current source.Once the voltage at CRWBR exceeds its
threshold (V
CR
) of 0.47V, the CRWBR current
immediately increases to 1.5mA and the circuit breaker
is tripped, necessitating a device reset by toggling the
ON pin LOW to HIGH.
Power-On Reset, Start-Up, and Overcurrent
TimerDelays
The Power-On Reset delay, t
POR
, is the time period for
the /POR pin to go HIGH once the voltage at the FB pin
exceeds the power-good threshold (V
TH
). A capacitor
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connected to CPOR sets the interval, t
POR
, and t
POR
is
equivalent to the start-up delay, t
START
(see Equation 1).
A capacitor connected to CFILTER is used to set the
timer which activates the circuit breaker during
overcurrent conditions. When the voltage across the
sense resistor exceeds the slow trip current limit
threshold of 48mV, the overcurrent timer begins to
charge for a period, t
OCSLOW
, determined by CFILTER. If
no capacitor is used at CFILTER, then t
OCSLOW
defaults
to 5µs. If t
OCSLOW
elapses, then the circuit breaker is
activated and the GATE output is immediately pulled to
ground. The following equation is used to determine the
overcurrent timer period, t
OCSLOW
.
()
µFC0.062
I
V
Ct
FILTER
TIMER
TH
FILTEROCSLOW
×≅×= (7)
where V
TH
, the CFILTER timer threshold, is 1.24V and
I
TIMER
, the overcurrent timer current, is 20µA. Tables 2
and 3 provide a quick reference for several timer
calculations using select standard value capacitors.
C
POR
t
POR
= t
START
0.01µF 6ms
0.02µF 12ms
0.033µF 18.5ms
0.05µF 30ms
0.1µF 60ms
0.33µF 200ms
Table 2. Selected Power-On Reset and
Start-Up Delays
C
FILTER
t
OCSLOW
1800pF 100µs
4700pF 290µs
8200pF 500µs
0.01µF 620µs
0.02µF 1.2ms
0.033µF 2.0ms
0.05µF 3.0ms
0.1µF 6.2ms
0.33µF 20.7ms
Table 3. Selected Overcurrent Timer Delays
Application Information
Output Undervoltage Detection
For output undervoltage detection, the first consideration
is to establish the output voltage level that indicates
“power is good.” For this example, the output value for
which a 12V supply will signal “good” is 11V. Next,
consider the tolerances of the input supply and FB
threshold (VFB). For this example, the 12V supply varies
±5%, thus the resulting output voltage may be as low as
11.4V and as high as 12.6V. Additionally, the FB
threshold has ±50mV tolerance and may be as low
as1.19V and as high as 1.29V. Thus, to determine the
values of the resistive divider network (R5 and R6) at the
FB pin, shown in Figure 5, use the following iterative
design procedure.
1) Choose R6 so as to limit the current through the
divider to approximately 100µA or less.
12.9k
100µ
1.29V
100µ
V
R6
(MAX)
FB
≥
Α
≥
Α
≥
R6 is chosen as 13.3k ± 1%
2) Next, determine R5 using the output “good”
voltage of 11V and the following equation:
()
⎥
⎦
⎤
⎢
⎣
⎡+
=R6
R6R5
VV
FBOUT(Good)
(8)
Using some basic algebra and simplifying Equation 8 to
isolate R5, yields:
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=1
V
V
R6R5
FB(MAX)
OUT(Good)
(8.1)
where V
FB(MAX)
= 1.29V, V
OUT(Good)
= 11V, and R6
is13.3k. Substituting these values into Equation 8.1
now yields R5 = 100.11k. A standard 100k ± 1% is
selected. Now, consider the 11.4V minimum output
voltage, the lower tolerance for R6 and higher tolerance
for R5, 13.17k and101k, respectively. With only
11.4V available, the voltage sensed at the FB pin
exceeds V
FB(MAX)
, thus the /POR and PWRGD
(MIC2086) signals will transition from LOW to HIGH,
indicating “power is good” given the worse case
tolerances of this example.
Input Overvoltage Protection
The external CRWBR circuit shown in Figure 5 consists
of capacitor C4, resistor R7, NPN transistor Q2, and
SCR Q3.The capacitor establishes a time duration for an
overvoltage condition to last before the circuit breaker
trips. The CRWBR timer duration is approximated by the
following equation:
(
)()
µFC40.01
I
VC4
t
CR
CR
OVCR
×≅
×
≅ (9)
where V
CR
, the CRWBR pin threshold, is 0.47V and I
CR
,
the CRWBR pin current, is 45µA during the timer period
(see the CRWBR timer pin description for further
description). A similar design approach as the previous
undervoltage detection example is recommended for the
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overvoltage protection circuitry, resistors R2 and R3 in
Figure 5. For input overvoltage protection, the first
consideration is to establish the input voltage level that
indicates an overvoltage triggering a sys-tem (output
voltage) shut down. For this example, the input value for
which a 12V supply will signal an “output shut down” is
13.2V (+10%). Similarly, from the previous example:
1) Choose R3 to satisfy 100µA condition.
11.9k
100µ
1.19V
100µ
V
R3
(MIN)
OV
≥
Α
≥
Α
≥
R3 is chosen as 13.7k ± 1%.
2) Thus, following the previous example and
substituting R2 and R3 for R5 and R6,
respectively, and 13.2V overvoltage for 11V
output “good”, the same formula yields R2 of
138.3k.The next highest standard 1% value is
140k.
Now, consider the 12.6V maximum input voltage (V
CC
+5%), the higher tolerance for R3 and lower tolerance
for R2, 13.84k and 138.60k, respectively. With a 12.6V
input, the voltage sensed at the OV pin is below V
OV(MIN)
,
and the MIC2085/86will not indicate an overvoltage
condition until VCC exceeds at least 13.2V.
Figure 5. Undervoltage/Overvoltage Circuit
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PCB Connection Sense
There are several configuration options for the
MIC2085/86’sON pin to detect if the PCB has been fully
seated in the backplane before initiating a start-up cycle.
In the typical applications circuit, the MIC2085/86 is
mounted on the PCB with a resistive divider network
connected to the ON pin. R2is connected to a short pin
on the PCB edge connector. Until the connectors mate,
the ON pin is held low which keeps the GATE output
charge pump off. Once the connectors mate, the resistor
network is pulled up to the input supply, 12V in this
example, and the ON pin voltage exceeds its threshold
(V
ON
) of 1.24V and the MIC2085/86 initiates a start-up
cycle. In Figure 6, the connection sense consisting of a
logic-level discrete MOSFET and a few resistors allows
for interrupt control from the processor or other signal
controller to shut off the output of the MIC2085/86. R4
keeps the GATE of Q2 at V
IN
until the connectors are
fully mated. A logic LOW at the/ON_OFF signal turns Q2
off and allows the ON pin to pull up above its threshold
and initiate a start-up cycle. Applying a logic HIGH at the
/ON_OFF signal will turn Q2 on and short the ON pin of
the MIC2085/86 to ground which turns off the GATE
output charge pump.
Figure 6. PCB Connection Sense with ON/OFF Control
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Higher UVLO Setting
Once a PCB is inserted into a backplane (power supply),
the internal UVLO circuit of the MIC2085/86 holds the
GATE output charge pump off until V
CC
exceeds 2.18V.
If VCC falls below 2V, the UVLO circuit pulls the GATE
output to ground and clears the overvoltage and/or
current limit faults. For a higher UVLO threshold, the
circuit in Figure 7 can be used to delay the output
MOSFET from switching on until the desired input
voltage is achieved. The circuit allows the charge pump
to remain off until VIN exceeds 1.24V
R2
R1
1×
⎟
⎠
⎞
⎜
⎝
⎛+. The
GATE drive output will be shut down when V
IN
falls
below 1.14V
R2
R1
1×
⎟
⎠
⎞
⎜
⎝
⎛+. In the example circuit (Figure
7), the rising UVLO threshold is set at approximately 11V
and the falling UVLO threshold is established as 10.1V.
The circuit consists of an external resistor divider at the
ON pin that keeps the GATE output charge pump off
until the voltage at the ON pin exceeds its threshold
(V
ON
) and after the start-up time relapses.
Figure 7. Higher UVLO Setting
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Fast Output Discharge for Capa citive Loads
In many applications where a switch controller is turned
off by either removing the PCB from the backplane or
the ON pin is reset, capacitive loading will cause the
output to retain voltage unless a ‘bleed’ (low impedance)
path is in place in order to discharge the capacitance.
The MIC2086 is equipped with an internal MOSFET that
allows the discharging of any load capacitance to ground
through a 550 path. The discharge feature is
configured by wiring the DIS pin to the output (source) of
the external MOSFET and becomes active (DIS pin
output is low) once the ON pin is deasserted. Figure 8(a)
illustrates the use of the discharge feature with an
optional resistor (R5) that can be used to provide added
resistance in the output discharge path. For an even
faster discharge response of capacitive loads, the
configuration of Figure 8(b) can be utilized to apply a
crowbar to ground through an external SCR (Q3) that is
triggered when the DIS pin goes low which turns on the
PNP transistor (Q2). See the different “Functional
Characteristic” curves for a comparison of the discharge
response configurations.
Figure 8. MIC2086 Fast Discharge of Cap acitive Load
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Auto-Retry Upon Overcurrent Faults
The MIC2085/86 can be configured for automatic restart
after a fault condition. Placing a diode between the ON
and/FAULT pins, as shown in Figure 9, will enable the
auto-restart capability of the controller. When an
application is configured for auto-retry, the overcurrent
timer should be set to minimize the duty cycle of the
overcurrent response to prevent thermal runaway of the
power MOSFET. See “MOSFET Transient Thermal
Issues” section for further detail. A limited duty cycle is
achieved when the overcurrent timer duration (t
OCSLOW
) is
much less than the start-up delay timer duration (t
START
)
and is calculated using the following equation:
100%
t
t
CycleDuty Retry Auto
START
OCSLOW
×=− (10)
An InfiniBand™ Application Circuit
The circuit in Figure 10 depicts a single 50W
InfiniBand™ module using the MIC2085 controller. An
InfiniBand™ backplane distributes bulk power to multiple
plug-in modules that employ DC/DC converters for local
supply requirements. The circuit in Figure 10 distributes
12V from the backplane to the MIC2182 DC/DC
converter that steps down +12V to+3.3V for local bias.
The pass transistor, Q1, isolates theMIC2182’s input
capacitance during module plug-in and allows the
backplane to accommodate additional plug-in modules
without affecting the other modules on the backplane.
The two control input signals are VBxEn_L (active LOW)
and a Local Power Enable (active HIGH). The MIC2085
in the circuit of Figure 10 performs a number of
functions. The gate output of Q1 is enabled by the two
bit input signal VBxEn_L, Local Power Enable = [0,1].
Also, the MIC2085 limits the drain current of Q1 to 7A,
monitors VB_In for an overvoltage condition greater than
16V, and enables the MIC2182 DC/DC converter
downstream to supply a local voltage rail. The
uncommitted comparator is used to monitor VB_In for an
undervoltage condition of less than 10V, indicated by a
logic LOW at the comparator output (COMPOUT).
COMPOUT may be used to control a downstream
device such as another DC/DC converter. Additionally,
the MIC2085 is configured for auto-retry upon an
overcurrent fault condition by placing a diode (D1)
between the /FAULT and ON pins of the controller.
Figure 9. Auto-Retry Configuration
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Figure 10. A 50W InfiniBand™ Application
Sense Resistor Selection
The MIC2085 and MIC2086 use a low-value sense
resistor to measure the current flowing through the
MOSFET switch (and therefore the load). This sense
resistor is nominally valued at 48mV/I
LOAD(CONT)
. To
accommodate worst-case tolerances for both the sense
resistor (allow ±3% over time and temperature for a
resistor with ±1% initial tolerance) and still supply the
maximum required steady-state load current, a slightly
more detailed calculation must be used. The current limit
threshold voltage (the “trip point”) for theMIC2085/86
may be as low as 40mV, which would equate to a sense
resistor value of 40mV/I
LOAD(CONT)
. Carrying the numbers
through for the case where the value of the sense
resistor is 3% high yields:
()
()
LOAD(CONT)LOAD(CONT)
SENSE(MAX)
I
38.8mV
I1.03
40mV
R== (11)
Once the value of R
SENSE
has been chosen in this
manner, it is good practice to check the maximum
I
LOAD(CONT)
which the circuit may let through in the case of
tolerance build-up in the opposite direction. Here, the
worst-case maximum cur-rent is found using a 55mV trip
voltage and a sense resistor that is 3% low in value. The
resulting equation is:
()
()
SENSE(NOM)SENSE(NOM)
MAX)LOAD(CONT,
R
56.7mV
R0.97
55mV
I==
(12)
As an example, if an output must carry a continuous
6Awithout nuisance trips occurring, Equation 11 yields:
6.5m
6A
38.8mV
R
SENSE(MAX)
==
The next lowest standard value is 6.0mW. At the other
set of tolerance extremes for the output in question:
9.45A,
6.0m
56.7mV
I
MAX)LOAD(CONT,
==
almost 10A. Knowing this final datum, we can determine
the necessary wattage of the sense resistor, using
P = I
2
R, where I will be I
LOAD(CONT, MAX)
, and R will be
(0.97)(R
SENSE(NOM)
). These numbers yield the following:
PMAX = (10A)
2
(5.82m) =0.582W.
In this example, a 1W sense resistor is sufficient.
MOSFET Selection
Selecting the proper external MOSFET for use with
theMIC2085/86 involves three straightforward tasks:
• Choice of a MOSFET which meets minimum
voltage requirements.
• Selection of a device to handle the maximum
continuous current (steady-state thermal
issues).
• Verify the selected part’s ability to withstand any
peak currents (transient thermal issues).
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MOSFET Voltage Requirements
The first voltage requirement for the MOSFET is that the
drain-source breakdown voltage of the MOSFET must
be greater than V
IN(MAX)
. For instance, a 16V input may
reasonably be expected to see high-frequency transients
as high as 24V.Therefore, the drain-source breakdown
voltage of the MOSFET must be at least 25V. For ample
safety margin and standard availability, the closest
minimum value should be 30V.
The second breakdown voltage criterion that must be
met is a bit subtler than simple drain-source breakdown
voltage. In MIC2085/86 applications, the gate of the
external MOSFET is driven up to a maximum of 21V by
the internal output MOSFET. At the same time, if the
output of the external MOSFET (its source) is suddenly
subjected to a short, the gate-source voltage will go to
(21V – 0V) = 21V. Since most power MOSFETs
generally have a maximum gate-source breakdown of
20V or less, the use of a Zener clamp is recommended
in applications with V
CC
8V. A Zener diode with 10V to
12V rating is recommended as shown in Figure11. At the
present time, most power MOSFETs with a 20V gate-
source voltage rating have a 30V drain-source break-
down rating or higher. As a general tip, choose surface-
mount devices with a drain-source rating of 30V or more
as a starting point.
Finally, the external gate drive of the MIC2085/86
requires a low-voltage logic level MOSFET when
operating at voltage slower than 3V. There are 2.5V
logic-level MOSFETs avail-able. Please see Table 4,
“MOSFET and Sense Resistor Vendors” for suggested
manufacturers.
MOSFET Steady-State Thermal Issues
The selection of a MOSFET to meet the maximum
continuous current is a fairly straightforward exercise.
First, arm yourself with the following data:
• The value of I
LOAD(CONT, MAX.)
for the output in
question (see “Sense Resistor Selection”).
• The manufacturer’s data sheet for the candidate
MOSFET.
• The maximum ambient temperature in which the
device will be required to operate.
• Any knowledge you can get about the heat
sinking available to the device (e.g., can heat be
dissipated into the ground plane or power plane,
if using a surface-mount part? Is any airflow
available?).
The data sheet will almost always give a value of on
resistance given for the MOSFET at a gate-source
voltage of 4.5V, and another value at a gate-source
voltage of 10V. As a first approximation, add the two
values together and divide by two to get the on-
resistance of the part with 8V of enhancement. Call this
value R
ON
. Since a heavily enhanced MOSFET acts as
an ohmic (resistive) device, almost all that’s required to
determine steady-state power dissipation is to calculate
I2R.The one addendum to this is that MOSFETs have a
slight increase in R
ON
with increasing die temperature. A
good approximation for this value is 0.5% increase in
R
ON
per °C rise in junction temperature above the point
at which R
ON
was initially specified by the manufacturer.
For instance, if the selected MOSFET has a calculated
R
ON
of 10m at a T
J
= 25°C, and the actual junction
temperature ends up at 110°C, a good first cut at the
operating value for R
ON
would be:
R
ON
≅ 10m[1 + (110 - 25)(0.005)] ≅14.3m
The final step is to make sure that the heat sinking
available to the MOSFET is capable of dissipating at
least as much power (rated in °C/W) as that with which
the MOSFET’s performance was specified by the
manufacturer. Here are a few practical tips:
1. The heat from a surface-mount device such a
san SO-8 MOSFET flows almost entirely out of
the drain leads. If the drain leads can be
soldered down to one square inch or more, the
copper will act as the heat sink for the part. This
copper must be on the same layer of the board
as the MOSFET drain.
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Figure 11. Zener Clamped MOSFET GATE
2. Airflow works. Even a few LFM (linear feet per
minute) of air will cool a MOSFET down
substantially. If you can, position the
MOSFET(s) near the inlet of a power supply’s
fan, or the outlet of a processor’s cooling fan.
3. The best test of a surface-mount MOSFET for
an application (assuming the above tips show it
to be a likely fit) is an empirical one. Check the
MOSFET's temperature in the actual layout of
the expected final circuit, at full operating
current. The use of a thermocouple on the drain
leads, or infrared pyrometer on the package, will
then give a reasonable idea of the device’s
junction temperature.
MOSFET Transient Thermal Issues
Having chosen a MOSFET that will withstand the
imposed voltage stresses, and the worse case
continuous I2R power dissipation which it will see, it
remains only to verify the MOSFET’s ability to handle
short-term overload power dissipation without
overheating. A MOSFET can handle a much higher
pulsed power without damage than its continuous
dissipation ratings would imply. The reason for this is
that, like everything else, thermal devices (silicon die,
lead frames, etc.) have thermal inertia.
In terms related directly to the specification and use of
power MOSFETs, this is known as “transient thermal
impedance,” or Z
(J-A)
. Almost all power MOSFET data
sheets give a Transient Thermal Impedance Curve. For
example, take the following case: VIN = 12V, t
OCSLOW
has been set to 100msec, I
LOAD(CONT. MAX)
is 2.5A, the
slow-trip threshold is 48mVnominal, and the fast-trip
threshold is 95mV. If the output is accidentally
connected to a 3 load, the output current from the
MOSFET will be regulated to 2.5A for 100ms (t
OCSLOW
)
before the part trips. During that time, the dissipation in
the MOSFET is given by:
P = E x I E
MOSFET
= [12V-(2.5A)(3)]=4.5V
PMOSFET = (4.5V x 2.5A) = 11.25W for 100msec.
At first glance, it would appear that a really hefty
MOSFET is required to withstand this sort of fault
condition. This is where the transient thermal impedance
curves become very useful. Figure 12 shows the curve
for the Vishay (Siliconix) Si4410DY, a commonly used
SO-8 power MOSFET.
Taking the simplest case first, we’ll assume that once a
fault event such as the one in question occurs, it will be
a long time – 10 minutes or more – before the fault is
isolated and the channel is reset. In such a case, we can
approximate this as a “single pulse” event, that is to say,
there’s no significant duty cycle. Then, reading up from
the X-axis at the point where “Square Wave Pulse
Duration” is equal to 0.1sec (=100msec), we see that the
Z
(J-A)
of this MOSFET to a highly infrequent event of this
duration is only 8% of its continuous R
(J-A)
.
This particular part is specified as having an R
(J-A)
of
50°C/W for intervals of 10 seconds or less. Thus:
Assume T
A
= 55°C maximum, 1 square inch of copper at
the drain leads, no airflow.
Recalling from our previous approximation hint, the part
Micrel, Inc.
MIC2085/2086
May 2006 25
M9999-050406
(408) 955-1690
has an R
ON
of (0.0335/2) = 17m at 25°C.
Assume it has been carrying just about 2.5A for some
time.
When performing this calculation, be sure to use the
highest anticipated ambient temperature (T
A(MAX)
) in
which the MOSFET will be operating as the starting
temperature, and find the operating junction temperature
increase (T
J
) from that point. Then, as shown next, the
final junction temperature is found by adding T
A(MAX
) and
T
J
. Since this is not a closed-form equation, getting a
close approximation may take one or two iterations, but
it’s not a hard calculation to perform, and tends to
converge quickly.
Then the starting (steady-state) T
J
is:
T
J
≅ T
A(MAX)
+ TJ
≅ T
A(MAX)
+ [R
ON
+ (T
A(MAX)
–T
A
)(0.005/°C)
(R
ON
)] x I
2
x R
(J-A)
T
J
≅ 55°C + [17m + (55°C-25°C)(0.005)
(17m)] x (2.5A)
2
x (50°C/W)
T
J
≅ (55°C + (0.122W)(50°C/W)
≅ 61.1°C
Iterate the calculation once to see if this value is within a
few percent of the expected final value. For this iteration
we will start with T
J
equal to the already calculated value
of 61.1°C:
T
J
≅ T
A
+ [17m + (61.1°C-25°C)(0.005)(17m)]
x (2.5A)
2
x (50°C/W)
T
J
≅ ( 55°C + (0.125W)(50°C/W)
≅ 61.27°C
So our original approximation of 61.1°C was very close
to the correct value. We will use TJ = 61°C.
Finally, add (11.25W)(50°C/W)(0.08) = 45°C to the
steady-state T
J
to get T
J(TRANSIENT MAX.)
= 106°C. This is
an acceptable maximum junction temperature for this
part.
Figure 12. Transient Thermal Impedance
Micrel, Inc.
MIC2085/2086
May 2006 26
M9999-050406
(408) 955-1690
PCB Layout Considerations
Because of the low values of the sense resistors used
with theMIC2085/86 controllers, special attention to the
layout must be used in order for the device’s circuit
breaker function to operate properly. Specifically, the
use of a 4-wire Kelvin connection to measure the voltage
across R
SENSE
is highly recommended. Kelvin sensing is
simply a means of making sure that any voltage drops in
the power traces connecting to the resistors does not get
picked up by the traces themselves. Additionally, these
Kelvin connections should be isolated from all other
signal traces to avoid introducing noise onto these
sensitive nodes. Figure 13 illustrates a recommended,
multi-layer layout for the R
SENSE
, Power MOSFET,
timer(s), overvoltage and feedback network connections.
The feed-back and overvoltage resistive networks are
selected for a12V application (from Figure 5). Many hot
swap applications will require load currents of several
amperes. Therefore, the power (VCC and Return) trace
widths (W) need to be wide enough to allow the current
to flow while the rise in temperature for a given copper
plate (e.g., 1 oz. or 2 oz.) is kept to a maximum of 10°C
~ 25°C. Also, these traces should be as short as
possible in order to minimize the IR drops between the
input and the load. For a starting point, there are many
trace width calculation tools available on the web such
as the following link:
http://www.aracnet.com/cgi-usr/gpatrick/trace.pl
Finally, plated-through vias are utilized to make circuit
connections to the power and ground planes. The trace
connections with indicated vias should follow the
example shown for the GND pin connection in Figure 13.
Figure 13. Recommend ed PCB L ayout for Sense Resistor, Power MOSFET,
and Feedback/Overvoltage Network
Micrel, Inc.
MIC2085/2086
May 2006 27
M9999-050406
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MOSFET and Sense Resistor Vendors
Device types and manufacturer contact information for
power MOSFETs and sense resistors is provided in
Table 4. Some of the recommended MOSFETs include a
metal heat sink on the bottom side of the package. The
recommended trace for the MOSFET Gate of Figure 13
must be redirected when using MOSFETs packaged in
this style. Contact the device manufacturer for package
information.
MOSFET Vendors Key MOSFET Type(s) *Applications Contact Information
Vishay (Siliconix) Si4420DY (SO-8 package)
Si4442DY (SO-8 package)
Si3442DV (SO-8 package)
Si7860DP (PowerPAK™ SO-8)
Si7892DP (PowerPAK™ SO-8)
Si7884DP (PowerPAK™ SO-8)
SUB60N06-18 (TO-263)
SUB70N04-10 (TO-263)
I
OUT
10A
I
OUT
= 10A – 15A, V
CC
5V
I
OUT
3A, V
CC
5V
I
OUT
12A
I
OUT
15A
I
OUT
15A
I
OUT
20A, V
CC
5V
I
OUT
20A, V
CC
5V
www.siliconix.com
(203) 452-5664
International Rectifier IRF7413 (SO-8 package)
IRF7457 (SO-8 package)
IRF7822 (SO-8 package)
IRLBA1304 (Super220™)
I
OUT
10A
I
OUT
10A
I
OUT
= 10A – 15A, V
CC
5V
I
OUT
20A, V
CC
5V
www.irf.com
(310) 322-3331
Fairchild Semiconductor FDS6680A (SO-8 package)
FDS6690A (SO-8 package)
I
OUT
10A
I
OUT
10A, V
CC
5V
www.fairchildsemi.com
(207) 775-8100
Philips PH3230 (SOT669-LFPAK) I
OUT
20A www.philips.com
Hitachi HAT2099H (LFPAK) I
OUT
20A www.halsp.hitachi.com
(408) 433-1990
* These devices are not limited to these conditions in many cases, but these conditions are provided as a helpful reference for customer applications
Resistor Vendors Sense Resistors Contact Information
Vishay (Dale) “WSL” Series www.vishay.com/docswsl_30100.pdf
(203) 452-5664
IRC “OARS” Series
”LR” Series
(second source to “WSL”)
www.irctt.com/pdf_files/OARS.pdf
www.irctt.com/pdf_files/LRS.pdf
(828) 264-8861
Table 4. MOSFET an d Sen se Resistor Vendors
Micrel, Inc.
MIC2085/2086
May 2006 28
M9999-050406
(408) 955-1690
Package Information
16-Pin QSOP (QS)
Micrel, Inc.
MIC2085/2086
May 2006 29
M9999-050406
(408) 955-1690
20-Pin QSOP (QS)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http:/www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its
use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant
into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully
indemnify Micrel for any damages resulting from such use or sale.
© 2003 Micrel, Incorporated.
