MIC2238
2.5MHz Dual Phase
PWM Buck Regulator
MLF and MicroLead Frame are registered trademarks of Amkor Technology, 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
April 2010 M9999-040810-C
General Description
The Micrel MIC2238 is dual output 2-phase synchronous
buck (step down) PWM DC/DC switching regulator. Power
conversion efficiencies of above 95% are easily obtainable
for a wide range of applications and loads. An ultra-low
quiescent current of 28µA at light loads assures minimum
current draw in battery powered applications that require
standby modes. The 2.5MHz PWM operation of MIC2238
allows the use of miniature 1µH, 2.2µH or 4.7µH inductor
with the use of 2.2µF ceramic output capacitor.
The MIC2238 operates from 2.5V to 5.5V input and is
available in fixed output voltage versions to save space
and reduce external component count. The MIC2238-AA’s
(adjustable version) output voltage versions can be
programmed as low as 0.8V
For applications that require the lowest noise performance,
the /FPWM pin allows the automatic Trickle mode to be
disabled, remaining in full PWM mode. To maximize
battery life in low-dropout conditions, MIC2238 can
operate with a maximum duty cycle of 100%.
The MIC2238 is available in a space-saving 3mm x 3mm
MLF®-12L package with a junction temperature range from
-40°C to +125°C.
Data sheets and support documentation can be found on
Micrel’s web site at www.micrel.com.
Features
• Input voltage range: 2.5V to 5.5V
• Dual output voltages running out of phase
• 28µA quiescent current
• Fixed output voltage versions
• Adjustable version down to 0.8V
• Low noise 2.5MHz PWM operation
• 800mA output current capability for each channel
• Stable with 2.2µH inductor, 2.2µF ceramic cap
• Automatic switching into light load mode of operation
• /FPWM pin allows low noise all-PWM mode operation
• Power good output with internal 5µA current source
allows sequencing with programmable delay time
• Internal soft-start
• 1μA shutdown current
• Built-in soft-start circuitry
• Current limit protection
• Pb-Free 3mm x 3mm MLF®-12L package
Applications
• Cellular phones
• PDAs
• Digital Cameras
• MP3 Players
___________________________________________________________________________________________________________
Typical Application
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Ordering Information
Part Number VOUT1 V
OUT2 Junction
Temperature Range Package Lead Finish
MIC2238-AAYML Adj. Adj. –40°C to +125°C 12-Pin 3mm x 3mm MLF® Pb-Free
MIC2238-521YML 1.28V 1.65V –40°C to +125°C 12-Pin 3mm x 3mm MLF® Pb-Free
MIC2238-G4YML 1.8V 1.2V –40°C to +125°C 12-Pin 3mm x 3mm MLF® Pb-Free
MIC2238-GF9YML 1.8V 1.545V
–40°C to +125°C 12-Pin 3mm x 3mm MLF® Pb-Free
MIC2238-GFHYML 1.8V 1.575V
–40°C to +125°C 12-Pin 3mm x 3mm MLF® Pb-Free
MIC2238-GSYML 1.8V 3.3V –40°C to +125°C 12-Pin 3mm x 3mm MLF® Pb-Free
MIC2238-GWYML 1.8V 1.6V –40°C to +125°C 12-Pin 3mm x 3mm MLF® Pb-Free
MIC2238-J4YML 2.5V 1.2V
–40°C to +125°C 12-Pin 3mm x 3mm MLF® Pb-Free
MIC2238-S4YML 3.3V 1.2V –40°C to +125°C 12-Pin 3mm x 3mm MLF® Pb-Free
MIC2238-SSYML 3.3V 3.3V –40°C to +125°C 12-Pin 3mm x 3mm MLF® Pb-Free
Note:
Other voltages available. Contact Micrel Marketing for details.
MLF® is a GREEN RoHS compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free.
Pin Configur ation
Adjustable
MIC2238-AAYML
12-Pin MLF® (ML)
(Top View)
Fixed
MIC2238-xxYML
12-Pin MLF® (ML)
(Top View)
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April 2010 3 M9999-040810-A
Pin Description
Pin Number
Adjustable Pin Number
Fixed
Pin Name Pin Name
1 – FB2
Feedback 2: For adjustable voltage options connect the
external resistor divider network to FB2 to set the output
voltage of regulator 2. Nominal value is 0.8V.
2 2 EN2
Enable 2 input. Logic low powers down regulator 2. Logic
high powers up regulator 2. MIC2238 features built-in soft-
start circuitry that reduces in-rush current and prevents the
output voltage from overshooting at start up.
3 3 AVIN
Analog Supply Voltage: Supply voltage for the analog control
circuitry. Requires bypass capacitor to GND.
4 4 SW2 Switch node for regulator 2, connected to external inductor.
5 5 AGND Analog (signal) ground.
6 6 PGND Power ground.
7 7 /FPWM
Forced PWM Mode Bar. Grounding this pin forces the device
to stay in constant frequency PWM mode only. Pulling this
pin high enables automatic Trickle mode operation.
8 8 SW1 Switch node for regulator 1, connected to external inductor.
9 9 VIN
Supply Voltage: Supply voltage for the internal switches and
drivers.
10 10 PGOOD
Power Good Output. This output is pulled down unless the
regulator 1 output voltage is within +6.25% and -8.5% of
regulation. After the output voltage is in regulation, the output
starts to go high with an internal 5μA current source. A delay
time could be programmed by tying a capacitor to this pin.
11 11 EN1
Enable 1 input. Logic low powers down regulator 1. Logic
high powers up regulator 1. MIC2238 features built-in soft-
start circuitry that reduces in-rush current and prevents the
output voltage from overshooting at start up.
12 – FB1
Feedback 1: For adjustable voltage options connect to the
external resistor divider network to FB1 to set the output
voltage of regulator 1. Nominal value is 0.8V.
– 1 OUT2
Output Voltage 2. For fixed output voltage options connect
OUT2 to the output voltage of regulator 2.
– 12 OUT1
Output Voltage 1. For fixed output voltage options connect
OUT1 to the output voltage of regulator 1.
EP EP GND Ground, backside pad.
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April 2010 4 M9999-040810-A
Absolute Maximum Ratings(1)
Supply Voltage (VIN).......................................................+6V
Enable 1 Voltage ………………………………………... +6V
Enable 2 Voltage ……………………………………….. +6V
Logic Input Voltage (VEN, VFPWM)............................ VIN to 0V
Storage Temperature (TS) .........................-65°C to +150°C
ESD .............................................................................. 2KV
Operating Ratings(2)
Supply Voltage (VIN) ........................................ 2.5V to 5.5V
Junction Temperature (TJ) .........................-40°C to +125°C
Package Thermal Resistance (θJA) ..........................60°C/W
Electrical Characteristics(4)
TA = 25°C with VIN = VEN1 = VEN2 =3.6V, VOUT1, VOUT2, L= 2.2μH, C = 2.2μF, unless otherwise specified. Bold values indicate -40ºC ≤ TJ
≤+125ºC..
Parameter Condition Min Typ Max Units
Supply Voltage and Current
Supply Voltage Range 2.5 5.5 V
UVLO (rising) 2.3 2.4 2.5 V
UVLO Hysteresis 100 mV
PWM Mode Supply
Current /FPWM = Low; VOUT1, VOUT2= 1.03 * VNOM (not switching) 560 950 µA
Trickle Mode Supply
Current /FPWM = High; VOUT1, VOUT2= 1.03 * VNOM (not switching) 28 50 µA
Shutdown Quiescent
Current VEN = 0V 0.1 1 µA
Output Voltage Accuracy
Feedback voltage, VFB Adjustable 0.780 0.8 0.820 V
Output voltage, VOUT Fixed Output Options
-2.5
+2.5 %
Feedback bias current 10 nA
Output Voltage Line
Regulation 2.5V ≤ VIN ≤ 5.5V 0.1
0.5 %
Output Voltage Load
Regulation
VIN = 5V, IOUT = 10mA to 800mA, /FPWM = 0V
VIN = 3V; IOUT = 10mA to 800mA, /FPWM = 0V 0.5 %
Ripple in Trickle Mode Vin=3.6V; Iout = 1mA; COUT = 2.2µF, L = 2.2µH. 40 mV
Logic Inputs
On 0.8 1.2 V
EN Input Threshold
Off 0.3 0.7 V
EN Input Current 0.01 1 µA
On 0.6xVIN V
/FPWM Input Threshold
Off 0.3xVIN V
/FPWM Input Current 0.01 1 µA
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Electrical Characteristics (cont.)(4)
Parameter Condition Min Typ Max Units
Protection
Current Limit Peak Switch Current, VOUT = 0V 0.9 1.2 1.8 A
Over Temperature
Shutdown Hysteresis 20
°C
Control
Maximum Duty Cycle VFB = 0.7V 100 %
Oscillator
PWM Mode Frequency 2.125 2.5 2.875 MHz
Power Good
Power Good Reset
Threshold
Upper Threshold
Lower Threshold
-14
6.25
-8.5
12
%
PGOOD Series
Resistance 1 1.4 kΩ
PGOOD Pull-Up Current Output within 8.5% of regulation 5 µA
Power Switch
Switch On-Resistance ISW = 150mA (PFET)
ISW = 150mA (NFET)
0.4
0.35
Ω
Notes:
1. Exceeding the absolute maximum rating may damage the device.
2. The device is not guaranteed to function outside its operating rating.
3. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5kΩ in series with 100pF.
4. Specification for packaged product only.
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Typical Characteristics
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Functional Characteristics
VIN = 3.6V, VOUT = 1.8V, L = 2.2µH, /FPWM = 0 VIN = 3.6V, VOUT = 1.8V, L = 2.2µH, /FPWM = 3.6V
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April 2010 8 M9999-040810-A
Functional Characteristics (continued)
Micrel, Inc. MIC2238
April 2010 9 M9999-040810-A
Functional Block Diagram
Functional Description
VIN
VIN provides power to the MOSFETs for the switch
mode regulator section, along with the current limiting
sensing. Due to the high switching speeds, a 10µF
capacitor is recommended close to VIN and the power
ground (PGND) pin for bypassing. Please refer to layout
recommendations.
AVIN
Analog VIN (AVIN) provides power to the analog supply
circuitry. AVIN and VIN must be tied together. Careful
layout should be considered to ensure high frequency
switching noise caused by VIN is reduced before
reaching AVIN. A 1µF capacitor as close to AVIN as
possible is recommended. See layout recommendations
for detail.
EN1
Enable 1 controls the on and off state of regulator 1. A
high logic on Enable 1 (EN1) activates regulator 1 while
a low logic deactivates regulator 1. MIC2238 features
built-in soft-start circuitry that reduces in-rush current
and prevents the output voltage from overshooting at
start up.
EN2
Enable 2 controls the on and off state of regulator 2. A
high logic on Enable 2 (EN2) activates regulator 2 while
a low logic deactivates regulator 2. MIC2238 features
built-in soft-start circuitry that reduces in-rush current
and prevents the output voltage from overshooting at
start up.
/FPWM
The Forced PWM Mode selects the mode of operation
for this device. Grounding this pin forces the device to
stay in constant frequency PWM mode only. Pulling this
pin high enables automatic selection of Trickle or PWM
mode operation, depending on the load. While /FPWM
is high and the load is below 100mA, the device will go
into Trickle mode. If the load is above 100mA, PWM
mode will automatically be selected. Do not leave this
pin floating.
PGOOD
The Power Good Output is pulled down unless the
regulator 1 output voltage is within +6.25% or -8.5% of
regulation. When the output voltage is in regulation, the
PGOOD capacitor will be charged to AVIN by an internal
5μA current source through a 1kΩ resistor. The charge
Micrel, Inc. MIC2238
April 2010 10 M9999-040810-A
time is approximately 1µs per 1pF of capacitance. For
example, a 390pF capacitor at the PGOOD pin will
cause the PGOOD pin voltage to rise from low to high in
around 390µs. A PGOOD capacitor is recommended to
prevent large output voltage transients from triggering
the PGOOD flag unexpectedly.
Figure 1. Power Good Circuit
FB1/FB2
The feedback pin (FB) provides the control path to
control the output. For adjustable versions, a resistor
divider connecting the feedback to the output is used to
adjust the desired output voltage. The output voltage is
calculated as follows:
VOUT =VREF ×R1
R2 +1
⎛
⎝
⎜
⎞
⎠
⎟
where VREF is equal to 0.8V.
A feedforward capacitor is recommended for most
designs using the adjustable output voltage option. To
reduce battery current draw, a 100KΩ feedback resistor
is recommended from the output to the FB pin (R1).
Also, a feedforward capacitor should be connected
between the output and feedback (across R1). The large
resistor value and the parasitic capacitance of the FB pin
can cause a high frequency pole that can reduce the
overall system phase margin. By placing a feedforward
capacitor, these effects can be significantly reduced.
Refer to the Feedback section for recommended
feedforward capacitor values.
SW1/SW2
The switch (SW) pin connects directly to the inductor
and provides the switching current necessary to operate
in PWM mode. Due to the high speed switching on this
pin, the switch node should be routed away from
sensitive nodes.
PGND
Power ground (PGND) is the ground path for the high
current PWM mode. The current loop for the power
ground should be as small as possible and separate
from the Analog ground (AGND) loop. Refer to the layout
considerations for more details.
AGND
Signal ground (AGND) is the ground path for the biasing
and control circuitry. The current loop for the signal
ground should be separate from the Power ground
(PGND) loop. Refer to the layout considerations for more
details.
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Applications Information
Input Capacitor
A minimum 2.2µF ceramic is recommended on the VIN
pin for bypassing. X5R or X7R dielectrics are
recommended for the input capacitor. Y5V dielectrics,
aside from losing most of their capacitance over
temperature, they also become resistive at high
frequencies. This reduces their ability to filter out high
frequency noise.
Output Capacitor
The MIC2238 was designed specifically for use with a
2.2µF or greater ceramic output capacitor. The output
capacitor requires either an X7R or X5R dielectric. Y5V
and Z5U dielectric capacitors, aside from the
undesirable effect of their wide variation in capacitance
over temperature, become resistive at high frequencies.
Inductor Selection
Inductor selection will be determined by the following
(not necessarily in the order of importance);
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
The MIC2238 was designed for use with a 1µH, 2.2µH,
or 4.7µH inductor. For a better load transient response,
a 1µH inductor is recommended. For better efficiency, a
4.7µH inductor is recommended.
Maximum current ratings of the inductor are generally
given in two methods; permissible DC current and
saturation current. Permissible DC current can be rated
either for a 40°C temperature rise or a 10% to 20% loss
in inductance. Ensure the inductor selected can handle
the maximum operating current. When saturation current
is specified, make sure that there is enough margin that
the peak current will not saturate the inductor.
The size requirements refer to the area and height
requirements that are necessary to fit a particular
design. Please refer to the inductor dimensions on their
datasheet.
DC resistance is also important. While DCR is inversely
proportional to size, DCR can represent a significant
efficiency loss. Refer to the Efficiency Considerations.
Compensation
The MIC2238 is an internally compensated, current
mode buck regulator. Current mode is achieved by
sampling the peak current and using the output of the
error amplifier to pulse width modulate the switch node
and maintain output voltage regulation.
The MIC2238 is designed to be stable with a 1µH, 2.2µH
or 4.7µH inductor with a 2.2µF ceramic (X5R) output
capacitor.
Feedback
The MIC2238 provides a feedback pin to adjust the
output voltage to the desired level. This pin connects
internally to an error amplifier. The error amplifier then
compares the voltage at the feedback to the internal
0.8V reference voltage and adjusts the output voltage to
maintain regulation. Calculating the resistor divider
network for the desired output is as follows;
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛−
=
1
V
V
R1
R2
REF
OUT
Where VREF is 0.8V and VOUT is the desired output
voltage.
A 100KΩ from the output to the feedback is
recommended for R1. Larger resistor values require an
additional capacitor (feed- forward) from the output to
the feedback. The large high side resistor value and the
parasitic capacitance on the feedback pin (~10pF) can
cause an additional pole in the control loop. The
additional pole can create a phase loss at high
frequencies. This phase loss degrades transient
response by reducing phase margin. Adding feed-
forward capacitance negates the parasitic capacitive
effects of the feedback pin. Refer to Table 1 for
recommended feedforward capacitor values.
Recommended CFF Total Feedback Resistance
22pF 1M - 2MΩ
47pF 500k -1MΩ
100pF 100k - 500kΩ
180pF 10k - 100kΩ
Table 1. Recommended Feed-Forward Capacitor
Large feedback resistor values increase impedance,
making the feedback node more susceptible to noise
pick-up. A feed-forward capacitor would also reduce
noise pick-up by providing a low impedance path to the
output.
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied.
100
IV
IV
_%Efficiency
ININ
OUTOUT ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
×
=
Maintaining high efficiency serves two purposes. It
reduces power dissipation in the power supply, reducing
the need for heat sinks and thermal design
considerations and it reduces consumption of current for
Micrel, Inc. MIC2238
April 2010 12 M9999-040810-A
battery powered applications. Reduced current draw
from a battery increases the devices operating time and
is critical in hand held devices.
There are two types of losses in switching converters;
DC losses and switching losses. DC losses are simply
the power dissipation of I2R. Power is dissipated in the
high side switch during the on cycle. Power loss is equal
to the high side MOSFET RDSON multiplied by the Switch
Current2. During the off cycle, the low side N-channel
MOSFET conducts, also dissipating power. Device
operating current also reduces efficiency. The product of
the quiescent (operating) current and the supply voltage
is another DC loss. The current required driving the
gates on and off at a constant 2.5MHz frequency and the
switching transitions make up the switching losses.
The Figure above shows an efficiency curve. From no
load to 100mA, efficiency losses are dominated by
quiescent current losses, gate drive and transition
losses. By Forcing the MIC2238 into Trickle Mode
(/FPWM=High), the buck regulator significantly reduces
the required switching current by entering into a PFM
(Pulse Frequency Modulation) mode. This significantly
increases efficiency at low output currents.
Over 100mA, efficiency loss is dominated by MOSFET
RDSON and inductor losses. Higher input supply
voltages will increase the Gate to Source threshold on
the internal MOSFETs, reducing the internal RDSON.
This improves efficiency by reducing DC losses in the
device. All but the inductor losses are inherent to the
device. In which case, inductor selection becomes
increasingly critical in efficiency calculations. As the
inductors are reduced in size, the DC resistance (DCR)
can become quite significant. The DCR losses can be
calculated as follows;
DCRIoutL_Pd 2×=
From that, the loss in efficiency due to inductor
resistance can be calculated as follows;
100
L_PdIV
IV
1_LossEfficiency
OUTOUT
OUTOUT ×
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+×
×
−=
Efficiency loss due to DCR is minimal at light loads and
gains significance as the load is increased. Inductor
selection becomes a trade-off between efficiency and
size in this case.
Trickle Mode Operation
Trickle Mode operation is achieved by clamping the
minimum peak current to approximately 150mA. This
forces a PFM mode by comparing the output voltage to
the internal reference. If the voltage is less than 0.8V,
the MIC2238 turns on the high side until the peak
inductor current reaches approximately 150mA. A
separate comparator then monitors the output voltage. If
the feedback voltage is greater than 0.8V, the high side
switch is used as a 10µA current source, never turning
completely off. This creates a highly efficient light load
mode by increasing the time it takes for the output
capacitor to discharge, delaying the amount of switching
required and increasing light load efficiency. When the
load current is greater than approximately 100mA, the
MIC2238 automatically switches to PWM mode.
FPWM Operation
In Forced PWM Mode (/FPWM=LOW) the MIC2238 is
forced to provides constant switching at 2.5MHz with
synchronous internal MOSFETs throughout the load
current.
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April 2010 13 M9999-040810-A
MIC2238 Adjustable Option (1.8V, 3.3V)
Bill of Materials
Item Part Number Manufacturer Description Qty
C1 C1608X5R0J106K TDK 10µF Ceramic Capacitor, 6.3V, X5R, Size 0603 1
C2 C1005X5R0J105K TDK 1µF Ceramic Capacitor, 6.3V, X5R, Size 0402 1
C3 C0603Y391KXXA Vishay 390pF Ceramic Capacitor, 25V, X7R, Size 0603 1
C4, C7 0603ZD225MAT AVX 2.2µF Ceramic Capacitor, 6.3V, X5R,. Size 0603 2
C5, C6 VJ0603A220KXXAT Vishay 22pF Ceramic Capacitor, 25V, NPO, Size 0603 2
CDRH2D11/HPNP-2R2NC Sumida 2.2µH, 1.1A Isat., 120mΩ, (1.2mmx3.2mmx3.2mm)
L1, L2 LQH43CN2R2M03 Murata 2.2µH, 900mA Isat., 110mΩ, (2.6mmx3.2mm, 4.5mm) 2
R1 CRCW06031374FT1 Vishay 1.37MΩ, 1%, Size 0603 1
R2, R4 CRCW06034423FT1 Vishay 442kΩ, 1%, Size 0603 2
R3 CRCW06035493FT1 Vishay 549kΩ, 1%, Size 0603 1
R5, R6 CRCW06031002FRT1 Vishay 10kΩ, 1%, Size 0603 2
U1 MIC2238-AAYML Micrel 2.5MHz Dual Phase PWM Buck Regulator 1
1. TDK: www.tdk.com
2. Murata: www.murata.com
3. Sumida: www.sumida.com
4. Vishay-Dale: www.vishay.com
5. AVX: www.avx.com
6. Micrel, Inc: www.micrel.com
Micrel, Inc. MIC2238
April 2010 14 M9999-040810-A
Layout Recomme ndations
Top Layer
AGND Layer
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April 2010 15 M9999-040810-A
Layout Recomme ndations
VIN and AVIN Layer
PGND Layer
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April 2010 16 M9999-040810-A
Package Information
12-Pin 3mm × 3mm MLF® (ML)
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.
© 2007 Micrel, Incorporated.
