October 2013 Rev. 1.5 www.microsemi.com 1
© 2013 Microsemi Corporation
1.4MHz, 1A Synchronous
Buck Converter
VOUT
R1
R2
L 2.2µH
VIN
EN
SW
FB
GND
LX7186A
ON OFF
CIN
4.7µF
VIN
COUT
22µF
1 (6)
3 (4)
2 (1, 5)
5 (2)
4 (3)
TSOT-5L (UDFN 2x2 6L)
Figure 1 · Typical Application of LX7186A
Features
Input Supply Range: 2.5V to 5.5V
Output Adjustable from 0.6V to VIN – 0.5V
100% Duty Cycle in Dropout
Integrated NMOS & PMOS Switches
Current Mode Control
1A Maximum Output Current
Fixed 1.4MHz Frequency
High Efficiency: Up To 96%
Built-in Soft-start
Built-in UV & OT Protection
Built-in Short Circuit Protection
RoHS Compliant & Halogen Free
TSOT-5L or UDFN 2x2 6L Packages
Applications
Datacom
Portable Devices
Smart Phone
Description
The LX7186A is 1.4MHz fixed frequency, current-
mode, synchronous PWM buck (step-down) DC-DC
converter, capable of driving a 1A load with high
efficiency, excellent line and load regulation. The
device integrates synchronous P-channel and N-
channel power MOSFET switches with low on-
resistance. They accept an input voltage range from
2.5V to 5.5V and will enter 100% duty cycle at
dropout making them ideal for powering portable
equipment that runs from a single Li-ion battery.
A standard series of inductors are available from
several different manufacturers optimized for use
with the LX7186A. This feature greatly simplifies the
design of switch-mode power supplies.
The converter includes standard safety features
such as over-current, short-circuit and thermal
shutdown protection. This device is available in
both TSOT-5L and UDFN 2x2 6L packages.
LX7186A
1.4MHz 1A Synchronous Buck Converter
2
Pin Configuration and Pinout
EN
VIN
GND
FB
SW
1
4
5
2
3
86A
YWWA = bottom mark
Figure 2 · Pinout TSOT-5 Top View
Marking: Front Mark 86A
Bottom Mark YWWA
Year/Work Week/Lot Code
Figure 3 · Pinout UDFN 2x2 6L Top View
Marking: Line1 186A
Line2 YWWA
Year/Work Week/Lot Code
Ordering Information
Pin Description
Pin Number
Pin Designator
Description
TSOT-5L
UDFN 2x2 6L
1
6
VIN
Supply Input Pin. A 4.7µF ceramic capacitor should be connected
between the VIN pin and GND pin to bypass the supply.
2
1, 5
GND
Ground Pin.
3
4
EN
Enable Input. Setting this pin above 1.5V enables the IC. Setting this pin
below 0.4V shuts down the IC. When the IC is in shutdown mode, all
functions are disabled to decrease the supply current below 1µA.
4
3
FB
Feedback Pin. This pin is connected to an external resistor divider to
program the system output voltage.
5
2
SW
Power Switch Output Pin. Inductor connection to drain of the internal
PFET and NFET switches.
Ambient
Temperature
Type
Package
Part Number
Packaging Type
-40°C to 85°C
RoHS Compliant,
Pb-free
TSOT-5L
LX7186AISG
Bulk / Tube
LX7186AISG-TR
Tape and Reel
UDFN 2x2 6L
LX7186AILU
Bulk / Tube
LX7186AILU-TR
Tape and Reel
Block Diagram
3
Block Diagram
VOLTAGE
REFERENCE OSCILLATOR
CURRENT
SENSE
ERROR
AMPLIFIER PWM
COMPARATOR MAX
CURRENT LIMIT
0.6V
VOCP
0.4V
LOGIC
CLK
DRIVER
SHORT
CIRCUIT
PROTECTION
REVERSE
COMPARATOR
EN
FB
GND
SW
GNDVIN VIN
3 (4)
4 (3)
2 (1, 5)
2 (1, 5)
1 (6) 1 (6)
5 (2)
TSOT-5L (UDFN 2x2 6L)
Figure 4 · Simplified Block Diagram of LX7186A
1.4MHz 1A Synchronous Buck Converter
4
Absolute Maximum Ratings
Parameter
Min
Max
Units
VIN to GND
-0.3
6
V
EN, FB to GND
-0.3
VIN + 0.3
V
SW to GND
-0.3
VIN + 0.3
V
Junction Temperature
150
°C
Storage Temperature
-65
150
°C
Peak Package Solder Reflow Temperature (40s, reflow)
260 (+0,-5)
°C
Lead Soldering Temperature (10 seconds)
260
°C
Note: Performance is not necessarily guaranteed over this entire range. These are maximum stress ratings only.
Exceeding these ratings, even momentarily, can cause immediate damage, or negatively impact long-term
operating reliability
Operating Ratings
Min
Max
Units
VIN
2.5
5.5
V
VOUT
0.6
VIN – 0.5
V
Ambient Temperature
-40
85
°C
Output Current
0
1
A
Thermal Properties
Package
Thermal Resistance
Typ
Units
TSOT-5
θJA
160
°C/W
UDFN 2x2 6L
θJA
86
°C/W
Note: The JA number assumes no forced airflow. Junction Temperature is calculated using TJ = TA + (PD x JA). In
particular, θJA is a function of the PCB construction. The stated number above is for a four-layer board in
accordance with JESD-51 (JEDEC).
Electrical Characteristics
Note: Unless otherwise specified, the following specifications apply at VIN = VEN = 3.3V. -40°C < TA < 85°C.
Symbol
Parameter
Test Condition
Min
Typ
Max
Units
Operating Current
IQ
Quiescent Current
VFB = 0.65V
62
100
µA
ISHDN
Shutdown Supply Current
VEN = GND
0.1
1
µA
VIN UVLO
VUVLO
Under Voltage Lockout
VIN rising
2.3
V
VHYS
UVLO Hysteresis
200
mV
Electrical Characteristics
5
Symbol
Parameter
Test Condition
Min
Typ
Max
Units
FEEDBACK VOLTAGE
VREF
Feedback Voltage
0.588
0.6
0.612
V
IFB
FB Pin Input Bias Current
VFB = VIN
-100
100
nA
∆VOUT
Output Voltage Accuracy
-2
2
%
OUTPUT
RDSON_P
PMOS Switch RDSON
ISW = 200mA
0.28
Ω
RDSON_N
NMOS Switch RDSON
ISW = -200mA
0.25
Ω
ILEAK
NMOS Switch Leakage
Current
VIN = 3.3V, VSW = 3.3V
0.1
µA
ILIM
Switch Current Limit
VFB = 0.55V
1.5
2.0
A
TOTSD
Thermal Shutdown
160
°C
THYS
Thermal Shutdown
Hysteresis
20
°C
OSCILLATOR
fOSC
Oscillator Frequency
1.12
1.40
1.68
MHz
DMAX
Maximum Duty Cycle
VFB = 0V
100
%
DMIN
Minimum Duty Cycle
VFB = 0.65V
0
%
SOFT START
TSS
Soft Start Time
1
ms
EN INPUT
VEN_H
EN Pin Threshold
1.5
V
VEN_L
0.4
V
1.4MHz 1A Synchronous Buck Converter
6
Typical Performance Curves -- (Efficiency in PSM)
Figure 5 · Efficiency vs. Output Current with 5V Input UDFN Package
Figure 6 · Output Voltage vs. Output Current with 3.3V Input UDFN Package
65%
70%
75%
80%
85%
90%
95%
100%
1 10 100 1000
Efficiency
Output Current (mA)
VOUT = 1V
VOUT = 1.2V
VOUT = 1.8V
VOUT = 3V
VOUT = 3.3V
70%
80%
90%
100%
1 10 100 1000
Efficiency
Output Current (mA)
VOUT = 1V
VOUT = 1.2V
VOUT = 1.8V
VOUT = 3V
Typical Performance Curves -- (VIN = 3.3V, VOUT = 2.5V)
7
Typical Performance Curves -- (VIN = 3.3V, VOUT = 2.5V)
Figure 7 · Efficiency vs. Input Voltage
Figure 8 · Output Voltage vs. Output Current
Figure 9 · Feedback Voltage vs. Temperature
Figure 10 · Frequency vs. Temperature
Figure 11 · OCP Current Limit vs. Temperature
IOUT= 0.5A
IOUT= 1.0A
68
76
84
92
100
2 3 4 5 6
Efficiency (%)
Input Voltage (V)
2.47
2.48
2.49
2.50
2.51
0 200 400 600 800 1000
Output Voltage (V)
Output Current (mA)
0.57
0.58
0.59
0.60
0.61
0.62
-40 0 40 80 120
Feedback Voltage (V)
Temperature (°C)
1.2
1.3
1.4
1.5
1.6
1.7
-40 0 40 80 120
Frequency (MHz)
Temperature (°C)
1.2
1.4
1.6
1.8
2.0
2.2
-40 0 40 80 120
OC Current Limit (A)
Temperature (°C)
1.4MHz 1A Synchronous Buck Converter
8
Theory of Operation / Application Information
Basic Operation
The LX7186A is a synchronous step-down converter operating with a typically 1.4MHz fixed
frequency pulse width modulation (PWM) at moderate to heavy load currents and in power-saving
mode (PSM) when operating at light load currents. It is capable of delivering a 1A output current over
a wide input voltage range from 2.5 to 5.5V.
At the beginning of each cycle initiated by the clock signal (from the internal oscillator), the P-channel
MOSFET switch is turned on, and the inductor current ramps up until the comparator trips and the
control logic turns off the switch. The current limit comparator also turns off the switch in case the
current limit of the P-channel MOSFET is exceeded. Then the N-channel synchronous switch is
turned on and the inductor current ramps down. The next cycle is initiated by the clock signal again,
turning off the N-channel synchronous switch and turning on the P-channel switch (See Figure 4).
Two operational modes are available: PSM and PWM. The internal synchronous rectifier with low
RDSON dramatically reduces conduction loss at PWM mode. No external Schottky diode is required in
practical application. The LX7186A enters PSM at extremely light load condition. The equivalent
switching frequency is reduced to increase the efficiency in PSM.
As the input supply voltage decreases to a value approaching the output voltage, the duty cycle
increases to the maximum. Further reduction of the supply voltage forces the P-channel main switch
to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be
determined by the input voltage minus the voltage drop across the P-channel MOSFET and the
inductor. This is particularly useful in battery powered applications to achieve longest operation time
by taking full advantage of the whole battery voltage range.
Typical Application
A general LX7186A application circuit is shown in Figure 12. External component selection is driven
by the load requirement, and begins with the selection of the inductor L. Once L is chosen, CIN and
COUT can be selected.
R1
R2
VIN
EN
SW
FB
L 2.2µH
CIN
4.7µF
VIN
GND
COUT
22µF
VOUT
LX7186A
1 (6)
3 (4)
5 (2)
4 (3)
2 (1, 5)
SOT23-5L (UDFN 2x2 6L)
Figure 12 · Typical Application
Theory of Operation / Application Information
9
Component Selection
Inductor Selection
Although the inductor does not influence the operating frequency, the inductor value has a direct
effect on ripple current. The inductor ripple current ∆IL decreases with higher inductance and
increases with higher VIN or VOUT.
 


Accepting larger values of ∆IL allows the use of low inductances, but results in higher output voltage
ripple, greater core losses, and lower output current capability. A typical ∆IL value is 20% to 40% of
output current.
Another important parameter for the inductor is the current rating. Exceeding an inductor's maximum
current rating may cause the inductor to saturate and overheat. Once the inductor value has been
selected, the peak inductor current can be calculated as the following:
   

It should be ensured that the current rating of the selected inductor is 1.5 times of the IPEAK.
Input Capacitor Selection
Because the buck converter has a pulsating input current, a low ESR input capacitor is required. This
results in the best input voltage filtering and minimizing the interference with other circuits caused by
high input voltage spikes. Also the input capacitor must be sufficiently large to stabilize the input
voltage during heavy load transients. Ceramic capacitors show a good performance because of the
low ESR value, and they are less sensitive to voltage transients and spikes. Place the input capacitor
as close as possible to the input pin of the device for best performance. The typical value is about
4.7µF. The X5R or X7R ceramic capacitors have the best temperature and voltage characteristics,
which is good for the input capacitor.
Output Capacitor Selection
The output capacitor is the most critical component of a switching regulator, it is used for output
filtering and keeping the loop stable. The selection of COUT is driven by the required ESR to minimize
voltage ripple and load step transients. Typically, once the ESR requirement is satisfied, the
capacitance is adequate for filtering. The output ripple (∆VOUT) is determined by:
 

The output ripple is highest at maximum input voltage since ∆IL increases with input voltage.
Once the ESR requirements for COUT have been met, the RMS current rating generally far exceeds
the IRIPPLE (P-P) requirement, except for an all ceramic solution. In most applications, a 22µF ceramic
capacitor is usually enough for these conditions.
At light load currents, the device operates in PSM mode, and the output voltage ripple is independent
of the output capacitor value. The output voltage ripple is set by the internal comparator thresholds.
The typical output voltage ripple is 1% of the output voltage VOUT.
1.4MHz 1A Synchronous Buck Converter
10
Feedback Divider Resistors
The LX7186A develops a 0.6V reference voltage between the feedback pin, FB, and the signal
ground as shown in Figure 13. The output voltage is set by a resistive divider according to the
following formula:
 

Keeping the current small (<40µA) in these resistors maximizes efficiency, but making them too small
(<20µA) may allow stray capacitance to cause noise problems and reduce the phase margin of the
error amp loop.
The Output resistor divider values are recommended below.
VOUT
R1
R2
L 2.2µH
SW
FB
COUT
22µF
5 (2)
4 (3)
LX7186A
VOUT
R1
R2
0.9V
12.1k
24.3k
1.2V
24.3k
24.3k
1.8V
47.5k
24.3k
2.5V
76.8k
24.3k
3.0V
95.3k
24.3k
3.3V
107k
24.3k
Figure 13 · Output Circuit
Layout Consideration
PCB layout is very important to the performance of the LX7186A. The traces where switching current
flows should be kept as short as possible. The external components (especially CIN) should be
placed as close to the IC as physically possible, therefore use wide and short traces for the main
current paths.
Try to route the feedback trace as far from the inductor and noisy power traces as possible. You
should also make the feedback trace connection as direct as possible and of reasonable thickness.
These two criteria sometimes involve a trade-off, but keeping the trace it away from the inductor and
other noise sources is the more critical of the two. Locate the feedback divider resistor network near
the feedback pin with short leads.
Flood all unused areas on all layers with copper. Flooding with copper will help to reduce the
temperature rise of power components. These copper areas should be connected to one of the input
supplies.
For detailed PCB layout consideration, please refer to LX7186A EVB User Guide.
Package Outline Dimensions
11
Package Outline Dimensions
e1
e
E
b
E1
D
A2
AA1
c
L2
L
Dim
MILLIMETERS
INCHES
MIN
MAX
MIN
MAX
A
-
1.00
-
0.039
A1
0.01
0.10
0.0004
0.004
A2
0.84
0.90
0.033
0.035
b
0.30
0.45
0.012
0.018
C
0.12
0.20
0.005
0.008
D
2.90 BSC
0.114 BSC
E
2.80 BSC
0.110 BSC
E1
1.60 BSC
0.063 BSC
e1
1.90 BSC
0.075 BSC
e
0.95 BSC
0.037 BSC
L
0.30
0.50
0.012
0.020
L2
0.25 BSC
0.010 BSC
Figure 14 · SG 5-Pin TSOT Package Dimensions
D
E
L
e
E2
b
Pin 1 ID
A
A1
D2
K
Dim
MILLIMETERS
INCHES
MIN
MAX
MIN
MAX
A
0.6
0.024
A1
0.00
0.05
0.000
0.002
K
0.15 MIN
0.006MIN
e
0.65 BSC
0.026 BSC
L
0.25
0.35
0.010
0.014
b
0.25
0.35
0.010
0.014
D2
1.35
1.55
0.059
0.067
E2
0.90
1.10
0.031
0.039
D
2.00 BSC
0.079 BSC
E
2.00 BSC
0.079 BSC
Figure 15 · LU 6-Pin Plastic UDFN 2mm x 2mm x 0.6mm Package Dimensions
Note: 1. Dimensions do not include mold flash or protrusions; these shall not exceed
0.155mm(.006”) on any side. Lead dimension shall not include solder coverage.
Note: 2. Dimensions are in mm, inches are for reference only.
1.4MHz 1A Synchronous Buck Converter
12
Land Pattern Recommendation
0.95mm
2.35mm
0.45mm
3.20mm
0.80mm
1.65mm
1.65mm 0.35mm
1.00mm 2.40mm
0.55mm
Figure 16 · SG 5-Pin TSOT Package Footprint
Figure 17 · LU 6-Pin UDFN Package Footprint
Disclaimer:
This PCB land pattern recommendation is based on information available to Microsemi by its suppliers. The actual land pattern to be used could be different depending on the
materials and processes used in the PCB assembly, end user must account for this in their final layout. Microsemi makes no warranty or representation of performance based
on this recommended land pattern.
PRODUCTION DATA – Information contained in this document is proprietary to Microsemi and is
current as of publication date. This document may not be modified in any way without the express
written consent of Microsemi. Product processing does not necessarily include testing of all
parameters. Microsemi reserves the right to change the configuration and performance of the product
and to discontinue product at any time.
LX7186A-6/1.5
Microsemi Corporation (NASDAQ: MSCC) offers a comprehensive portfolio of semiconductor
solutions for: aerospace, defense and security; enterprise and communications; and industrial
and alternative energy markets. Products include high-performance, high-reliability analog and
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complete subsystems. Microsemi is headquartered in Aliso Viejo, Calif. Learn more at
www.microsemi.com.
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