January 16, 2008
5A, 500kHz PowerWise® Synchronous Buck Regulator
The LM20125 is a full featured 500 kHz synchronous buck
regulator capable of delivering up to 5A of continuous output
current. The current mode control loop can be compensated
to be stable with virtually any type of output capacitor. For
most cases, compensating the device only requires two ex-
ternal components, providing maximum flexibility and ease of
use. The device is optimized to work over the input voltage
range of 2.95V to 5.5V making it suited for a wide variety of
low voltage systems.
The device features internal over voltage protection (OVP)
and over current protection (OCP) circuits for increased sys-
tem reliability. A precision enable pin and integrated UVLO
allows the turn-on of the device to be tightly controlled and
sequenced. Start-up inrush currents are limited by both an
internally fixed and externally adjustable Soft-Start circuit.
Fault detection and supply sequencing is possible with the
integrated power good circuit.
The LM20125 is designed to work well in multi-rail power
supply architectures. The output voltage of the device can be
configured to track a higher voltage rail using the SS/TRK pin.
If the output of the LM20125 is pre-biased at startup it will not
sink current to pull the output low until the internal soft-start
ramp exceeds the voltage at the feedback pin.
The LM20125 is offered in a 16-pin eTSSOP package with an
exposed pad that can be soldered to the PCB, eliminating the
need for bulky heatsinks.
■ Input voltage range 2.95V to 5.5V
■ Accurate current limit minimizes inductor size
■ 97% efficiency with 500 kHz switching frequency
■ 32 mΩ integrated FET switches
■ Starts up into pre-biased loads
■ Output voltage tracking
■ Peak current mode control
■ Adjustable output voltage down to 0.8V
■ Adjustable Soft-Start with external capacitor
■ Precision enable pin with hysteresis
■ Integrated OVP, UVLO, power good and thermal
■ eTSSOP-16 exposed pad package
■ Simple to design, high efficiency point of load regulation
from a 5V or 3.3V bus
■ High Performance DSPs, FPGAs, ASICs and
■ Broadband, Networking and Optical Communications
Typical Application Circuit
PowerWise® is a registered trademark of National Semiconductor Corporation.
© 2008 National Semiconductor Corporation 300302
NSC Package Drawing
92 Units of Rail
250 Units of Tape an Reel
2500 Units of Tape and Reel
Soft-Start or Tracking control input. An internal 5 µA current source charges an external capacitor to
set the soft-start ramp rate. If driven by a external source less than 800 mV, this pin overrides the
internal reference that sets the output voltage. If left open, an internal 1ms soft-start ramp is activated.
Feedback input to the error amplifier from the regulated output. This pin is connected to the inverting
input of the internal transconductance error amplifier. An 800 mV reference connected to the non-
inverting input of the error amplifier sets the closed loop regulation voltage at the FB pin.
Power good output signal. Open drain output indicating the output voltage is regulating within
tolerance. A pull-up resistor of 10 kΩ to 100 kΩ is recommend for most applications.
External compensation pin. Connect a resistor and capacitor to this pin to compensate the device.
These pins must be connected to GND to ensure proper operation.
Input voltage to the power switches inside the device. These pins should be connected together at the
device. A low ESR capacitor should be placed near these pins to stabilize the input voltage.
Switch pin. The PWM output of the internal power switches.
Power ground pin for the internal power switches.
Precision enable input for the device. An external voltage divider can be used to set the device turn-
on threshold. If not used the EN pin should be connected to PVIN.
Internal 2.7V sub-regulator. This pin should be bypassed with a 1 µF ceramic capacitor.
Analog input supply that generates the internal bias. Must be connected to VIN through a low pass
Quiet analog ground for the internal bias circuitry.
Exposed metal pad on the underside of the package with a weak electrical connection to ground. It is
recommended to connect this pad to the PC board ground plane in order to improve heat dissipation.
wwAwb.DsatoaSlhueette4UM.coamximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Power Dissipation (Note 2)
Lead Temperature (Soldering,
Minimum ESD Rating (Note 3)
Voltages from the indicated pins to GND
AVIN, PVIN, EN, PGOOD, SS/
TRK, COMP, FB, SW
-0.3V to +6V
-65°C to 150°C
PVIN, AVIN to GND
2.95V to 5.5V
−40°C to + 125°C
Electrical Characteristics Unless otherwise stated, the following conditions apply: AVIN = PVIN = VIN = 5V.
Limits in standard type are for TJ = 25°C only, limits in bold face type apply over the junction temperature (TJ) range of -40°C to
+125°C. Minimum and Maximum limits are guaranteed through test, design, or statistical correlation. Typical values represent the
most likely parametric norm at TJ = 25°C, and are provided for reference purposes only.
Min Typ Max Unit
Feedback pin voltage
Switch Current Limit Threshold
High-Side Switch On Resistance
VIN = 2.95V to 5.5V
IOUT = 100 mA to 5A
VIN = 3.3V
ISW = 3.5A
0.788 0.8 0.812
6.7 7.4 8.1
Low-Side Switch On Resistance
ISW = 3.5A
32 52 mΩ
Operating Quiescent Current
Shutdown Quiescent current
VIN Under Voltage Lockout
VIN Under Voltage Lockout Hysteresis
Soft-Start Pin Source Current
SS/TRK Accuracy, VSS - VFB
Non-switching, VFB = VCOMP
VEN = 0V
IVCC = 0 µA
VSS/TRK = 0V
VSS/TRK = 0.4V
Maximum Duty Cycle
Minimum On Time
Current Sense Blanking Time
Error Amplifier and Modulator
ILOAD = 0A
After Rising VSW
440 500 560 kHz
Feedback pin bias current
COMP Output Source Current
COMP Output Sink Current
Error Amplifier Transconductance
Error Amplifier Voltage Gain
VFB = 0.8V
VFB = VCOMP = 0.6V
VFB = 1.0V, VCOMP = 0.6V
ICOMP = ± 50 µA
1 100 nA
450 510 600 µmho
Over Voltage Protection Rising Threshold
Over Voltage Protection Hysteresis
PGOOD Rising Threshold
PGOOD Falling Hysteresis
PGOOD deglitch time
PGOOD Low Sink Current
PGOOD High Leakage Current
With respect to VFB
With respect to VFB
VPGOOD = 0.4V
VPGOOD = 5V
105 108 111
92 94 96
webmaste! click here