AN-1693| Application Note

LM20145 Evaluation Board

National Semiconductor Application Note 1693 Dennis Hudgins October 30, 2007

Introduction
The LM20145 is a full featured buck switching regulator capable of driving up to 5A of load current. The switching frequency of the LM20145 can be varied from 250kHz to 750kHz with an external resistor allowing the size of the power stage components to be reduced while still providing efficient operation. The LM20145 is capable of converting an input voltage between 2.95V and 5.5V down to an output voltage as low as 0.8V. Fault protection features include cycle-by-cycle current limit, output power good, and output over-voltage protection. The dual function soft-start/tracking pin can be used to control the startup response of the LM20145, and the precision enable pin can be used to easily sequence the LM20145 in applications with sequencing requirements. The LM20145 is available in an eTSSOP-16 package with an exposed pad for enhanced thermal performance.

The LM20145 evaluation board has been designed to balance overall solution size with the efficiency of the regulator. The evaluation board measures just under 1.3" x 1.1" on a two layer PCB, with all components placed on the top layer. The power stage and compensation components of the LM20145 evaluation board have been optimized for an input voltage of 5V and a switching frequency of 500 kHz, but for testing purposes, the input can be varied across the entire operating range. The output voltage of the evaluation board is nominally 1.2V, but this voltage can be easily changed by replacing one of the feedback resistors (RFB1 or RFB2). The control loop compensation of the LM20145 evaluation board has been designed to provide a stable solution over the entire input and output voltage range with a reasonable transient response. The EN pin must be above 1.18V (typ) on the board to initiate switching. If the EN function is not necessary, the EN pin should be externally tied to VIN.

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FIGURE 1. Evaluation Board Schematic

AN-1693

2007 National Semiconductor Corporation

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