AN-1884| Application Note

LM26003 Evaluation Board

National Semiconductor Application Note 1884 Frederik Dostal August 28, 2008

Introduction
The LM26003 evaluation board is designed to demonstrate the capabilities of the LM26003 switching regulator. The LM26003 board, schematic shown in Figure 1, is configured to provide an output of 3.3V at up to 3A from an input range of 4V to 38V. The nominal operating frequency is 300 kHz and can be synchronized from +30% to -20% of nominal using the SYNC connection post. The evaluation board is designed to operate at ambient temperatures up to 75 C. Typical evaluation board waveforms and performance curves are shown in Figure 2 through Figure 4. Figure 5 and Figure 6 show the pcb trace layout. To aid in the design and evaluation of dc/dc buck converters based on the LM26003 regulator, the evaluation board can be re-configured for different output voltages and operating frequencies. Test points are also provided to enable easy connection and monitoring of critical signals. For more information about device function and circuit design, refer to the LM26003 datasheet.

with single resistors R2 and R5 respectively. However, large changes to the default settings may require other changes to the inductor, output capacitor and compensation network. Several optional component pads have been provided for application flexibility. The C8 pad is provided for an additional ceramic output capacitor, This capacitor is used to filter noise on the output line to prevent noise from coupling into the Vbias line when the IC is supplied by Vout. If the Vbias function is not used, C8 is not necessary. C11 is a phase lead capacitor which can be installed to increase phase and gain margin. See the compensation section of the datasheet for more detailed information. The resistor R8 can be used as an injection point for a loop stability measurement. R7 is populated with a 0 resistor. A higher resistor value can be used on this location to build an RC filter in combination with C2 bypass capacitor.

Jumper Settings
The FPWM jumper is used to disable the sleep mode function. For normal operation, select `off' which connects FPWM to GND. For FPWM operation (sleep mode disabled), select `on'. The Vbias jumper connects the VBIAS pin to Vout. When Vout is greater than 3V, the VBIAS function will be activated for improved efficiency. To disable VBIAS, or if Vout is set to less than 3V, set the jumper to `GND'.

Powering Up
Before powering up the LM26003 evaluation board, all external connections should be verified. The power supply input must be turned off and connected with proper polarity to the VIN and GND posts. The load should be connected between the VOUT post and GND post. Both the VIN and VOUT connections should use the GND post closest to VIN or VOUT. Output voltage can be monitored with a DVM or oscilloscope at the VOUT post. Once all connections have been verified, input power can be applied. The load can be on or off at startup. If the EN post is left open, the output voltage will ramp up when VIN is applied.

Optional Components
Before changing the default components, please refer to the product datasheet for information regarding component selection. Output voltage and frequency are easily adjustable

AN-1884

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FIGURE 1. Evaluation Board Schematic

2008 National Semiconductor Corporation

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