AN-1381| Application Note

LM3475 Evaluation Board

National Semiconductor Application Note 1381 Wei Gu May 2005

Introduction
The LM3475 evaluation board is provided as a tool for developing DC/DC converters based on the LM3475 IC. As shown in Figure 1, the evaluation board is configured to provide an output of 2.5V at up to 2A from an input up to 10V. The corresponding bill of material is given in Table 1. Typical efficiencies are shown in Figure 2 and Figure 3. Figure 4 and Figure 5 show the board layout. To aid in the design and evaluation of dc/dc buck converters based on the LM3475 controller, the LM3475 Evaluation Board can be easily re-configured for different output voltages.

VOUT = VFB x (RFB1 + RFB2) / RFB2 Where VFB is 0.8V typically. Refer to the datasheet before changing any component values, since additional design adjustments may be required.

Optional Components
A feed-forward capacitor CFF is placed on the board, which will increase operating frequency. However, the speed up effect decreases with lower output voltage and is negligible below 1.6V output. A zero Ohm is used to pull up the EN pin for always on operation. The enable pin can be pulled low at the EN post to shutdown the device. If this resistor is removed, any analog level signal can be used to enable and disable the device.

Setting Vout
Vout can be set using RFB1, as shown in the following equation:

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FIGURE 1. Full Demo Board Schematic

AN-1381

2005 National Semiconductor Corporation

AN201523

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