R2 = 1.225 x

R1

VIN(min) + (5 x 10

-6 x R1) ± 1.225

§

¨

©

21

LM5576, LM5576-Q1

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SNVS447J – JANUARY 2007 – REVISED NOVEMBER 2014

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For a 5 V output, the R5/R6 ratio calculates to 3.082. The resistors should be chosen from standard value

resistors, a good starting point is selection in the range of 1.0 k

Ω - 10 kΩ. Values of 5.11 kΩ for R5, and 1.65 kΩ

for R6 were selected.

8.2.2.12 R1, R2, C12

A voltage divider can be connected to the SD pin to set a minimum operating voltage VIN(min) for the regulator. If

this feature is required, the easiest approach to select the divider resistor values is to select a value for R1

(between 10 k

Ω and 100 kΩ recommended) then calculate R2 from:

(16)

Capacitor C12 provides filtering for the divider. The voltage at the SD pin should never exceed 8 V, when using

an external set-point divider it may be necessary to clamp the SD pin at high input voltage conditions. The

reference design utilizes the full range of the LM5576 (6V to 75V); therefore these components can be omitted.

With the SD pin open circuit the LM5576 responds once the VCC UVLO threshold is satisfied.

8.2.2.13 R7, C11

A snubber network across the power diode reduces ringing and spikes at the switching node. Excessive ringing

and spikes can cause erratic operation and couple spikes and noise to the output. Voltage spikes beyond the

rating of the LM5576 or the re-circulating diode can damage these devices. Selecting the values for the snubber

is best accomplished through empirical methods. First, make sure the lead lengths for the snubber connections

are very short. For the current levels typical for the LM5576 a resistor value between 5 and 20 Ohms is

adequate. Increasing the value of the snubber capacitor results in more damping but higher losses. Select a

minimum value of C11 that provides adequate damping of the SW pin waveform at high load.

8.2.2.14 R4, C5, C6

These components configure the error amplifier gain characteristics to accomplish a stable overall loop gain. One

advantage of current mode control is the ability to close the loop with only two feedback components, R4 and C5.

The overall loop gain is the product of the modulator gain and the error amplifier gain. The DC modulator gain of

the LM5576 is as follows:

DC Gain(MOD) = Gm(MOD) x RLOAD = 2 x RLOAD

(17)

The dominant low frequency pole of the modulator is determined by the load resistance (RLOAD,) and output

capacitance (COUT). The corner frequency of this pole is:

fp(MOD) = 1 / (2π RLOAD COUT)

(18)

For RLOAD = 5 Ω and COUT = 177 µF then fp(MOD) = 180 Hz

DC Gain(MOD) = 2 x 5 = 10 = 20 dB

For the design example of Figure 18 the following modulator gain vs. frequency characteristic was measured as

shown in Figure 20.