R2 = 1.225 x

R1

§

¨

©

C4 x 1.225V

10 PA

LM5574, LM5574-Q1

www.ti.com

SNVS478F – JANUARY 2007 – REVISED APRIL 2013

The forward voltage drop has a significant impact on the conversion efficiency, especially for applications with a

low output voltage. “Rated” current for diodes vary widely from various manufacturers. The worst case is to

assume a short circuit load condition. In this case the diode will carry the output current almost continuously. For

the LM5574 this current can be as high as 0.7A. Assuming a worst case 1V drop across the diode, the maximum

diode power dissipation can be as high as 0.7W. For the reference design a 100V Schottky in a SMA package

was selected.

C1

The regulator supply voltage has a large source impedance at the switching frequency. Good quality input

capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the switch current

during the on-time. When the buck switch turns on, the current into the VIN pin steps to the lower peak of the

inductor current waveform, ramps up to the peak value, then drops to zero at turn-off. The average current into

VIN during the on-time is the load current. The input capacitance should be selected for RMS current rating and

Quality ceramic capacitors with a low ESR should be selected for the input filter. To allow for capacitor

tolerances and voltage effects, one 1.0µF, 100V ceramic capacitor will be used. If step input voltage transients

are expected near the maximum rating of the LM5574, a careful evaluation of ringing and possible spikes at the

device VIN pin should be completed. An additional damping network or input voltage clamp may be required in

these cases.

C8

of C8 should be no smaller than 0.1µF, and should be a good quality, low ESR, ceramic capacitor. A value of

0.47µF was selected for this design.

C7

The bootstrap capacitor between the BST and the SW pins supplies the gate current to charge the buck switch

gate at turn-on. The recommended value of C7 is 0.022µF, and should be a good quality, low ESR, ceramic

capacitor.

C4

The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage and the output

voltage, to reach the final regulated value. The time is determined from:

(10)

For this application, a C4 value of 0.01µF was chosen which corresponds to a soft-start time of 1ms.

R5, R6

R5 and R6 set the output voltage level, the ratio of these resistors is calculated from:

(11)

For a 5V 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 1.0k

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

R6 were selected.

R1, R2, C2

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

(between 10k

Ω and 100kΩ recommended) then calculate R2 from:

(12)

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