R2 = 1.225 x

R1

§

¨

©

C4 x 1.225V

10 PA

LM25005

SNVS411C – JANUARY 2006 – REVISED MARCH 2013

www.ti.com

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 0.022 µF, 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:

(8)

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

R5, R6

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

(9)

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 1.0 k

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

for R6 were selected.

R1, R2, C12

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:

(10)

Capacitor C12 provides filtering for the divider. The voltage at the SD pin should never exceed 8V, 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 LM25005 (7V to 42V); therefore these components can be omitted.

With the SD pin open circuit the LM25005 responds once the Vcc UVLO threshold is satisfied.

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. In the limit, spikes beyond the

rating of the LM25005 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 LM25005 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.

16

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