SC412AEVB datasheet(15/22 Pages) SEMTECH

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SC412A

POWER MANAGEMENT

Applications Information (continued)

by increasing the duty cycle (more time is spent drawing

energy from VBAT as losses increase). Since the on-time

is constant for a given VOUT/VBAT combination, the way

to increase duty cycle is to gradually shorten the off-time.

The net effect is that switching frequency increases slightly

with increasing load.

The typical operating frequency is 325kHz. It is possible to

raise the frequency by placing a resistor divider between

the output and the VOUT pin, see Figure 7. This reduces

the voltage at the VOUT pin which is used to generate the

on-time according to the previous equation. Note that

this places a small minimum load on the output. The new

frequency is approximated by the following equation:

FREQ (kHz) = 325 ∙ (1 + R1/R2)

COUT

ESR

VOUT

VLX

pin 10

(VOUT)

1nF

Power Output

Figure 7.

It is also possible to lower the frequency using a resistive

divider to the 5V bias supply, see Figure 3. This raises the

voltage at the VOUT pin which will increase the on-time.

Note that this results in a small leakage path from the 5V

supply to the output voltage. The resistor values should be

fairly large (>50kOhm) large to prevent the output voltage

from drifting up during shutdown conditions. Note that

the feedback resistors act as a dummy load to limit how

far the output can rise.

The new operating frequency is approximated by the

equation:

FREQ (kHz) = 325 ∙ ((R1 + R2) / (R1 + R2 ∙ VCC/VOUT))

COUT

ESR

VOUT

VLX

pin 10

(VOUT)

1nF

Power Output

VCC

Figure 8.

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