SC412AEVB datasheet(8/22 Pages) SEMTECH

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SC412A

POWER MANAGEMENT

SC412A Synchronous Buck Controller

The SC412A is a synchronous power supply controller which

simpliﬁes the task of designing a power supply suitable for

powering low voltage circuits.

Battery and +5V Bias Supplies

The SC412A requires an external +5V bias supply in ad-

dition to the battery. If stand-alone capability is required,

the +5V supply can be generated with an external linear

regulator.

Pseudo-Fixed-Frequency Constant On-Time

PWM Controller

The PWM control method is a constant-on-time, pseudo-

ﬁxed-frequency PWM controller, see Figure 1. The ripple

voltage seen across the output capacitor’s ESR provides

the PWM ramp signal, eliminating the need for a current

sense resistor. The on-time is determined by an internal

one-shot whose period is proportional to output voltage,

and inversely proportional to input voltage. A separate

one-shot sets the minimum off-time (typically 350ns).

The typical operating frequency is 325kHz. It is possible

to raise or lower the operating frequency with external

components, refer to the section on Switching Frequency

Variations.

COUT

VBAT

ESR

CIN

VOUT

FB Threshold

0.75V

VOUT/FB Ripple

VPHASE

TON

Figure 1.

On-Time One-Shot (T

ON)

The internal on-time one-shot comparator has two inputs.

One input looks at the output voltage via the VOUT pin,

while the other input samples the input voltage via the LX

pin and converts it to a proportional current which charges

an internal on-time capacitor.

The TON time is the time required for this capacitor to

charge from zero volts to VOUT, thereby making the on-

time directly proportional to output voltage and inversely

proportional to input voltage. This implementation results

in a fairly constant switching frequency without the need of

a clock generator. The internal frequency is optimized for

325kHz. The general equation for the on-time is:

TON (nsec) = 2560 • (VOUT/VBAT) + 35

VOUT Voltage Selection

Output voltage is regulated by comparing VOUT as seen

through a resistor divider to the internal 0.75V reference,

see Figure 2. The output voltage is set by the equation:

VOUT = 0.75 • (1 + R1/R2)

VOUT

To FB pin

Figure 2.

Applications Information

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