SC475AEVB datasheet(9/27 Pages) SEMTECH

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SC475A

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POWER MANAGEMENT

Applications Information

SC475A Synchronous Buck Controller

The SC475A is a synchronous power supply controller

which simpliﬁes the task of designing a dual-level power

supply suitable for controlling video chip sets and other

dual-voltage circuits. The SC475A provides an input (G0)

which controls an internal pull-down transistor used to

select from two adjustable output voltages.

Battery and +5V Bias Supplies

The SC475A requires an external +5V bias supply in addi-

tion to the VBAT supply. If stand-alone capability is required,

the +5V bias 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).

COUT

VBAT

ESR

CIN

VOUT

FB Threshold

0.75V

VFB

VLX

TON

Figure 1

On-Time One-Shot (T

ON)

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

One input senses output voltage via the VOUT pin, while

the other input samples VBAT via the LX pin and creates

a proportional current which charges an internal capaci-

tor. The TON time is the time required for this capacitor to

charge from zero volts to VOUT, thereby making TON 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 • (V

OUT/VBAT) + 35

Immediately after the DH on-time, the DL output drives high

to energize the low-side MOSFET. DL has a minimum high

time of typically 350nsec, after which DL will continue to

stay high until one of the following occur:

FB drops to the 0.75V reference

The Zero Cross detector trips if power save is active

The Negative Current Limit detector trips

The Zero Cross detector monitors the voltage across the

low-side MOSFET and trips when it reaches zero. If this oc-

curs on eight consecutive cycles, then DL will subsequently

shut off when the Zero Cross detector trips. See the PSAVE

Operation section. Both MOSFETS will then stay off until

FB drops to 0.75V, which will begin the next DH on-time.

This is normal operation at light load.

The Negative Current Limit detector trips when the drain

voltage at the low-side MOSFET reaches typically +80mV,

indicating a large negative current is being drawn through

the inductor from VOUT. When this occurs, DL drives low.

Both MOSFETS will then stay off until FB drops to 0.75V,

which will begin the next DH on-time. Tripping the Negative

Current detector is rare.

If DL drives low because FB has dropped to the 0.75V refer-

ence, then another DH on-time is started: this is normal

operation at heavy load. If DL drives low because of the

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