SC493 datasheet(14/28 Pages) SEMTECH

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SC493

The switching frequency is changed to the programmed

value by scaling the on time as needed.

Power Save Mode Programming

The SC493 provides selectable power-save operation at

light loads. When register bits PSV1,0 are set to 01 or 10

the power save mode is enabled. With power save

enabled, the zero crossing comparator monitors the

inductor current via the voltage across the low-side

MOSFET. If the inductor current falls to zero for 8 consecu-

tive cycles then the controller enters power save and turns

off the low-side FET on each subsequent cycle as long as

the current crosses zero. If the inductor current does not

reach zero for 8 consecutive switching cycles the control-

ler immediately exits power save. The controller counts

zero crossings and therefore the converter can sink current

as long as the current does not cross zero on 8 consecu-

tive consecutive cycles. This allows the output voltage to

recover quickly in response to negative load steps.

The SC493 can also be operated in forced Continuous

Conduction Mode (CCM) by setting PSV1,0 = 00 or 11.

With these settings the device will not enter PSAVE and

operates at programmed frequency even at light loads.

This feature provides user flexibility for system design.

Figure 1 shows operation under power save and continu-

ous conduction mode at light loads.

FB Ripple

Voltage

(VFB)

FB threshold

Inductor

Current

Zero (0A)

DH On-time is triggered when

VFB reaches the FB Threshold.

(500mV)

On-time (TON)

DL drives high when on-time is completed.

DL remains high until inductor current reaches zero.

Dead time varies

according to load

Figure 1 — Power-save Operation

capacitor from 0V to the voltage at the positive input. This

makes the on-time proportional to the output voltage and

inversely proportional to the input voltage, providing a near-

constant switching frequency when the input and output

voltage vary. A second comparator compares the voltage at

the feedback pin to a fixed internal reference voltage to

determine when to turn on the high side MOSFET.

Power-on Delay Programming

The power-on delay is programmable via the I2C interface.

The power-on delay is defined as the time from when the

ENSW bit is set to when the PWM control is enabled and the

part begins switching. At the end of the power-on delay time,

the PWM control circuit is enabled in a phased manner to

ensure proper operation. The phased enabling of internal

circuitry adds up to 48μs to the power-on delay time. On start-

up (controller enabled by applying power and driving the EN

pin high). The controller can take up to 1ms for internal

initialization following which the power-on delay is applied.

Soft-Start Operation and Programming

Soft-start is achieved in the PWM controller by using an

internalvoltagerampasthereferencefortheFBComparator.

The voltage ramp is generated using an internal charge

pump which drives the reference from zero to 500mV in ~

2mV increments, using an internal oscillator. When the

ramp voltage reaches 500mV, the ramp is ignored and the

FB comparator switches over to a fixed 500mV threshold.

During soft-start the output voltage tracks the internal

ramp, which limits the start-up inrush current and provides

a controlled soft-start profile for a wide range of applica-

tions. Soft-start programmability is achieved by changing

the frequency of the oscillator. The soft-start ramp reaches

500mV in 90% of the programmed soft-start time. The

remaining 10% of the programmed soft-start time is used

to allow the system to stabilize before enabling the PGOOD

comparator to drive the PGOOD pin high.

During soft-start the controller turns off the low-side

MOSFET on any cycle if the inductor current falls to zero.

This prevents negative inductor current, allowing the

device to start into a pre-biased output.

Frequency Programming

The nominal switching frequency in continuous

conduction mode is programmable via the I2C interface.

Applications Information (continued)

SC493 Datasheet(PDF) 14 Page - Semtech Corporation

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