SC493 datasheet(16/28 Pages) SEMTECH

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SC493

abled or set to ultrasonic before the output is slewed. This

will ensure that the low side MOSFET will turn back on at

the end of the slewing sequence even if there is no load

and bring the output voltage back into regulation. Figure

4 demonstrates this with a 1000µF output capacitor and

with psave disabled before slewing from VFBadj = +9% to

VFBadj = -9%.

SCL (5V/div)

Time (500μs/div)

OUT (100mV/div)

PGOOD (5V/div)

Figure 4 — Slew Down in CCM with Large Output

Capacitors

Soft-shutdown

SC493 features a soft-shutdown feature: whenever switch-

ing is disabled, an internal 10Ω pull-down activates on the

Vout pin which discharges the output capacitor and holds

Vout low. The pull-down stays active even if the EN pin is

subsequently driven low. Note that the pull-down will not

activate if EN pin has not been high after power has been

applied or power is cycled with EN pin low.

SmartDriveTM

For each DH pulse, the DH driver initially turns on the

high-side MOSFET at a slower speed, allowing a softer,

smooth turn-off of the low-side diode. Once the diode is

off and the LX voltage has risen 0.5V above PGND, the

SmartDrive circuit automatically drives the high-side

MOSFET on at a rapid rate. This technique reduces switch-

ing noise while maintaining high efficiency, reducing the

need for snubbers or series resistors in the gate drive.

Dynamic Output Voltage Control

SC493 allows changing the output voltage when the part

is already switching. Whenever the output voltage is

changed via margining or by fine adjust, the controller

changes the internal reference by small intermediate steps

using 32us per step. This ensures that the output keeps up

with internal slewing and excessive current does not build

up in the inductor. The faults are blanked till the end of the

slewing. Blanking is released when 8 switching cycles

occur after the end of internal slewing since the switching

cycles indicate that the part is in regulation. The part is

enabled to operate in DCM mode to prevent negative

current build up in the inductor when slewing down. The

part returns to the previous operational state (DCM or

CCM) at the end of blanking.

Since the controller operates in DCM, it depends on the

load to discharge the output when slewing down.Therefore,

no switching cycles may occur if the load is very light and

the output will not keep pace with the internal slew. In such

cases, the blanking at the end of slewing will end after a

1ms timeout whether any switching cycles occur or not and

the part will return to its previous operational state.

For applications that need to have the output slew down at

the same rate as the internal slewing, register bit Enable

Internal Load (EnIntLd) can be set. This activates an internal

10Ω pull down on Vout when the part is slewing down. This

resistance discharges the output capacitor and helps the

output keep pace with internal slewing. To help maximize

efficiency, the pulldown will not activate when slewing up

even if the feature is enabled via register settings.

When the internal pull down is enabled, it is active only for

the time it takes the internal reference to reach its target.

For example, when slewing from VFBadjust of +9% to 0%,

the pull down is active only for 12 x 32µs = 384µs. In appli-

cations with large output capacitors, this time may not be

sufficient to discharge the output to keep up with the

internal reference. Therefore, the output can be higher

than the desired regulation value at the end of the slewing

sequence. If the part was in DCM before slewing began,

the output will stay at the higher value at the end of the

sequence. If there is no load, the output will return to the

desired value very slowly. In such applications, to ensure

that the controller quickly brings the output voltage into

regulation, it is recommended that either PSAVE be dis-

Applications Information (continued)

SC493 Datasheet(PDF) 16 Page - Semtech Corporation

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