SC508 datasheet(19/32 Pages) SEMTECH

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SC508/SC508A

Pre-Bias Start-up

SC508 can support soft-start with an output pre-bias. The

SS ramp time is the same as a normal start-up when the

output voltage starts from zero. Under a pre-bias start-up,

the DH and DL drivers inhibit switching until 40% of the

ramp at the SS pin equals the pre-bias FB voltage level.

Pre-bias start-up is achieved by turning off the lower

MOSFET when the inductor current reaches zero during

the soft-start cycle. This method prevents the output

voltage from decreasing.

Power Good Output

The PGOOD (power good) output is an open-drain output

which requires a pull-up resistor. During start-up, PGOOD

is held low and is not allowed to transition high until the

output voltage is in regulation and the SS pin has reached

67% of VDDA. The time from EN going high to PGOOD

going high is typically 11ms for CSS = 10nF.

When the voltage at the FB pin is 10% below the nominal

voltage, PGOOD is pulled low. Once PGOOD pulls low

there is typically 2% hysteresis to prevent chatter on the

PGOOD output.

PGOOD will transition low if the FB voltage exceeds +20%

of nominal (720mV), which is also the over-voltage shut-

down threshold. PGOOD also pulls low if the EN pin is low

and VDDA is present.

Output Over-Voltage Protection

Over-voltage protection (OVP) becomes active as soon as

the device is enabled. The OVP threshold is set at 600mV

+ 20% (720mV). There is a 5μs delay built into the OVP

detector to prevent false transitions. When V

FB exceeds the

OVP threshold, DL is driven high and the low-side MOSFET

is turned on. DL remains high and the controller remains

off while the device goes through the automatic fault

recovery cycle. When the automatic recovery cycle is

completed, the device will attempt a new soft-start cycle.

At the start of the soft-start cycle, the DL output will go

low for typically 30usec while the controller initializes the

soft-start sequence. PGOOD is also low after an OVP

event.

Output Under-Voltage Protection

When V

FB falls 25% below its nominal voltage (falls to

450mV) for eight consecutive clock cycles, the switcher is

shut off and the DH and DL drives are pulled low to tri-

state the MOSFETs. The controller stays off while the

device goes through the automatic fault recovery cycle.

Automatic Fault Recovery

The SC508 includes an automatic recovery feature (hiccup

mode upon fault). If the switcher output is shut down due

to a fault condition, the device uses the SS capacitor as a

timer. Upon fault detection the SS pin is pulled low and

then begins charging through the internal 3μA current

source. When the SS capacitor reaches 67% of VDDA, the

SS pin is again pulled low, after which the SS capacitor

begins another charging cycle. The SS capacitor will be

used for 15 cycles of charging from 0 to 67% of VDDA.

During these cycles the switcher is off and there is no

MOSFET switching.

During the 16th SS charging cycle, the normal soft-start

routine is implemented and the MOSFETs begin switch-

ing. Switching continues until the Power Good Start-up

Delay Time is reached. If the switcher output is still in a

fault condition, the switcher will again shut down and

force 15 cycles of SS charging before attempting another

soft-start. The long delay between soft-start cycles

reduces the average power loss in the power

components.

The automatic recovery timing is shown in Figure 12.

15 cycles

tHICCUP = 15 x tEN_PGOOD

tEN_PGOOD

67% x VDDA

1 soft-start cycle

fault

applied

15 cycles

1 soft-start cycle

tEN_PGOOD

tHICCUP = 15 x tEN_PGOOD

Figure 12 — Automatic Recovery Timing

If the fault was due to an over-voltage condition, the DL

output will remain high during the 15 SS charging cycles.

For all other faults, the DL output will remain low. In all

cases, during the 16th SS charging cycle, DL will drive low

for typically 30usec as the controller initializes a soft-start

cycle.

Applications Information (continued)

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