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SC412A

POWER MANAGEMENT

Applications Information (continued)

Enable Input

The EN is used to disable or enable the SC412A. When

EN is low (grounded), the SC412A is off and in its lowest-

power state. When EN is high the controller is enabled and

switching will begin.

PSAVE Operation

The SC412A provides automatic power save operation at

light loads. The internal Zero-Cross comparator looks for

inductor current (via the voltage across the lower MOSFET)

to fall to zero on 8 consecutive cycles. Once observed, the

controller then enters power save and turns off the low-side

MOSFET on each cycle when the current crosses zero. To

add hysteresis, the on-time is increased by 25% in power-

save. The efﬁciency improvement at light loads more than

offsets the disadvantage of slightly higher output ripple. If

the inductor current does not cross zero on any switching

cycle, the controller immediately exits power save. Since

the controller counts zero crossings, the converter can

sink current as long as the current does not cross zero on

eight consecutive cycles. This allows the output voltage to

recover quickly in response to negative load steps.

Smart Power Save Protection

In some applications, active loads can leak current from a

higher voltage and thereby cause VOUT to slowly rise and

reach the OVP threshold, leading to a hard shutdown. The

SC412A uses Smart Power Save to prevent this. When the

feedback signal exceeds 8% above nominal (810mV), the IC

exits power save operation (if already active) and DL drives

high to turn on the low-side MOSFET, which draws current

from VOUT via the inductor. When FB drops back to the

0.75V trip point, a normal TON switching cycle begins. This

method cycles energy from VOUT back to VBAT and prevents

a hard OVP shutdown, and also minimizes operating power

by avoiding continuous conduction-mode operation.

Current Limit Circuit

Current limiting can be accomplished in two ways. The RD-

SON of the lower MOSFET can be used as a current sensing

element, or a sense resistor at the lower MOSFET source

can be used if greater accuracy is needed. RDSON sensing

is more efﬁcient and less expensive. In both cases, the R

ILIM

resistor sets the over-current threshold. The R

ILIM connects

from the ILIM pin to either the lower MOSFET drain (for RD-

SON sensing) or the high side of the current-sense resistor.

ILIM connects to a 10μA current source from the ILIM pin

which turns on when the low-side MOSFET turns on, after

the on-time DH pulse has completed. If the voltage drop

across the sense resistor or low-side MOSFET exceeds the

voltage across the R

ILIM resistor, current limit will activate.

The high-side MOSFET will then not turn on until the voltage

drop across the sense element (resistor or MOSFET) falls

below the voltage across the R

ILIM resistor.

This current sensing scheme actually regulates the inductor

valley current, see Figure 3. This means that if the current

limit is set to 10A, the peak current through the inductor

would be 10A plus the peak ripple current, and the average

current through the inductor would be 10A plus 1/2 the

peak-to-peak ripple current.

I LIMIT

I LOAD

IPEAK

TIME

Valley Current Limit

Figure 3.

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