SC475AEVB datasheet(14/27 Pages) SEMTECH

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SC475A

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POWER MANAGEMENT

Note that the presence of Rs/Cs will affect the effective

resistance at the FB pin, and therefore modiﬁes the VOUT

setpoints. If Rs is used, the following table shows the

calculated values for VOUT.

VOUT Equation

0.75 • (1 + R1/R2)

0.75 • (1 + R1/R2 + R1/(R3+Rs))

Enable Input

The EN is used to disable or enable the SC475A. When

EN is low (grounded), the SC475A is off and in its lowest-

power state. When EN is high the controller is enabled

and switching will begin.

PSAVE Operation

The SC475A 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 eight consecutive switching cycles. Once

observed, the controller 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, or to voltage transitions from a higher to a

lower voltage where the change exceeds 8%.

Smart Power Save Protection

In some applications, active loads on VOUT can leak

current from a higher voltage and thereby cause VOUT to

slowly rise and reach the OVP threshold, causing a hard

shutdown; the SC475A uses Smart Power Save to prevent

this. When FB exceeds 8% above nominal (810mV), the

IC exits power save (if already active) and DL drives high to

turn on the low-side MOSFET, which starts to draw current

from VOUT via the inductor. When FB drops to the 0.75V

trip point, a normal TON switching cycle begins. This cycles

energy from VOUT back to VBAT and prevents a hard OVP

shutdown, and also minimizes operating power by avoiding

continuous conduction-mode operation. If a light load is

present, the switching continues for 8 consecutive clock

cycles and then the IC will re-enter power save to reduce

operating power.

Current Limit Circuit

Current limiting can be accomplished in two ways. The

RDSON 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 RDSON sensing) or the high side of the

current-sense resistor. R

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

is held off until the voltage drop across the sense element

(resistor or MOSFET) falls below the voltage across the

ILIM resistor.

This current sensing scheme actually regulates the

inductor valley current, (see Figure 10). 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.

Applications Information (continued)

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