SC415_08 datasheet(12/30 Pages) SEMTECH

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SC415

Applications Information (continued)

Smart Psave 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 SC415 uses Smart Power Save to prevent

this. When the output voltage exceeds 8% above nominal

(810mV at FB), that converter then exits PSAVE (if already

active), and DL drives high to energize the low-side

MOSFET. This will draw current from VOUT via the inductor

causing VOUT to fall. When FB drops to the 750mV trip

point, a normal TON switching cycle begins. This method

cycles energy from VOUT back to VIN and prevents a hard

OVP shutdown, and also minimizes operating power

by avoiding continuous conduction-mode operation.

If a light load is present, DH/DL switching continues for

8 consecutive cycles and then the IC re-turns to PSAVE

mode to reduce operating power.

Current Limit

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 RILIM resistor sets the over-current threshold.

RILIM connects from the ILIM pin to either the lower

MOSFET drain (for RDSON sensing) or the high side of the

current-sense resistor. RILIM connects to a 10μA current

source from the ILIM pin which turns on when the low-

side MOSFET turns on (DL is high). If the voltage drop

across the sense resistor or low-side MOSFET exceeds

the voltage across RILIM, then the voltage at the ILIM

pin will be negative or below GND, and current limit will

activate. The high-side MOSFET is not allowed to turn on

until the voltage drop across the sense resistor or MOSFET

falls below the voltage across the RILIM resistor (ILIM pin

reaches GND). If the overload at the output continues, the

DH pulses will get farther apart, and the output voltage

will fall. Eventually the output will fall enough to cause FB

to drop to 525mV, activating the under-voltage protection

and shutting down the converter.

The current sensing scheme actually regulates the

inductor valley current, (see Figure 4). 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 ½ the peak-to-peak ripple current.

Figure 4

The RDSON sensing circuit is shown in Figure 5 with RILIM

= R1 and RDSON of Q2.

Figure 5

I LIMIT

I LOAD

IPEAK

TIME

Valley Current Limit

VOUT

+5V

VIN

PAD

VDD

ILIM

BST

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