SC416 datasheet(12/34 Pages) SEMTECH

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SC416

Applications Information (continued)

Smart Psave Protection

In some applications, circuits connected to 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 SC416 uses Smart Psave 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 MOS-

FET. 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 meth-

od cycles energy from VOUT back to VIN and prevents

a hard OVP shutdown, and also minimizes operating

power by avoiding continuous conduction-mode opera-

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

for 8 consecutive cycles and then the IC returns to Psave

mode to reduce operating power.

Current Limit

For current limiting, 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 accu-

racy 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 ei-

ther 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 is on (DL is high). If the

voltage drop across the sense resistor or low-side MOS-

FET 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 resis-

tor 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, activat-

ing the under-voltage protection and shutting down the

converter.

The current sensing scheme actually regulates the induc-

tor 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.

I LIMIT

I LOAD

IPEAK

TIME

Valley Current Limit

Figure 4

The RDSON sensing circuit is shown in Figure 5 with RIL-

IM = R1 and RDSON of Q2.

VOUT

+5V

VIN

CIN

PAD

VDD

ILIM

BST

Figure 5

The resistor sensing circuit with RILIM = R1 and RSENSE =

R4 is shown in Figure 6.

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