XC9237A2ACML datasheet(10/28 Pages) TOREX

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XC9235/XC9236/XC9237 Series

■OPERATIONAL DESCRIPTION (Continued)

<Short-Circuit Protection>

The short-circuit protection circuit monitors the internal R1 and R2 divider voltage from the VOUT pin (refer to FB point in

the block diagram shown in the previous page). In case where output is accidentally shorted to the Ground and when

the FB point voltage decreases less than half of the reference voltage (Vref) and a current more than the ILIM flows to

the driver transistor, the short-circuit protection quickly operates to turn off and to latch the driver transistor. In latch

mode, the operation can be resumed by either turning the IC off and on via the CE/MODE pin, or by restoring power

supply to the VIN pin.

When sharp load transient happens, a voltage drop at the VOUT is propagated to the FB point through CFB, as a result,

short circuit protection may operate in the voltage higher than 1/2 VOUT voltage.

<U.V.L.O. Circuit>

When the VIN pin voltage becomes 1.4V or lower, the P-channel output driver transistor is forced OFF to prevent false

pulse output caused by unstable operation of the internal circuitry. When the VIN pin voltage becomes 1.8V or higher,

switching operation takes place. By releasing the U.V.L.O. function, the IC performs the soft start function to initiate

output startup operation. The soft start function operates even when the VIN pin voltage falls momentarily below the

U.V.L.O. operating voltage. The U.V.L.O. circuit does not cause a complete shutdown of the IC, but causes pulse output

to be suspended; therefore, the internal circuitry remains in operation.

In PFM control operation, until coil current reaches to a specified level (IPFM), the IC keeps the P-ch MOSFET on. In this

case, time that the P-ch MOSFET is kept on (TON) can be given by the following formula.

TON= L

×IPFM (VIN−VOUT)

→IPFM①

In PFM control operation, the maximum duty ratio (MAXIPFM) is set to 200% (TYP.). Therefore, under the condition that

the duty increases (e.g. the condition that the step-down ratio is small), it’s possible for P-ch MOSFET to be turned off even

when coil current doesn’t reach to IPFM.

→IPFM②

IPFM①

Ton

I Lx

IPFM

0mA

IPFM②

IPFM

0mA

I Lx

FOSC

Maxumum IPFM Current

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