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XCL205/XCL206/XCL207 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 the latch state, 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.

<UVLO 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 UVLO 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 UVLO operating voltage. The UVLO circuit does

not cause a complete shutdown of the IC, but causes pulse output to be suspended; therefore, the internal circuitry remains in

operation.

<PFM Switch Current>

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

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

tON = L×IPFM / (VIN－VOUT)

→IPFM①

<PFM Duty Limit>

In the PFM control operation, the PFM Duty Limit (DTYLIMIT_PFM) 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

tON

Lx

IPFM

0mA

IPFM①

IPFM②

0mA

Lx

fOSC

Maximum IPFM Limit

ILx

ILx

IPFM