9/35

XC9511

Series

■

OPERATIONAL EXPLANATION (Continued)

<Synchronous / Non-synchronous>

The XC9511 series automatically switches between synchronous / non-synchronous according to the state of the DC/DC

converter. Highly efficient operations are achievable using the synchronous mode while the coil current is in a continuous

state.

The series enters non-synchronous operation when the built-in N-ch switching transistor for synchronous

operation is shutdown, which happens when the load current becomes low and the operation changes to a discontinuous

state. The IC can operate without an external schottky diode because the parasitic diode in the N-ch switching transistor

provides the circuit's step-down operation. However, since VF of the parasitic diode is a high 0.6V, the efficiency level

during non-synchronous operation shows a slight decrease. Please use an external Schottky diode if high efficiency is

required during light load current.

●

Continuous Mode: Synchronous

●

Discontinuous Mode: Non-Synchronous

<Current Limit>

The current limiter circuit of the XC9511 series monitors the current flowing through the P-channel MOS driver transistor

connected to the Lx pin and features a combination of the constant-current type current limit mode and the operation

suspension mode.

①

When the driver current is greater than a specific level, the constant-current type current limit function operates to turn

off the pulses from the Lx pin at any given timing.

②

When the driver transistor is turned off, the limiter circuit is then released from the current limit detection state.

③

At the next pulse, the driver transistor is turned on. However, the transistor is immediately turned off in the case of an

over current state.

④

When the over current state is eliminated, the IC resumes its normal operation.

The IC waits for the over current state to end by repeating the steps ① through ③. If an over current state continues

for 8msec* and the above three steps are repeatedly performed, the IC performs the function of latching the OFF state of

the driver transistor, and goes into operation suspension mode. Once the IC is in suspension mode, operations can be

resumed by either turning the IC off via the CE pin, or by restoring power to the VIN pin. The suspension mode does not

mean a complete shutdown, but a state in which pulse output is suspended; therefore, the internal circuitry remains in

operation. The constant-current type current limit of the XC9511 series can be set at 1.1A.