Synchronous Step-Down DC/DC Converter with built-in LDO Regulator in parallel plus Voltage Detector

Preliminary

< U.V.L.O. Circuit>

# High Speed LDO Voltage Regulator

< Reference Voltage Source >

The reference voltage source provides the reference voltage to ensure stable output voltage of the regulator.

< Error Amplifier >

The error amplifier compares the reference voltage with the signal from VROUT, and the amplifier controls the output of the Pch driver transistor.

<Current Limit Circuit>

# Voltage Detector

The detector block of the XC9511 series detects output voltage from the VDOUT pin while sensing either VDD, DCOUT, or VROUT internally.

(N channel Open Drain Type)

XC9511 Series

When the VIN pin voltage becomes 1.4 V 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 1.8 V 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.

The voltage regulator block includes a combination of a constant current limiter circuit and a foldback circuit. The voltage regulator senses

output current of the built-in P channel output driver transistor inside. When the load current reaches the current limit level, the current limiter

circuit operates and the output voltage of the voltage regulator block drops. As a result of this drop in output voltage, the foldback circuit

operates, output voltage drops further and the load current decreases. When the VROUT and GND pin are shorted, the load current of about

30mA flows.

The voltage regulator block of the XC9511 series consists of a reference voltage source, error amplifier, and current limiter circuit.

The voltage divided by split resistors is compared with the internal reference voltage by the error amplifier. The P-Channel MOSFET, which is

connected to the VROUT pin, is then driven by the subsequent output signal. The output voltage at the VROUT pin is controlled and stabilized

by a system of negative feedback. A stable output voltage is achievable even if used with low ESR capacitors as a phase compensation circuit

is built-in.

Semiconductor Ltd.

Data Sheet

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