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XCL205/XCL206/XCL207

Series

■OPERATIONAL DESCRIPTION

The XCL205/XCL/206/XCL207 series consists of a reference voltage source, ramp wave circuit, error amplifier, PWM

comparator, phase compensation circuit, output voltage adjustment resistors, P-channel MOSFET driver transistor, N-channel

MOSFET switching transistor for the synchronous switch, current limiter circuit, UVLO circuit with control IC, and an inductor.

(See the block diagram above.)

Using the error amplifier, the voltage of the internal voltage reference source is compared with the feedback voltage from the

VOUT pin through split resistors, R1 and R2. Phase compensation is performed on the resulting error amplifier output, to input a

signal to the PWM comparator to determine the turn-on time during PWM operation. The PWM comparator compares, in terms

of voltage level, the signal from the error amplifier with the ramp wave from the ramp wave circuit, and delivers the resulting

output to the buffer driver circuit to cause the Lx pin to output a switching duty cycle. This process is continuously performed to

ensure stable output voltage.

The current feedback circuit monitors the P-channel MOS driver transistor current for each switching operation, and modulates

the error amplifier output signal to provide multiple feedback signals. This enables a stable feedback loop even when a low ESR

capacitor such as a ceramic capacitor is used ensuring stable output voltage.

<Reference Voltage Source>

The reference voltage source provides the reference voltage to ensure stable output voltage of the DC/DC converter.

<Ramp Wave Circuit>

The ramp wave circuit determines switching frequency. The frequency is fixed internally 3.0MHz. Clock pulses generated in this

circuit are used to produce ramp waveforms needed for PWM operation, and to synchronize all the internal circuits.

<Error Amplifier>

The error amplifier is designed to monitor output voltage. The amplifier compares the reference voltage with the feedback voltage

divided by the internal split resistors, R1 and R2. When a feedback voltage is lower than the reference voltage, the output voltage

of the error amplifier is increased. The gain and frequency characteristics of the error amplifier output are fixed internally to deliver

an optimized signal to the mixer.

<Current Limit>

The current limiter circuit of the XCL205/XCL206/XCL207 series monitors the current flowing through the P-channel MOS driver

transistor connected to the Lx pin, and features a combination of the current limit mode and the operation suspension mode.

① When the driver current is greater than a specific level, the 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 a few

milliseconds 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 state. Once the IC is in suspension state, operations can be resumed by

either turning the IC off via the CE/MODE pin, or by restoring power to the VIN pin. The suspension state does not mean a complete

shutdown, but a state in which pulse output is suspended; therefore, the internal circuitry remains in operation. The current limit

of the XCL205/XCL206/XCL207 series can be set at 1050mA at typical.

Depending on the state of the PC Board, latch time may become longer and latch operation may not work. In order to avoid the

effect of noise, an input capacitor is placed as close to the IC as possible.

Limit > # ms

Current Limit LEVEL

0mA

I

OUT

V

Lx

IN

V

CE

V

Lx

Vss

Restart

Limit < # ms