9/28

XC9235/XC9236/XC9237

Series

■OPERATIONAL DESCRIPTION

The XC9235/XC9236/XC9237 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, U.V.L.O. circuit and others. (See the block

diagram above.) The series ICs compare, using the error amplifier, the voltage of the internal voltage reference source with the

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 and can be selected from 1.2MHz or

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 voltage lower than the reference voltage is fed back, the

output voltage of the error amplifier increases. 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 XC9235/XC9236/XC9237 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 ms 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

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 XC9235/XC9236/XC9237 series can be set at 1050mA at typical. Besides, care must

be taken when laying out the PC Board, in order to prevent misoperation of the current limit mode. 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, the

board should be laid out so that input capacitors are placed as close to the IC as possible.