XC9236 Torex Semiconductor, XC9236 Datasheet - Page 9

• Manufacturer Part Number: XC9236
• Description: 600mA Driver Tr. Built-In, Synchronous Step-Down DC/DC Converters
• Manufacturer: Torex Semiconductor

■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
feedback voltage from the V
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
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
④ When the over current state is eliminated, the IC resumes its normal operation.
pin at any given timing.
current state.
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
resumed by either turning the IC off via the CE/MODE pin, or by restoring power to the V
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.
OUT
pin through split resistors, R1 and R2. Phase compensation is performed on the resulting
XC9235/XC9236/XC9237
IN
pin. The suspension mode does
Series
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