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XC9128/XC9129
Series
■OPERATIONAL EXPLANATION
The XC9128/XC9129 series consists of a reference voltage source, ramp wave circuit, error amplifier, PWM comparator,
phase compensation circuit, N-channel driver transistor, P-channel synchronous rectification switching transistor and current
limiter circuit. The error amplifier compares the internal reference voltage with the FB pin feed back voltage via resistors
R
FB1 and RFB2. Phase compensation is performed on the resulting error amplifier output, to input a signal to the PWM
comparator to determine the turn-on time of the N-channel driver transistor 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 N-channel driver
transistor’s turn-on 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 source provides the reference voltage to ensure stable output of the DC/DC converter.
<Ramp Wave Circuit>
The ramp wave circuit determines switching frequency. The frequency is fixed internally at 1.2MHz. The Clock generated
is 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 resistors (R
FB1 and RFB2). When the FB pin is lower than the reference voltage, output
voltage of the error amplifier increases. The gain and frequency characteristics of the error amplifier are optimized
internally.
< Maximum Current Limit>
The current limiter circuit monitors the maximum current flowing through the N-channel driver transistor connected to the Lx
pin, and features a combination of the current limit and latch function.
1When the driver current is greater than a specific level (equivalent to peak coil current), the maximum current limit
function starts to operate and the pulses from the Lx pin turn off the N-channel driver transistor at any given time.
2When the driver transistor is turned off, the limiter circuit is then released from the maximum current limit detection state.
3At 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 XC9128B/XC9129B series waits for the over current state to end by repeating the steps 1 through 3. If an over current
state continues for several ms and the above three steps are repeatedly performed, the IC performs the function of latching the
OFF state of the N-channel driver transistor, and goes into operation suspension mode. After being put into suspension mode,
the IC can resume operation by turning itself off once and then re-starting via the EN pin, or by restoring power to the V
IN pin.
The XC9128D/XC9129D series does not have this latch function, so operation steps 1 through 3 repeat until the over current
state ends.
Integral latch time may be released from a over current detection state because of the noise. Depending on the state of a
substrate, it may result in the case where the latch time may become longer or the operation may not be latched. Please locate
an input capacitor as close as possible.
Limit<# ms
Limit<# ms
