XC9119D10A
Series
■OPERATIONAL
EXPLANATION
The XC9119D10A series consists of a reference voltage source, ramp wave circuit, error amplifier, PWM comparator, phase
compensation circuit, driver transistor, current limiter circuit and others. The series ICs compare, using the error amplifier, the
voltage of the internal reference voltage source with the feedback voltage from the FB pin. 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 switching.
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 drive 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 detects the N-channel
MOS driver transistor's 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 IC.
<Ramp Wave Circuit>
The ramp wave circuit determines switching frequency. The 1MHz (TYP.) of frequency is fixed internally. Clock pulses
generated in this circuit are used to produce ramp waveforms needed for PWM operation.
<Error Amplifier>
The error amplifier is designed to monitor output voltage. The amplifier compares the reference voltage with the FB pin
voltage. When a voltage lower than the reference voltage is fed back, the output voltage of the error amplifier increases.
Gain and frequency characteristics of the error amplifier output are fixed internally as an optimize signal.
<Current Limit >
The current limit circuit of the XC9119D10A series monitors the current flowing through the N-channel MOS driver
transistor connected to the Lx pin, and features a combination of the constant-current type current limit mode and the
duty cycle limit of the next pulse.
①When
the driver current is greater than a specific level, the constant-current type current limit function operates to turn
off the pulses from the Lx pin at any given timing.
②The
IC controls the next pulse to be smaller than the first pulse.
Current Limit
Current Limit
IL
Lx
1
The current will be off when the coil current
reaches the value of the constant current limit.
2
Limit some duty pulses after the limit.
<CE Pin Function>
The operation of the XC9119D10A series will enter into the shut down mode when a low level signal is input to the CE
pin. During the shut down mode, the supply current is 0µA (TYP.), with high impedance at the Lx pin. The IC starts its
operation with a high level signal to the CE pin. The input to the CE pin is a CMOS input and the sink current is 0µA
(TYP.). The hysteresis between the chip enable and the chip disable is 50mV (TYP.).
<Soft-Start Time>
Soft-start function operates when capacitors and resistors are connected to the CE/SS pin. With the Vref voltage limited
by the CE/SS pin start-up voltage and applying the input to the error amps, the operation maintains a balance between
the two inputs of the error amps. and controls the Lx pin’s ON time so that it doesn’t increase more than is necessary.
Depending of current limit function, load current, step-up ratio, and external components, the IC takes about 500uS to 5mS
to attain the setting voltage after applying the CE ‘H’ voltage even though the R
SS
is 0Ω and a soft start capacitor C
SS
is not
connected. (For a numerical constant, please refer to Note on Use.) For longer soft-start time, please connect R
SS
and C
SS
.
Soft-start function operates while the CE pin voltage is between 0V to around 1.9V. Please be noted that if the CE/SS pin voltage
does not start from 0V but is in intermediate potential when the power is turned on etc., soft start function may lose an effect and
that will cause a high inrush current and ripple voltage.
7
TOREX [ TOREX SEMICONDUCTOR ]
