XC9220/XC9221
Series
■TYPICAL
APPLICATION CIRCUITS
●
External Components
Pch MOSFET: 2SJ646 (SANYO)
SBD: DE5PC3 (SHINDENGEN)
L: CDRH8D28-4R7 (4.7μH, SUMIDA / Fosc=1.0MHz)
CDRH8D43-100 (10μH, SUMIDA / Fosc=500kHz)
CDRH127-220 (22μH, SUMIDA / Fosc=300kHz)
CIN: 47μF (OS-CON, SANYO)
CL: 47μF (OS-CON, SANYO)
■OPERATIONAL
EXPLANATION
The XC9220/XC9221 series consists of a reference voltage source, ramp wave circuit, error amplifier, PWM comparator,
phase compensation circuit, protection circuits, U.V.L.O. circuit and others. The series ICs compare, using the error
amplifier, the voltage of the internal voltage reference source with the feedback voltage from the V
OUT
pin through split
resistors. 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 EXT pin to output a switching duty cycle. This process is continuously performed
to ensure stable output voltage.
<Reference Voltage Source>
The reference voltage source provides the reference voltage to ensure stable output voltage of the DC/DC converter.
<Oscillator>
The oscillator determines switching frequency. The frequency is fixed internally and can be selected from 300kHz,
500kHz and 1.0MHz. 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 (FB pin voltage) divided by the internal split resistors. 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 PWM comparator.
<Control Methods>
The XC9220 series is PWM control, and the XC9221 series is PWM/PFM automatic switching mode. In the XC9220
series, it is controlled at a constant frequency from light load to heavy load. When a noise etc. is concerned, it is easy
to set up a filter etc. since the frequency is fixed. On the other hand, the efficiency at the time of light load may fall.
In the XC9221 series, the high efficiency can be drawn from PFM control at the time of light load. In PWM/PFM
automatic switching mode, a control method is automatically changed from PWM control to PFM control at the time of
light load. If coil current becomes discontinuous at the time of light load, ON time duty intends to reduce less than
25%. Therefore, the PFM circuit operates to output the pulse, which ON time duty fixed to 25% from the EXT/pin. The
ON time duty is fixed when PFM operation. The pulse is outputted with the cycle suitable for the conditions at that
time. In order that the number of times of switching per unit time may decrease, the efficiency, which it is at the light
load time is improved. However, output cycle of the pulse cannot be fixed. For this, the circuit should be designed
with this point in mind when using a noise filter etc. The conditions shifting to the PFM operation is depend on values
of input voltage, load current, coil and so on.
6/22
TOREX [ TOREX SEMICONDUCTOR ]
