XCL208/XCL209 Series
■OPERATIONAL DESCRIPTION
The XCL208/XCL209 series consists of a reference voltage source, ramp wave circuit, error amplifier, PWM comparator,
phase compensation circuit, output voltage adjustment resistors, P-ch MOSFET driver transistor, N-ch MOSFET switching
transistor for the synchronous switch, current limiter circuit, UVLO circuit with control IC, and an inductor. (See the block
diagram below.) Using the error amplifier, the voltage of the internal voltage reference source is compared with the feedback
voltage from the VOUT pin through split resistors, R1 and R2. 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 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-ch 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.
Type A
L1
L2
VOUT
LX
VIN
PVSS
AVSS
CE
<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 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
(Type F: FB pin voltage) divided by the internal split resistors, R1 and R2. When a feed back voltage is lower than the reference
voltage, the output voltage of the error amplifier is increased. 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 XCL208/XCL209 series monitors the current flowing through the P-ch 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 current limit 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 latch
time 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 state. Once the IC is in suspension state, operations can be resumed by either
turning the IC off via the CE pin, or by restoring power to the VIN pin. The suspension state does 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
XCL208/XCL209 series can be set at 800mA at typical. 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, an input capacitor is placed as close to the IC as possible.
Limit＜#ms
Limit＞#ms
Current Limit Level
0mA
ILx
VOUT
VSS
Lx
VCE
Restart
VIN
10/22
TOREX [ Torex Semiconductor ]
