UTC L2800
LINEAR INTEGRATED CIRCUIT
FUNCTIONAL DESCRIPTION
1. SWITCHING REGULATOR FUNCTION
(1) REFERENCE VOLTAGE CIRCUIT
The reference voltage circuit generates a temperature-compensated reference voltage(≒1.25V)from voltage
supplied from the power supply pin(pin 30).In addition to providing the reference voltage for the switching
regulator, the circuit also sets the idle period.
(2) SAWTOOTH WAVE OSCILLATOR
The sawtooth oscillator generates a sawtooth wave(up to 1 MHz)that is stable with respect to the supply
voltage and temperature. The capacitor and resistor that set the oscillation frequency are connected to the
OSC pin(pin 7).
(3) ERROR AMPLIFIER (ERROR AMP.)
The error amplifier detects the output voltage of the switching regulator and outputs the PWM control signal.
The voltage gain is fixed, and connecting a phase compensation capacitor to the FB pin(pin 8)provides stable
phase compensation for the system.
(4) PWM COMPARATOR (PWM COMP.)
The voltage comparator has one inverting and three non-inverting inputs. The comparator is a voltage /pulse
width converter that controls the ON time of the output pulse depending on the input voltage. The output level
is high(H) when the sawtooth wave is lower than the error amplifier output voltage, soft start setting voltage,
and idle period setting voltage.
(5) OUTPUT CIRCUIT
The output circuit has a totem pole type configuration and can drive an external NPN transistor directly. The
value of the ON/OFF current can be set by a resistor connected to the BR/CTL pin(pin 4).
2. POWER SUPPLY CONTROL FUNCTION
Stand-by mode(supply current 1µA or less) can be set by connecting the BR/CTL pin(pin 4) to Vcc or by
making the pin open circuit.
UTC L2800
4
RB
SW
SW
OFF
ON
MODE
Stand-by mode
Operating mode
BR/CTL
3. OTHER FUNCTIONS
(1) SOFT START AND SHORT CIRCUIT DETECTION
Soft start operation is set by connecting capacitor C
PE
to SCP pin(pin 2).Soft start prevents a current spike on
start-up.
On completion of soft start operation, the SCP pin(pin 2) stays low and enters the short circuit detection wait
state. When an output short circuit occurs, the error amplifier output is fixed at V
OM
+ and capacitor C
PE
starts
charging. After charging to approximately 0.8V,the output pin(pin 5) is set low and the SCP pin(pin 2) stays low.
Once the protection circuit operates, the circuit can be restored by resetting the power supply.(See “HOW TO
SET THE TIME CONSTANT FOR SOFT START AND SHORT CIRCUIT DETECTION”)
UTC
UNISONIC TECHNOLOGIES CO., LTD.
8
QW-R121-010,A
UTC-IC [ UNISONIC TECHNOLOGIES ]
