MB3817 Datasheet, PDF (11/29 Pages) Fujitsu Component Limited. – Switching Regulator Controller

MB3817
s FUNCTIONAL DESCRIPTION
1. Switching Regulator Functions
(1) Reference voltage circuit (Ref)
The reference voltage circuit generates a temperature-compensated stable voltage (.=.1.50 V). This reference
voltage is used as the reference voltage and bias level for the power control unit.
(2) Triangular-wave oscillator circuit
By connecting a timing capacitor and a resistor to the CT (pin1) and the RT (pin2) terminals, it is possible to
generate any desired triangular oscillation waveform.
(3) Error amplifier
The error amp. is an amplifier circuit that detects the output voltage from the switching regulator and produces
the PWM control signal. The broad in-phase input voltage range of 0 V to Vcc – 0.9 V provides easy setting
from external power supplies and enables use with applications such as DC motor speed control systems.
Also, it is possible to provide stable phase compensation for a system by setting up any desired level of loop
gain, by connecting feedback resistance and a capacitor between the error amp. output pin (FB pin (pin 5))
and the inverse input pin (–IN pin (pin 4)).
(4) PWM comparator (PWM Comp.)
This is a voltage comparator with one inverted input and three non-inverted inputs, and operates as a voltage-
pulse width modulator controlling output duty in relation to input voltage.
The output transistor is turned on during the interval in which the triangular waveform is lower than any of three
voltages: the error amp. output voltage (FB pin (pin 5)), soft start set voltage (CS pin (pin 13)), or dwell time
setting voltage (DTC pin (pin 6)).
(5) Output circuits (OUT)
The output circuit has totem pole type configuration, and can drive an external PNP transistor.
The on current value can be set up to a maximum of 30 mA using the resistance (RE) connected to the VE pin
(pin 11).
The off current is set by connecting a bootstrap capacitor CB between the CP1 pin (pin 7) and CP2 pin (pin 8).
2. Power Supply Control Functions
The output is switched on and off according to the voltage level at the CTL pin (pin 15).
CTL pin voltage level
Channel on/off status
L ( 0.7 V)
Standby mode*
H ( 2.1 V)
Operating mode
* : Supply current in standby mode is 10 µA or less.
3. Protective Circuit Functions
(1) Soft start and short protection circuits (CS, SCP)
Soft starting, by preventing a rush current at power-on, can be provided by connecting a capacitor CS to the
CS pin (pin 13).
After the soft start operation is completed, the CSCP pin (pin 14) is held at “L” level (standby voltage VSTB),
which functions as short detection standby mode. If an output short causes the error amp. output to rise above
1.5 V, capacitor CSCP begins charging, and after reaching threshold voltage VTH of 0.65 V causes the OUT pin
(pin 10) to be fixed at “H” level and the dwell time to be set to 100%, and the CSCP pin (pin 14) is held at “L” level.
Once the protection circuit has been activated, the power supply must be reset to restore operation.
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