BD9870FPS_09 Datasheet, PDF (4/12 Pages) Rohm – Simple Step-down Switching Regulator with Built-in Power MOSFET

BD9870FPS
Technical Note
●Block Function Explanations
・ VREF
Generates the regulated voltage from Vcc input, compensated for temperature.
・ OSC
Generates the triangular wave oscillation frequency (900kHz) using an internal resistors and capacitor. Used for PWM
comparator input.
・ Error AMP
This block, via the INV pin, detects the resistor-divided output voltage, compares this with the reference voltage, then
amplifies and outputs the difference.
・ PWM COMP
Outputs PWM signals to the Driver block, which converts the error amp output voltage to PWM form.
・ DRIVER
This push-pull FET driver powers the internal Pch MOSFET, which accepts direct PWM input.
・ STBY
Controls ON/OFF operation via the STBY pin. The output is ON when STBY is High.
・ Thermal Shutdown (TSD)
This circuit protects the IC against thermal runaway and damage due to excessive heat. A thermal sensor detects the
junction temperature and switches the output OFF once the temperature exceeds a threshold value (175°C). Hysteresis is
built in (15°C) in order to prevent malfunctions due to temperature fluctuations.
・ Over Current Protection (OCP)
The OCP circuit detects the voltage difference between Vcc and OUT by measuring the current through the internal Pch
MOSFET and switches the output OFF once the voltage reaches the threshold value. The OCP block is a self-recovery
type (not latch).
・ Soft Start (SS)
This block conducts soft start operations. When STBY is High and the IC starts up the internal capacitor begins charging.
The soft start time is fixed at 5ms.
●Notes for PCB layout
C3
R2:1kΩ
R1:4kΩ
4
INV
STBY 5
1 _VCC
C2
GND
FIN
OUT ²
L1
D1 ^C1
5.0V
Fig.3
• Place capacitors between Vcc and Ground, and the Schottky diode as close as possible to the IC to reduce noise and
maximize efficiency.
• Connect resistors between INV and Ground, and the output capacitor filter at the same Ground potential in order to stabilize
the output voltage.
（If the patterning is longer or thin, it’s possible to cause ringing or waveform crack.）
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4/11
2009.05 - Rev.A