BD9853AFV

Technical Note

7/16

www.rohm.com

2009.05 - Rev.A

© 2009 ROHM Co., Ltd. All rights reserved.

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Block functional descriptions

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Triangular wave oscillator

Connecting the resistor that sets the triangular wave oscillation frequency at the RT terminal generates the triangular wave,

which is input into the CH1 and CH2 PWM comparator.

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Error amp

The INV pin detects the output voltage, compares it to the programmed output voltage and amplifies the difference for

output by the FB pin. (The 0.8V reference is the comparison voltage. The tolerance is 1.25%.)

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PWM comparator

The PWM comparator converts the error amp (FB) voltage into a pulse width modulated waveform that goes to the FET

driver and turns FET output ON.

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FET driver

The push-pull FET driver directly drives the external MOSFET,providing high-side(OUT1H,OUT2H) switching at voltages

between Vcc⇔REGB, and low-side switching in the 0⇔REGA voltage range. (REGA = 5V; REGB= VCC -5V internal power)

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Standby function

The standby function enables output ON/OFF control by the STB pin. Output is ON when STB voltage is HIGH.

With the STB pin set HIGH, the output ON/OFF for each channel can be independently controlled by one of the

SCP/SOFT1, 2 pins.

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Soft Start/Short Circuit Protection (SCP)

Connecting the external capacitor on the SCP/SOFT1, 2 pins sets the SCP delay time and soft start time.

When STB is HIGH and the IC starts up, the capacitors on the SCP/SOFT1, 2 pins charge up at 2uA, stabilizing when the

system reaches 1.3V.

If load conditions change rapidly, causing the output voltage in either channel to fall to 70% or less of the set output voltage

(INV voltage 0.56 or lower), the SCP/SOFT1, 2 external capacitors will charge further until output for both channels

switches OFF at 2.3V.

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Under Voltage Lockout (UVLO)

Under Voltage lockout prevents IC malfunctions that could otherwise occur due to intermittent or fluctuating power supply

voltage, or insufficient voltage during start-up.When the VCC voltage falls to 4.1V or below, both channel outputs are

turned OFF, while the SCP/SOFT1, 2 pins are simultaneously set LOW.

The UVLO detection voltage includes 0.1V hysteresis width to prevent malfunctions from input voltage fluctuations.

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Thermal Shutdown (TSD)

The TSD circuit protects the IC against thermal runaway and heat damage.

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The TSD thermal sensor detects junction temperature. When the temperature reaches the TSD threshold (175℃), the

circuit switches the output of both channels OFF, and also switches REGA and REGB OFF. At the same time, it sets the

SCP/SOFT1, 2 pins LOW. The hysteresis width (15℃) provided between the TSD function start temperature (threshold)

and the stop temperature serves to prevent malfunctions from temperature fluctuations.