BD2046AFJ, BD2056AFJ

Technical Note

9/13

www.rohm.com

2009.05 - Rev.A

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Functional description

1. Switch operation

IN terminal and OUT terminal are connected to the drain and the source of switch MOSFET respectively. And the IN

terminal is used also as power source input to internal control circuit.

When the switch is turned on from EN/EN control input, IN terminal and OUT terminal are connected by a 100mΩ switch.

In on status, the switch is bidirectional. Therefore, when the potential of OUT terminal is higher than that of IN terminal,

current flows from OUT terminal to IN terminal.

Since a parasitic diode between the drain and the source of switch MOSFET is canceled, in the off status, it is possible to

prevent current from flowing reversely from OUT to IN.

2. Thermal shutdown circuit (TSD)

Thermal shut down circuit have dual thermal shutdown threshold. Since thermal shutdown works at a lower junction

temperature when an overcurrent occurs, only the switch of an overcurrent state become off and error flag is output.

Thermal shut down action has hysteresis. Therefore, when the junction temperature goes down, switch on and error flag

output automatically recover. However, until cause of junction temperature increase such as output shortcircuit is removed

or the switch is turned off, thermal shut down detection and recovery are repeated. The thermal shut down circuit works

when the switch of either OUT1 or OUT2 is on (EN,/EN signal is active).

3. Over current detection (OCD)

MOSFET exceeds a specified value. There are three types of response against over current. The over current detection

circuit works when the switch is on (EN,/EN signal is active).

3-1. When the switch is turned on while the output is in shortcircuit status

When the switch is turned on while the output is in shortcircuit status or so, the switch gets in current limit status soon.

3-2. When the output shortcircuits while the switch is on

When the output shortcircuits or large capacity is connected while the switch is on, very large current flows until the

over current limit circuit reacts. When the current detection, limit circuit works, current limitation is carried out.

3-3. When the output current increases gradually

When the output current increases gradually, current limitation does not work until the output current exceeds the over

current detection value. When it exceeds the detection value, current limitation is carried out.

4. Under voltage lockout (UVLO)

the switch turns on, then UVLO shuts off the switch. UVLO has hysteresis of a 100mV(Typ).

Under voltage lockout circuit works when the switch of either OUT1 or OUT2 is on (EN,/EN signal is active).

5. Error flag (/OC) output

Error flag output is N-MOS open drain output. At detection of over current, thermal shutdown, low level is output.

Over current detection has delay filter. This delay filter prevents instantaneous current detection such as inrush current at

switch on, hot plug from being informed to outside.