BD2045AFJ, BD2055AFJ

Technical Note

8/12

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2011.06 - Rev.C

© 2011 ROHM Co., Ltd. All rights reserved.

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I/O circuit

Symbol

Pin No

Equivalent circuit

EN(/EN)

4

/OC

5

OUT

6,7,8

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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 80mΩ 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)

If over current would continue, the temperature of the IC would increase drastically. If the junction temperature were

beyond 140°C (typ.) in the condition of over current detection, thermal shutdown circuit operates and makes power switch

turn off and outputs error flag (/OC). Then, when the junction temperature decreases lower than 120°C (typ.), power switch

is turned on and error flag (/OC) is cancelled. Unless the fact of the increasing chips temperature is removed or the output

of power switch is turned off, this operation repeats.

The thermal shutdown circuit operates when the switch 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.