Technical Note

7/11

BD5650AFVM

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2011.11 - Rev.B

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

2. Over current detection

BD5650AFVM has a function regarding over current detection. When over current your set limit freely flow during a

continuous time you also set capacitance in CP terminal, open collector in OCP terminal is driving (ON). Once turned to ON,

its state keep(latch) in internal. When you want to release latch state, you need that CP terminal fall to GND, or, VCC

voltage apply lower under about 1V.

An application circuit in Fig.16 has a function that adaptor output stop due to stop feedback to primary side.

(2-1) Limit current

Overcurrent detection is composited from detection comparator, sensing resistance RS.

Limit current “IL” is defined by expression (2).

IL = VSE / RS

(2)

IL means Limit current, VSE means current detected voltage(73mV: a potential difference from ICT toVSE).

We show a reference value below.

When IL=1A, RS=73mΩ

You need to decide RS value sufficiently considering maximum load current IL,max in application.

Pl=VSE×IL,max

(3)

For example, when IL,max set to 2A, the maximum power loss “Pl.max” is 200mW in RS resistance. Since BD5650AFVM

itself can’t limit IL,max, considering a characteristics on module, you need to select resistance includes enough margin for

power loss. But for mostly small power adaptor, selecting 1/4 watt or 1/2 watt resistance is sufficiently suitable.

(2-2) CP charge

A delay time from a occur of over current to turn ON in OCP terminal is below expression (4).

Toth=CP×VREF/Ichg

(4)

Timing chart when over current detection is shown in Fig.19.

Fig.19

CP

ΔVS

IOUT

IL

(73mV/RS)

Toth

VREF

73ｍV

Iocp

IL,max