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Datasheet

Datasheet

BM1P107FJ

TSZ02201-0F2F0A200170-1-2

2.Oct.2013.Rev.001

© 2013 ROHM Co., Ltd. All rights reserved.

www.rohm.com

TSZ22111・15・001

Figure 17. Operation with FB pin voltage

・mode1: Burst operation

・mode2: Frequency reduction operation (reduces maximum frequency.)

・mode3: Fixed frequency operation (operates at maximum frequency.)

・mode4: Overload operation (overload status is detected and pulse operation is stopped.)

(4-3) Overcurrent detection operation

Ω.typ) is used as pull-up resistance for the FB pin with regard to the internal power supply (4.0 V).

When the load of the secondary output voltage (secondary load power) changes, the photo-coupler current changes, and

so the FB pin voltage also changes.

For example, when the load becomes heavier, the FB current is reduced, so the FB voltage rises.

When the load becomes lighter, the FB current is increased, so the FB voltage drops.

In this way, secondary voltage is monitored by the FB pin.

As the FB pin voltage is monitored, if the load becomes lighter (if FB voltage drops), a burst mode operation or frequency

reduction operation is executed.

Figure 18 shows the CS detection voltage with regard to FB voltage.

Figure 18

FB Voltage and CS Voltage Characteristics

When FB voltage is less than 2.0 V or when the CS voltage exceeds the FB voltage / Gain (typ = 4), the MOSFET is

turned off.

(See time point C in Figure 14.)

When the FB voltage exceeds 2.0 V, the CS voltage = Vcs + Kcs * Ton. Kcs * Ton depends on AC voltage compensation.

(See 4-4.)

Therefore, peak current Ip is determined as Ip = Vcs1 / Rs.

The current value for the MOSFET should be set with a margin with regard to the Ip value obtained from this formula.

Maximum power is determined as Pmax = 1/2 x Lp x Ip

Fsw: switching frequency)

Vcs1 is determined as Vcs1 = Vcs (typ = 0.4 V) + Kcs (typ = 20) * Ton + Vdelay.

Vdelay is the amount of CS voltage increase during the delay time Rondelay between B and C in Figure 14.

This is calculated as Vdelay = Vin / Lp * Tondelay * Rs.

⊿CS/⊿FB Gain : 1/4