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Datasheet

Datasheet

BM1P105FJ

TSZ02201-0F2F0A200170-1-2

2.Oct.2013.Rev.001

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

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TSZ22111・15・001

E:

Since the switching frequency within the IC is predetermined, SET signal output from the internal oscillator occurs

for a set period starting from point A, and the MOSFET is turned on.

(5-2) Frequency operations

Figure 17. PWM Operations in IC

The PWM frequency is generated by the OSC block (internal oscillator) in Figure 17.

This oscillator has a switching frequency hopping function and the switching frequency fluctuates such as is shown in

Figure 18.

The fluctuation cycle is 125 Hz. Due to this frequency hopping function, the frequency spectrum is dispersed and the

frequency spectrum peak is lowered. This increases the margin for EMI testing.

100

Switching Frequency

[kHz]

106

125 Hz(8ms)

Time

94

500us

Figure 18. Frequency Hopping Function

In Figure 18, the duty is calculated as Ton * Switching frequency * 100. The maximum duty value is Dmax (typ = 75%).

Since the PWM current mode method is being used, if the duty exceeds 50% sub harmonic oscillation may occur. 22

mV/us slope compensation is built in as a countermeasure to this.

To reduce power consumption when there is a light load, a burst mode circuit and frequency reduction circuit are built in.

These operations are illustrated in Figure19. As shown in this figure, frequency fluctuates according to the FB voltage.

If the FB voltage is in the range shown for mode2, switching loss is reduced by reducing internal oscillations based on the

FB voltage.