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BM2PXX2F
(4) DCDC driver (PWM comparator, frequency hopping, slope compensation, OSC, burst)
BM2PXX2F is current mode PWM control.
An internal oscillator sets a fixed switching frequency (65 kHz typ).
BM2PXX2F is integrated switching frequency hopping function which changes the switching frequency to fluctuate as
shown in figure-10 below.
The fluctuation cycle is 125 Hz typ.
Switching Frequency
[kHz]
500us
69
68
67
66
65
64
63
62
61
125 Hz(8ms)
Time
Figure 10. Frequency hopping function
Max duty cycle is fixed as 75% (typ) and MIN pulse width is fixed as 400 ns (typ).
With current mode control, when the duty cycle exceeds 50% sub harmonic oscillation may occur.
As a countermeasure to this, BM2PXX2F is built in slope compensation circuits.
BM2PXX2F is built in burst mode circuit and frequency reduction circuit to achieve lower power consumption, when the load
is light.
FB pin is pull up by RFB (30 kΩ typ).
FB pin voltage is changed by secondary output voltage (secondary load power).
FB pin is monitored, burst mode operation and frequency detection start.
Figure 11 shows the FB voltage, and switching frequency, DCDC operation
・mode1: Burst operation
・mode2: Frequency reduction operation.
・mode3: Fixed frequency operation.
・mode4: Over load operation.
(operate at the max frequency)
(detect the over load state and stop the pulse operation)
Figure 11. Switching operation state changes by FB pin voltage
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TSZ02201-0F2F0A200040-1-2
7. Mar.2017.Rev.007
© 2012 ROHM Co., Ltd. All rights reserved.
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TSZ22111・15・001
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