Technical Note
8/22
BD60A00NUX, BD60A60NUX
www.rohm.com
2011.12 - Rev.A
© 2011 ROHM Co., Ltd. All rights reserved.
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Functional Descriptions
1) PWM current mode DC/DC converter
This IC keeps output voltage invariably by setting PWM duty to make FB terminal 0.7V when the power is on. As for the
inputs of the PWM comparator as the feature of the PWM current mode, one is overlapped with error components from
the error amplifier, and the other is overlapped with a current sense signal that controls the inductor current into Slope
waveform to prevent sub harmonic oscillation. This output controls internal Nch Tr via the RS latch. In the period where
internal Nch Tr gate is ON, energy is accumulated in the external inductor, and in the period where internal Nch Tr gate
is OFF, energy is transferred to the output capacitor via external SBD.
This IC has many safety functions, and their detection signals stop switching operation at once.
2) Soft Start
This IC has soft start function, which prevents rush current at turning on.
After EN and PWM are changed L
H, soft start becomes effective within 1ms. Soft start doesn't become effective
even if Enable is changed L
H, after that. By changing EN and PWM L
H, soft start can become effective.
Since a soft start function stops working at the time of following PWM=H when EN=H is held with PWM=L
After EN=H should input a PWM signal within 10ms.
PWM
OFF
ON ON
OFF OFF
OFF
ON
OFF
OFF
EN
Soft Start
Soft Start
Reset
Reset
Reset
Soft start time Finish
PWM
EN
Within 10ms
Fig.3 Soft start