MB39C313 Datasheet, PDF (23/52 Pages) Fujitsu Component Limited. – 4ch System Power Management IC for LCD Panel

MB39C313
(3) Rectifier Diode Selection
Schottky diode should be used to attain high efficiency. The reverse voltage rating of the diode must be
higher then the maximum output voltage of the converter. The required averaged rectified forward current
of diode is the product of off-time of Buck converter and the maximum switch current at SWB pin.
Vout
Off-time of Buck converter: D = 1 −
=1−D
Vin
( ) ^Vin
Maximum output current: Iavg = (1 − D) × ISWLIM = 1 −
Vout
× ISWLIM
A Schottky diode with maximum rectified forward-current of 1.5 A to 2 A should be sufficient for most of
applications. The diode forward voltage should be less than 0.7 V in order to prevent damage to IC.
Another requirement for Schottky diode is the power dissipation. The power dissipation can be calculated
from the formula below:
PD = Iavg × VF = (1 − D) × ISWLIM × VF
Where
PD = Power dissipation of the diode [W]
VF = Diode forward voltage [V]
ISWLIM = Minimum over current protection of SWB-pin [A] (2.5 A)
(4) Bootstrap Capacitor Selection
Bootstrap capacitor connected to BOOT pin is charged by integrated synchronous diode with 4 V internal
supply. Ceramic capacitor is recommended for less leakage current. The minimum bootstrap capacitor can
be calculated by following equation:
QGATE +
CBOOT ≥
IDRV (dynamic)
f
+ QDRV (static)
VB − Vf − VLS − Vmin
ICBOOT (leak)
f
Where:
CBOOT = bootstrap capacitor value
QGATE = gate charge of integrated power transistor
f = switching frequency (500 kHz or 750 kHz)
IDRV(dynamic) = dynamic current of power transistor driver
QDRV(static) = static current of power transistor driver
ICBOOT(leak) = bootstrap capacitor leakage current
VB = internal regulated voltage 4 V
Vf = forward voltage drop of bootstrap diode
VLS = voltage drop of low-side diode of Buck converter
Vmin = minimum voltage between BOOT pin and SWB pin
Practically, bootstrap capacitor is selected more than ten times of its minimum value, such that providing
sufficient charge for driver and gate of power transistor. With assumption on power used is dominated by
charging the gate capacitor of power transistor, the equation can be simplified:
CBOOT ≥
QGATE
, where ΔV is the change of boot voltage in switching cycle.
ΔV
0.1 μF bootstrap capacitor is recommended for Buck converter in MB39C313. The bootstrap capacitor voltage
rating is suggested to be high than input voltage.
DS04–27267–1E
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