MB39C313 FUJITSU [Fujitsu Component Limited.], MB39C313 Datasheet - Page 23

• Manufacturer Part Number: MB39C313
• Description: 4ch System Power Management IC for LCD Panel
• Manufacturer: FUJITSU [Fujitsu Component Limited.]

(3) Rectifier Diode Selection
(4) Bootstrap Capacitor Selection
DS04–27267–1E
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.
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:
P
Where
P
V
I
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:
Where:
C
Q
f = switching frequency (500 kHz or 750 kHz)
I
Q
I
V
V
V
V
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:
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.
SWLIM
DRV(dynamic)
CBOOT(leak)
Off-time of Buck converter: D = 1 −
Maximum output current: I
C
C
D
D
F
B
f
LS
min
BOOT
GATE
DRV(static)
= forward voltage drop of bootstrap diode
BOOT
BOOT
= Diode forward voltage [V]
= internal regulated voltage 4 V
= I
= Power dissipation of the diode [W]
= voltage drop of low-side diode of Buck converter
= minimum voltage between BOOT pin and SWB pin
avg
= Minimum over current protection of SWB-pin [A] (2.5 A)
= gate charge of integrated power transistor
= bootstrap capacitor value
≥
≥
× V
= static current of power transistor driver
= bootstrap capacitor leakage current
= dynamic current of power transistor driver
Q
F
GATE
Q
= (1 − D) × I
ΔV
GATE
+
, where ΔV is the change of boot voltage in switching cycle.
I
DRV (dynamic)
V
f
B
− V
SWLIM
avg
f
= (1 − D) × I
− V
+ Q
× V
LS
DRV (static)
F
− V
V
V
min
out
in
SWLIM
I
= 1 − D
CBOOT (leak)
=
f
(
1 −
V
V
out
in
)
× I
SWLIM
MB39C313
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