MB39C313 Datasheet, PDF (28/52 Pages) Fujitsu Component Limited. – 4ch System Power Management IC for LCD Panel

MB39C313
(6) Soft Start Capacitor Selection
A soft start function is to slow the rate of rising output voltage and minimize the large inrush current at startup.
The soft start time is adjustable by connecting external capacitor to SS pin. Soft start capacitor can be
estimated by defining the soft start time thought equation below:
Iss × tss
C=
,
VFB
Where:
Iss = soft start charging current;
tss = soft start time;
VFB = voltage at FB pin.
In general, startup time for power supply is larger than 10 us. The startup time of Boost converter of
MB39C313 is defined as 1.5 ms.
Iss × tss
15 μA × 1.5 ms
C=
=
= 19.6 nF, therefore, 22 nF soft start capacitor is selected.
VFB
1.146 V
3. Positive Charge Pump Design
(1) Positive Charge Pump Block Diagram
D
R7
FBP
14
R8
enb4
L priority
−
+
+
Error
Amp4
VTH
1.213 V ± 2.1%
OSC
<< CH4(Positive Charge Pump) >>
Current
Control
Logic
DRV
SUP
8
Vs (17.7 V)
DRP
D
10
VGH
(32 V / 50 mA Max)
(2) Output Voltage Selection
Theoretically, the maximum output voltage is the sum of input voltage and pumping clock voltage of a charge
pump. In MB39C313, the maximum output voltage is VS (Boost converter output voltage) + VSUP − 2Vdiode
which is 17.7 V + 17.7 V + 2(0.4 V) = 34.6 V with typical setting. Due to the regulated voltage control, the
output voltage can be configured by equation below:
( ) ( ) ^R7
VGH = VREF × 1 +
R8
R7
= 1.213 × 1 +
R8
Typically, multiple 2 (x2) function for Positive Charge Pump. Its output voltage will be limited by VS − 2Vdiode
≤ VGH ≤ Vs + VSUP − 2 Vdiode. For other application that requires higher output voltage, MB39C313 allows
adding pumping stage by using SW pin. With multiple 3 (x3) function of Positive Charge Pump, the output
voltage should be limited by 2VS + Vdiode(Vs) − 2Vdiode ≤ VGH ≤ 2VS + Vdiode(Vs) + VSUP − 4Vdiode.
28
DS04–27267–1E