MB39C313 Datasheet, PDF (25/52 Pages) Fujitsu Component Limited. – 4ch System Power Management IC for LCD Panel

MB39C313
2. Boost Converter Design
(1) Boost Converter Block Diagram
• Figure 3. Boost converter block diagram
B
COMP
2
R3
FB
1
R4 Cff2
SS
28
B
L priority
Error
Amp2
−
OVP
COMP
+
+
VTH
1.146 V ± 0.9%
1.146 V
18.7 V
<< CH2(Boost) >>
LEVEL
CONV
OS
3
RON = (10 Ω
at VGS = -12 V)
VIN
Vs
(17.7 V / 1.5 A Max)
enb2
Saw tooth
Generator
AVIN
+
−
PWM
COMP2
PWM
Logic
Control
DRV
+
−
GD
1.03 V COMP
OSC
Current
Limit ILIM
COMP
SW
45
RON = (110 mΩ
at VGS = 5 V)
6 7 ^PGND
27 ^GD
It is necessary to verify the maximum output current of this converter whether it meets the application
requirements. The efficiency of the Boost converter can be read from the graph or employ a worst-case
assumption of 80 %.
Vin × η
Duty cycle: D = 1 −
Vout
Vin
Maximum output current: Iavg = (1 − D) × ISWLIM
× ISWLIM
Vout
Vin × D
Iout
Peak switch current: ISWPEAK =
+
2 × fOSC × L
1−D
Where
D = duty cycle
fOSC = switching frequency [Hz] (500 kHz or 750 kHz)
L = inductor value [H]
η = estimated Boost converter efficiency (typically 80 % minimum)
ISWLIM = minimum switch current limit of SW-pin [A] ( = 2.8 A)
The selected components, including the embedded switch, the inductor and external Schottky Diode
must be able to handle the peak switching current. The estimation should be based on the minimum
input voltage, since the switching current will be the highest in this case.
Limited by the power FET maximum switching current, the maximum output current depends on input voltage
and output voltage configuration. Refer to “REFERENCE DATA” section for graphical information. For data
reading from reference data, margin is suggested to avoid activating current limit.
Inductor Selection
The inductor can range from 6.8 μH to 22 μH. When selecting the inductor, its saturation current must be
higher than the peak switch current (ISWPEAK) as shown above. Extra margin is required to cope with high
current transients. A more conservative design is to use the maximum SW current limit of 3.5 A as saturation
current rating of inductor. Another parameter for choosing inductor is the DC resistance.
Usually, lower the DC resistance can result in higher converter efficiency.
DS04–27267–1E
25