MB39C313 Datasheet, PDF (24/52 Pages) Fujitsu Component Limited. – 4ch System Power Management IC for LCD Panel

MB39C313
(5) Output Capacitor Selection
This IC is designed to work best with ceramic output capacitor. Two 10 μF ceramic output capacitors are
recommended for most application. More capacitance can be added so as to reduce voltage drop during
load transients.
(6) Output Voltage and Feed Forward Capacitor Selection
• Equivalent circuit of Buck converter error amp block
CH1 output
R1
Cff1
L priority
Error
15
− ^Amp1
R2
FBB
enb1
++
1.213 V
VTH
1.213 V ± 1.5%
The output voltage of Buck converter can be set by external resistor divider as shown below:
( ) ( ) ^R1
VLOGIC = VREF × 1 +
R2
R1
= 1.213 × 1 +
R2
R2 is around 1.2 kΩ, and the reference voltage (VREF) = 1.213 V
The lower feedback resistor (R2) should be around 1.2 kΩ to maintain a minimum load current of 1 mA.
If the loading current is less than 1 mA, the output voltage will rise slightly above the nominal voltage in light
load or no load condition.
A feed forward capacitor (Cff1) is added parallel to the upper resistor (R1). The Cff1 sets a zero in the transfer
function. This will improve the load transient response and stabilize the converter loop. The value of Cff1 is
depending on the inductor and zero frequency (fz1) required.
For 10 μH inductor, set fz1 = 8 kHz; for 15 μH inductor, set fz1 = 17 kHz.
1
1
Cff =
=
= 9.9 nF =: 10 nF (Example of 3.3 V output voltage)
2 × π × R1 × fZ
2 × π × 2 kΩ × 8 kHz
A capacitor value close to the calculated value is chosen.
24
DS04–27267–1E