MB39C313 Datasheet, PDF (27/52 Pages) Fujitsu Component Limited. – 4ch System Power Management IC for LCD Panel

MB39C313
(4) Output Voltage and Feed Forward Capacitor Selection
• Equivalent circuit of Boost converter error amp block
CH2 output
(step up converter)
COMP
2
R3
FB
Cff2
R4
1
SS
28
L priority
Error
Amp2
−
+
+
1.146 V
VTH
1.146 V ± 0.9%
enb2
The Boost converter output voltage of can be set by external resistor divider as shown below:
( ) ^R3
VS = 1.146 × 1 +
R4
Note : Output overshot due to large input voltage change may be high enough to trigger OVP under certain
condition when output setting is close to 18 V.
A feed forward capacitor (Cff2) is added parallel to the upper resistor (R3). The Cff2 sets a zero in the control
loop transfer function. This improves the load transient response and stabilizes the converter loop. The value
of Cff2 is depending on the inductor and zero frequency (fz2) required.
For 6.8 μH and 10 μH inductor, set fz = 10 kHz; for 22 μH inductor, set fz = 7 kHz.
1
1
Cff2 =
=
= 23.4 pF=: 20 pF (Example of 16.5 V output voltage)
2 × π × R3 × fZ2 2 × π × 680 kΩ × 10 kHz
A capacitor value close to the calculated value can be used.
(5) Compensation (COMP) Capacitor Selection
The regulator compensation is adjusted by an external component connected to the COMP-pin. This pin is
the output of internal trans-conductance error amplifier. By adding a resistor in series will change the internal
zero and increases the high-frequency gain. The formula below give the frequency (Fz) at which the resistor
increases the high-frequency gain.
1
FZ =
2 × π × CC × (RC + 10 k)
Typically, a 22 nF capacitor is suitable for most applications. If the input voltage is lower, it requires a smaller
capacitor value so that it has higher regulator gain.
DS04–27267–1E
27