Datasheet

Multi-Output Power Supplies with VCOM Amplifier
and High-Voltage Gamma Reference for LCD TVs
MAX17126/MAX17126A
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the output voltage of the positive charge-pump regula-
tor. The charge pump includes a high-side p-channel
MOSFET (P1) and a low-side n-channel MOSFET (N1) to
control the power transfer as shown in Figure 3.
During the first half cycle, N1 turns on and charges flying
capacitors C12 and C13 (Figure 3). During the second
half cycle, N1 turns off and P1 turns on, level shifting C12
and C13 by V
SUPP
volts. If the voltage across C15 (V
GH
)
plus a diode drop (VD) is smaller than the level-shifted
flying-capacitor voltage (VC13) plus V
SUPP
, charge
flows from C13 to C15 until the diode (D3) turns off. The
amount of charge transferred to the output is determined
by the error amplifier that controls N1’s on-resistance.
Each time it is enabled, the positive charge-pump regu-
lator goes through a soft-start routine by ramping up its
internal reference voltage from 0 to 1.25V in 128 steps.
The soft-start period is 2ms (typ) and FBP fault detec-
tion is disabled during this period. The soft-start feature
effectively limits the inrush current during startup.
Negative Charge-Pump Regulator
The negative charge-pump regulator is typically used to
generate the negative supply rail for the TFT LCD gate
driver ICs. The output voltage is set with an external
resistive voltage-divider from its output to REF with the
midpoint connected to FBN. The number of charge-pump
stages and the setting of the feedback divider determine
the output of the negative charge-pump regulator. The
charge-pump controller includes a high-side p-channel
MOSFET (P2) and a low-side n-channel MOSFET (N2) to
control the power transfer as shown in Figure 4.
During the first half cycle, P2 turns on, and flying capacitor
C10 charges to V
SUPN
minus a diode drop (Figure 4).
During the second half cycle, P2 turns off, and N2 turns
on, level shifting C10. This connects C10 in parallel with
reservoir capacitor C11. If the voltage across C11 minus
a diode drop is greater than the voltage across C10,
charge flows from C11 to C10 until the diode (D4) turns
off. The amount of charge transferred from the output is
determined by the error amplifier that controls N2’s on-
resistance.
Figure 4. Negative Charge-Pump Regulator Block Diagram
MAX17126
MAX17126A
D4
REF
C11
VGOFF
REF
0.25V
R5
IN2
ERROR
AMPLIFIER
OSC
R6
NEGATIVE CHARGE-PUMP REGULATOR
C10
P2
N2
DRVN
FBN
CPGND
SUPN