Datasheet
Multi-Output Power Supplies with VCOM Amplifier
and High-Voltage Gamma Reference for LCD TVs
MAX17126/MAX17126A
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Step-Down Regulator
The step-down regulator consists of an internal n-chan-
nel MOSFET with gate driver, a lossless current-sense
network, a current-limit comparator, and a PWM con-
troller block. The external power stage consists of a
Schottky diode rectifier, an inductor, and output capaci-
tors. The output voltage is regulated by changing the
duty cycle of the high-side MOSFET. A bootstrap circuit
that uses a 0.1FF flying capacitor between LX2 and BST
provides the supply voltage for the high-side gate driver.
Although the MAX17126/MAX17126A also include a 10I
(typ) low-side MOSFET, this switch is used to charge the
bootstrap capacitor during startup and maintains fixed-
frequency operation at light load and cannot be used as
a synchronous rectifier. An external Schottky diode (D2
in Figure 1) is always required.
PWM Controller Block
The heart of the PWM control block is a multi-input, open-
loop comparator that sums three signals: the output-
voltage signal with respect to the reference voltage, the
current-sense signal, and the slope-compensation signal.
The PWM controller is a direct-summing type, lacking a
traditional error amplifier and the phase shift associated
with it. This direct-summing configuration approaches
ideal cycle-by-cycle control over the output voltage.
The step-down controller always operates in fixed-fre-
quency PWM mode. Each pulse from the oscillator sets
the main PWM latch that turns on the high-side switch
until the PWM comparator changes state. As the high-
side switch turns off, the low-side switch turns on. The
low-side switch stays on until the beginning of the next
clock cycle.
Current Limiting and Lossless Current Sensing
The current-limit circuit turns off the high-side MOSFET
switch whenever the voltage across the high-side
MOSFET exceeds an internal threshold. The actual cur-
rent limit is typically 3.2A for MAX17126 and 3.5A for
MAX17126A.
For current-mode control, an internal lossless sense
network derives a current-sense signal from the inductor
DCR. The time constant of the current-sense network is
not required to match the time constant of the inductor
and has been chosen to provide sufficient current ramp
signal for stable operation at both operating frequencies.
The current-sense signal is AC-coupled into the PWM
comparator, eliminating most DC output-voltage varia-
tion with load current.
Dual-Mode Feedback
The step-down regulator of the MAX17126/MAX17126A
support both fixed output and adjustable output. Connect
FB2 to GND to enable the 3.3V fixed-output voltage.
Connect a resistive voltage-divider between OUT and
GND with the center tap connected to FB2 to adjust the
output voltage. Choose RB (resistance from FB2 to GND)
to be between 5kI and 50kI, and solve for RA (resis-
tance from OUT to FB2) using the equation:
OUT
FB2
V
RA RB - 1
V
= ×
where V
FB2
= 1.25V, and V
OUT
may vary from 1.5V to 5V.
Because FB2 is a very sensitive pin, a noise filter is gen-
erally required for FB2 in adjustable-mode operation.
Place an 82pF capacitor from FB2 to GND to prevent
unstable operation. No filter is required for 3.3V fixed-
mode operation.
Soft-Start
The step-down regulator includes a 7-bit soft-start DAC
that steps its internal reference voltage from zero to
1.25V in 128 steps. The soft-start period is 3ms (typ)
and FB2 fault detection is disabled during this period.
The soft-start feature effectively limits the inrush current
during startup (see the Step-Down Regulator Soft-Start
Waveforms in the Typical Operating Characteristics).
Step-Up Regulator
The step-up regulator employs a current-mode, fixed-fre-
quency PWM architecture to maximize loop bandwidth
and provide fast-transient response to pulsed loads
typical of TFT LCD panel source drivers. The integrated
MOSFET and the built-in digital soft-start function reduce
the number of external components required while
controlling inrush currents. The output voltage can be
set from V
IN
to 16.5V with an external resistive voltage-
divider. The regulator controls the output voltage and the
power delivered to the output by modulating duty cycle
D of the internal power MOSFET in each switching cycle.
The duty cycle of the MOSFET is approximated by:
AVDD DIODE IN
AVDD DIODE LX1
V V - V
D
V V - V
+
≈
+
where V
AVDD
is the output voltage of the step-up regu-
lator, V
DIODE
is the voltage drop across the diode, and
V
LX1
is the voltage drop across the internal MOSFET.