Datasheet

Step-Up, Step-Down Regulator, Gate-On Charge Pump,
and Boost-Buck Regulator for TV TFT LCD Display
MAX17122
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Since this is an open-loop control, gate-drain capacitor,
C8 is always required to reduce the inrush current during
startup; 22nF is suitable for this purpose.
When not using this feature, leave GATE high imped-
ance, and connect IN2 to the step-up regulator power
inductor (L1) directly.
Soft-Start
The step-up regulator achieves soft-start by linearly
ramping up its internal current limit. Connect a soft-start
capacitor (C
SS
) of at least 1nF between SS and AGND.
The SS pin voltage initially follows the FB1 pin voltage.
Once the GATE pin voltage reaches the GATE_OK
threshold (typically V
IN
- 5.5V), C
SS
is charged by a
10FA constant current. The soft-start terminates when
the SS pin voltage reaches 1.25V. Calculate C
SS
with the
following equation:
SS SS
10 A
C t
1.25V
= ×
F
where t
SS
is the desired soft-start duration. The soft-start
feature effectively limits the inrush current during startup
(see the Step-Up Regulator Soft-Start Waveforms in the
Typical Operating Characteristics).
Positive Charge-Pump Power Good (GPGD)
The GPGD power-good block is an open-drain type
design. The block monitors the positive charge-pump
feedback FBP with a 1.15V threshold. The threshold has
a 125mV (typ) hysteresis. GPGD goes low when FBP is
below the threshold.
Positive Charge-Pump Linear Regulator
The positive linear regulator controller is an analog gain
block with an open-drain n-channel output. It drives an
external pnp pass transistor (P1) with a 510I base-to-
emitter resistor. Its guaranteed base-drive sink current is
at least 10mA. The output voltage is set with an external
resistive voltage-divider from the charge-pump output to
AGND, with the midpoint connected to FBP. The regu-
lator in Figure 1 uses a 1FF ceramic output capacitor
and is designed to deliver 100mA at 28V. Other output
voltages and currents are possible with the proper pass
transistor, output capacitor, number of charge-pump
stages, and the setting of the feedback divider. The posi-
tive charge-pump regulator output (V
GON
) is typically
used to generate the positive supply rail for the TFT LCD
gate-driver ICs.
The regulator utilizes the step-up regulator switching
node (LX1) to toggle the charge-pump flying capacitor.
Therefore, to have a good output regulation, it requires
LX1 to toggle with a known duty cycle. In other words,
the step-up regulator needs to be working in continuous
mode. The regulator achieves its loop control by limit-
ing the current available through P1 to charge the flying
capacitor (C
FLY
). This topology eliminates the high-volt-
age stress on the DRVP pin. However, flying capacitor
charging-current pulses could cause early termination
of the step-up regulator switching pulses and cause
unstable performance of the step-up regulator. A small
resistor (R
P
) in series with charging diode D4 can reduce
the magnitude of these current pulses and prevent this
behavior. The value of this small resistor is determined
by the available headroom loss.
The positive linear regulator is enabled after the step-
down regulator finishes its soft-start and EN2 is pulled
high. Each time it is enabled, the regulator 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 3ms (typ) and FBP fault detection is disabled during
this period. The soft-start feature effectively limits the
inrush current during startup.
Negative Linear Regulator
The negative linear-regulator controller is an analog gain
block with an open-drain p-channel output. It drives an
external npn pass transistor (N1) with a 510I base-
to-emitter resistor. Its guaranteed base-drive source
current is at least 10mA. The output voltage is set with
an external resistive voltage-divider from its output to
3.3V reference with the midpoint connected to FBN. The
regulator in Figure 1 uses a 1FF ceramic output capaci-
tor and is designed to deliver 100mA at -7.5V. Other out-
put voltages and currents are possible with the proper
pass transistor, output capacitor, and the setting of the
feedback divider. The negative linear-regulator output
(GOFF1) is typically used to linearly derive a negative
gate-driver supply between the boost-buck regulator’s
output GOFF2 and ground.
The negative linear regulator is enabled after the step-
down regulator finishes its soft-start and EN2 is pulled high.