Datasheet

Multi-Output Power Supplies with VCOM Amplifier
and High-Voltage Gamma Reference for LCD TVs
MAX17126/MAX17126A
______________________________________________________________________________________ 27
Power-Down Sequence
The step-down regulator, step-up regulator, positive
charge pump, negative charge pump, and high-voltage
switching block all start to shut down when INVL drops
below its UVLO threshold. VL stays flat until INVL does
not have enough headroom. Reference REF starts to fall
after VL drops below its UVLO threshold.
Gamma reference GREF stays flat until AVDD does not
have enough headroom. A pMOS switch turns on after
VL drops below its UVLO threshold to guarantee GREF
does not go over AVDD.
PGOOD is pulled low after its input voltage (buck output
in this case) drops below the designed threshold. After
VL drops below its UVLO threshold, PGOOD gives up
control and is resistively pulled up to its input voltage.
The high-voltage switching block output VGHM falls until
VL drops below its UVLO threshold, after which it is in
high impedance.
Fault Protection
During steady-state operation, if any output of the four
regulators’ output (step-down regulator, step-up regulator,
positive charge-pump regulator, and negative charge-pump
regulator) goes lower than its respective fault-detection
threshold, the MAX17126 activates an internal fault timer. If
any condition or the combination of conditions indicates a
continuous fault for the fault timer duration (50ms typ), the
MAX17126A latches off all its outputs while the MAX17126
latches off all the outputs except the buck regulator (latched
off only when the fault happens on its output).
If a short has happened to any of the four regulator
outputs, no fault timer is applied; the part latches off
immediately. Pay special attention to shorts on the step-
up regulator and positive charge pump. Make sure when
a short happens, negative ringing on VREF_I (connected
to step-up regulator output) and VGH (connected to
positive charge-pump output) does not exceed Absolute
Maximum Ratings. Otherwise, physical damage of the
part may occur. Cycle the input voltage to clear the fault
latch and restart the supplies.
Thermal-Overload Protection
The thermal-overload protection prevents excessive
power dissipation from overheating the MAX17126/
MAX17126A. When the junction temperature exceeds
T
J
= +160NC, a thermal sensor immediately activates
the fault protection that shuts down all the outputs. Cycle
the input voltage to clear the fault latch and restart the
MAX17126/MAX17126A.
The thermal-overload protection protects the controller in
the event of fault conditions. For continuous operation, do
not exceed the absolute maximum junction temperature
rating of T
J
= +150NC.
Design Procedure
Step-Down Regulator
Inductor Selection
Three key inductor parameters must be specified:
inductance value (L), peak current (I
PEAK
), and DC
resistance (R
DC
). The following equation includes a
constant, LIR, which is the ratio of peak-to-peak inductor
ripple current to DC load current. A higher LIR value
allows smaller inductance, but results in higher losses
and higher ripple. A good compromise between size
and losses is typically found at a 30% ripple current-to-
load current ratio (LIR = 0.3) that corresponds to a peak
inductor current 1.15 times the DC load current:
( )
OUT IN2 OUT
2
IN2 SW OUT(MAX)
V V - V
L
V f I LIR
×
=
× × ×
Figure 8. Power-Down Sequence
TIME
TIME
REF
INVL
NEGATIVE
CHARGE-PUMP
REGULATOR
OUTPUT
POSITIVE
CHARGE-PUMP
REGULATOR
OUTPUT
BUCK
INVL UVLO
AVDD
TIME
VGHM UNCONNECTED
VGHM DEPENDS
ON GVOFF
VGHM
TIME
VL UVLO
GREF
TIME
PGOOD
VL
OUTPUT