Rainbow Electronics MAX8712 - Manual (Page 17)

MAX8710/MAX8711/MAX8712
Low-Cost Linear-Regulator
LCD Panel Power Supplies
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ed, the fault-protection circuits for the linear regulator
and the negative charge-pump regulator are activated.
When the linear regulator is enabled, the positive
charge-pump-regulator delay block is enabled. An
internal current source starts charging the DLP capaci-
tor. The voltage on DLP linearly rises because of the
constant charging current. When V
DLP goes above
V REF / 2, the switch control block is enabled, and the
positive charge-pump regulator begins its soft-start.
After the positive charge-pump regulator’s soft-start is
completed, the fault protection of the positive charge-
pump regulator is also enabled.
The MAX8710/MAX8711/MAX8712 enter into shutdown
when SHDN is pulled low or REF falls below 4.5V. In
shutdown, OUTL, GON and OUTB are all internally
pulled to ground with 1kΩ resistors. FBN, FBP, and DLP
are all internally pulled to ground with 10Ω resistors in
shutdown. The DLP current source is disabled in shut-
down and a switch discharges C
DLP to ground. REF
remains on in shutdown. Pulling SHDN high when REF
is above 4.5V reactivates the IC. Output fault protection
and thermal-overload protection can also turn off the
IC’s outputs. See the respective sections for details.
Output Fault Protection
During steady-state operation, if the output of the linear
regulator or any of the charge-pump regulator outputs
does not exceed its respective fault-detection thresh-
old, the MAX8710/MAX8711/MAX8712 activate an inter-
nal fault timer. If any condition or the combination of
conditions indicates a continuous fault for the fault-
timer duration (44ms typ), the MAX8710/MAX8711/
MAX8712 set the fault latch, shutting down all the out-
puts except the reference. Once the fault condition is
removed, cycle the input voltage or toggle SHDN to
clear the fault latch and reactivate the device. Each
regulator’s fault-detection circuit is disabled during the
regulator’s soft-start time.
Thermal-Overload Protection
The thermal-overload protection prevents excessive
power dissipation from overheating the IC. When the
junction temperature exceeds +160°C, a thermal sensor
immediately activates the fault protection, which shuts
down all the outputs except the reference, allowing the
device to cool down. Once the device cools down by
approximately 15°C, the IC restarts automatically.
Switch Control (MAX8710)
The MAX8710’s switch-control block (Figure 6) consists
of a high-voltage P-channel MOSFET Q1 between SRC
and GON, and a common-source-connected P-channel
MOSFET pair Q2 between GON and DRN. The switch-
control block is enabled when V
DLP goes above V REF /
2. Q1 and Q2 are controlled by CTL and MODE. There
are two different modes of operation.
Activate the first mode by connecting MODE to REF.
When CTL is logic high, Q1 turns on and Q2 turns off,
connecting GON to SRC. When CTL is logic low, Q1
turns off and Q2 turns on, connecting GON to DRN.
GON can then be discharged through a resistor con-
nected between DRN and GND or OUTL. Q2 turns off
and stops discharging GON when V
GON reaches 10
times the voltage on THR.
When V
MODE
is less than 0.9 x V
REF
, the switch-control
block works in the second mode. The rising edge of V CTL
turns on Q1 and turns off Q2, connecting GON to SRC.
An internal N-channel MOSFET Q5 between MODE and
GND is also turned on to discharge an external capacitor
between MODE and GND. The falling edge of V
CTL turns
off Q5, and an internal 50µA current source starts charg-
ing the MODE capacitor. Once V
MODE
exceeds 0.5 x
V REF , the switch-control block turns off Q1 and turns on
Q2, connecting GON to DRN. GON can then be dis-
charged through a resistor connected between DRN and
GND or OUTL. Q2 turns off and stops discharging GON
when V
GON reaches 10 times the voltage on THR.