MAX8753ETI-T Maxim Integrated, MAX8753ETI-T Datasheet - Page 15

• Manufacturer Part Number: MAX8753ETI-T
• Description: LCD Drivers
• Manufacturer: Maxim Integrated

charge pump, and the positive charge pump remain
disabled until LCDON is high. When LCDON is logic-
high, the main DC-DC step-up converter powers up with
soft-start enabled. Once the main step-up converter
reaches regulation, the negative charge pump turns on.
When the main step-up converter reaches regulation,
the positive charge-pump regulator delay block is
enabled. An internal current source starts charging the
DLP capacitor. The voltage on DLP linearly rises
because of the constant-charging current. When V
goes above V
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.
A logic-low level on LCDON disables the main BOOST
converter, the negative charge pump, and the positive
charge pump. The output capacitance and load current
determine the rate at which each output voltage
decays. The linear regulator and the reference remain
enabled unless SHDN drops below its logic-low thresh-
old. When shut down, the reference turns off and the IC
supply current drops to 0.1µA to maximize battery life
in portable applications. Do not leave SHDN floating. If
unused, connect SHDN to IN.
During steady-state operation if the output of the linear
regulator, the step-up regulator, or either of the charge-
pump regulator outputs, does not exceed its respective
fault-detection threshold, the MAX8753 activates an
internal fault timer. If any condition or combination of
conditions indicates a continuous fault for the fault timer
duration (50ms typ), the MAX8753 sets the fault latch,
shutting down all the outputs 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 dis-
abled during the regulator’s soft-start time.
The thermal-overload protection prevents excessive
power dissipation from overheating the IC. If the junc-
tion temperature exceeds +160°C, a thermal sensor
immediately activates the thermal fault protection,
which shuts down all the outputs, allowing the device to
cool down. Once the device cools down, cycle the
input voltage to clear the thermal fault latch and reacti-
vate the device.
REF
, the switch control block is enabled,
Thermal-Overload Protection
______________________________________________________________________________________
Output Fault Protection
TFT LCD DC-DC Converter with
DLP
Integrated Charge Pumps
The minimum inductance value, peak current rating, and
series resistance are factors to consider when selecting
the inductor. These factors influence the converter’s effi-
ciency, maximum output load capability, transient
response time, and output voltage ripple. Physical size
and cost are also important factors to be considered.
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance values minimize the cur-
rent ripple and therefore reduce the peak current,
which decreases core losses in the inductor and I
losses in the entire power path. However, large induc-
tor values also require more energy storage and more
turns of wire, which increase physical size and can
increase I
ues decrease the physical size but increase the current
ripple and peak current. Finding the best inductor
involves choosing the best compromise between circuit
efficiency, inductor size, and cost.
The equations used here include a constant LIR, which
is the ratio of the inductor peak-to-peak ripple current
to the average DC inductor current at the full-load cur-
rent. The best trade-off between inductor size and cir-
cuit efficiency for step-up regulators generally has an
LIR between 0.3 and 0.5. However, depending on the
AC characteristics of the inductor core material and
ratio of inductor resistance to other power-path resis-
tances, the best LIR can shift up or down. If the induc-
tor resistance is relatively high, more ripple can be
accepted to reduce the number of turns required and
increase the wire diameter. If the inductor resistance is
relatively low, increasing inductance to lower the peak
current can decrease losses throughout the power
path. If extremely thin high-resistance inductors are
used, as is common for LCD panel applications, the
best LIR can increase to between 0.5 and 1.0.
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficien-
cy improvements in typical operating regions.
In Figure 1’s typical operating circuit, the LCD’s gate-
on and gate-off voltages are generated from two
charge pumps powered by the step-up regulator. The
additional load on V
in the inductance calculation. The effective maximum
output current I
mum load current on the step-up regulator’s output
2
R losses in the inductor. Low inductance val-
MAIN(EFF)
MAIN
Main Step-Up Regulator
becomes the sum of the maxi-
must therefore be considered
Design Procedure
Inductor Selection
2
15
R