MAX1513 Datasheet, PDF (17/28 Pages) Maxim Integrated Products – TFT-LCD Power-Supply Controllers

TFT-LCD Power-Supply Controllers
Soft-Start
Each positive regulator (step-up regulator, REG P, REG
L, and REG G) includes a 7-bit soft-start DAC whose
input is the reference, and whose output is stepped in
128 steps from zero up to the reference voltage. The
soft-start DAC of the negative regulator (REG N) steps
from the reference down to 250mV in about 100 steps.
The outputs of the soft-start DACs determine the set
points of each regulator. The soft-start duration is 2.7ms
(typ) for each positive regulator and about 2.2ms for the
negative regulator. The soft-start is independent of the
selected operating frequency.
Fault Protection
During steady-state operation, if the step-up regulator
output or any of the linear-regulator outputs does not
exceed its respective fault detection threshold, the
MAX1513/MAX1514 activate an internal fault timer. If
any condition or the combination of conditions indi-
cates a continuous fault for the fault-timer duration
(43.6ms typ), the MAX1513/MAX1514 set the fault
latch, shutting down all the outputs except the refer-
ence. Once the fault condition is removed, toggle SDFR
(below 0.4V) or cycle the input voltage (below 2.2V) to
clear the fault latch and reactivate the device. The fault-
detection circuit is disabled during the soft-start time of
each regulator.
Thermal-Overload Protection
The thermal-overload protection prevents excessive
power dissipation from overheating the MAX1513/
MAX1514. When the junction temperature exceeds
+160°C, a thermal sensor immediately activates the
fault-protection circuit, which shuts down all the outputs
except the reference, allowing the device to cool down.
Once the device cools down by approximately 15°C,
cycle the input voltage (below the UVLO falling thresh-
old) to clear the fault latch and reactivate the device.
The thermal-overload protection protects the controller
in the event of fault conditions. For continuous opera-
tion, do not exceed the absolute maximum junction
temperature rating of T_J = +150°C.
Design Procedure
Main Step-Up Regulator
Inductor Selection
The minimum inductance value, peak current rating,
and DC series resistance (DCR) are factors to consider
when selecting the inductor. These factors influence the
converter’s efficiency, maximum output load capability,
transient-response time, and output voltage ripple. Size
and cost are also important factors to consider.
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 I²R
losses in the entire power path. However, large induc-
tor values also require more energy storage and more
turns of wire, which increases size and can increase
I²R losses in the inductor. Low inductance values
decrease the size but increase the current ripple and
peak current. Finding the best inductor involves choos-
ing 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 the
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 an inductor is chosen, higher and lower values
for the inductor should be evaluated for efficiency
improvements in typical operating regions.
Determine the inductor value and peak current require-
ment as follows:
Since the current delivered by charge pumps connect-
ed to LX adds to the inductor current, calculate the
effective maximum output current, I_MAIN(EFF):
I_MAIN(EFF) = I_MAIN(MAX) + n_NEG × I_NEG
+ ₍n_POS + 1₎ × I_POS
where I_MAIN(MAX) is the maximum output current
including any gamma-regulator current, n_NEG is the
number of negative charge-pump stages, n_POS is the
number of positive charge-pump stages, I_NEG is the
negative charge-pump output current, and I_POS is the
positive charge-pump output current, assuming the
pump source for I_POS is V_MAIN.
Calculate the approximate inductor value using the typ-
ical input voltage (V_IN), the expected efficiency (η_TYP)
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