MAX8795A_1106 Datasheet, PDF (20/31 Pages) Maxim Integrated Products – TFT-LCD DC-DC Converter with Operational Amplifiers 2.5V to 5.5V Input Supply Range

TFT-LCD DC-DC Converter with
Operational Amplifiers
30Ω p-channel switch (Q2) between DRN and COM
turns off. If CTL is low, Q1 turns off and Q2 turns on.
Fault Protection
During steady-state operation, if the output of the main
regulator or any of the linear-regulator outputs does not
exceed its respective fault-detection threshold, the
MAX8795A activates an internal fault timer. If any condi-
tion or combination of conditions indicates a continuous
fault for the fault-timer duration (200ms typ), the
MAX8795A sets the fault latch to shut down all the outputs
except the reference. Once the fault condition is removed,
cycle the input voltage (below the UVLO falling threshold)
to clear the fault latch and reactivate the device. The fault-
detection circuit is disabled during the soft-start time.
Thermal-Overload Protection
Thermal-overload protection prevents excessive power dis-
sipation from overheating the MAX8795A. When the junc-
tion temperature exceeds +160°C, a thermal sensor
immediately activates the fault protection, which shuts
down all outputs except the reference, allowing the device
to cool down. Once the device cools down by approximate-
ly 15°C, cycle the input voltage (below the UVLO falling
threshold) 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 +150°C.
Design Procedure
Main Step-Up Regulator
Inductor Selection
The minimum inductance value, peak current rating,
and series resistance are factors to consider when
selecting the inductor. These factors influence the con-
verter’s efficiency, maximum output load capability,
transient-response time, and output voltage ripple. Size
and cost are also important factors to consider.
average DC inductor current at the full load current. The
best trade-off between inductor size and circuit efficiency
for step-up regulators generally has an LIR between 0.3
and 0.6. However, depending on the AC characteristics of
the inductor core material and ratio of inductor resistance
to other power-path resistances, the best LIR can shift up
or down. If the inductor 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.
Calculate the approximate inductor value using the typ-
ical input voltage (V_IN), the maximum output current
(I_MAIN(MAX)), the expected efficiency (η_TYP) taken from
an appropriate curve in the Typical Operating
Characteristics section, and an estimate of LIR based
on the above discussion:
L
=
⎛
⎝⎜
V_IN
V_MAIN
⎞
⎠⎟
2
⎛
⎜
⎝
V_MAIN −
I_MAIN(MAX)
V_IN
× f_OSC
⎞
⎟
⎠
⎛
⎝⎜
η_TYP
LIR
⎞
⎠⎟
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input cur-
rent at the minimum input voltage (V_IN(MIN)) using con-
servation of energy and the expected efficiency at that
operating point (η_MIN) taken from the appropriate curve
in the Typical Operating Characteristics:
I_IN(DC,MAX)
=
I_MAIN(MAX) × V_MAIN
V_IN(MIN) × η_MIN
Calculate the ripple current at that operating point and
the peak current required for the inductor:
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance values minimize the current
ripple, and therefore, reduce the peak current, which
decreases core losses in the inductor and conduction
losses in the entire power path. However, large inductor
values also require more energy storage and more turns of
wire, which increase size and can increase conduction
losses in the inductor. Low inductance values decrease
the size, but increase the current ripple and peak current.
Finding the best inductor involves choosing the best com-
promise 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
I_RIPPLE
=
V_IN(MIN) × (V_MAIN
L × V_MAIN ×
− V_IN(MIN))
f_OSC
I_PEAK
=
I_IN(DC,MAX)
+
I_RIPPLE
2
The inductor’s saturation current rating and the
MAX8795A’s LX current limit (I_LIM) should exceed I_PEAK,
and the inductor’s DC current rating should exceed
I_IN(DC,MAX). For good efficiency, choose an inductor with
less than 0.1Ω series resistance.
Considering the typical operating circuit, the maximum
load current (I_MAIN(MAX)) is 500mA with a 14V output and
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