MAX8795A Maxim Integrated Products, MAX8795A Datasheet - Page 20

• Manufacturer Part Number: MAX8795A
• Description: TFT-LCD DC-DC Converter
• Manufacturer: Maxim Integrated Products

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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 con-
dition or combination of conditions indicates a continu-
ous fault for the fault-timer duration (200ms typ), the
MAX8795A sets the fault latch to shut down all the out-
puts 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 prevents excessive power
dissipation from overheating the MAX8795A. When the
junction temperature exceeds +160°C, a thermal sen-
sor 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 approximately 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.
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.
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 con-
duction 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 induc-
tance values decrease the size, but increase the cur-
rent ripple and peak current. Finding the best inductor
involves choosing the best compromise between circuit
efficiency, inductor size, and cost.
TFT-LCD DC-DC Converter with
Operational Amplifiers
20
______________________________________________________________________________________
Thermal-Overload Protection
Main Step-Up Regulator
Design Procedure
Fault Protection
Inductor Selection
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 0.6. 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.
Calculate the approximate inductor value using the typ-
ical input voltage (V
(I
an appropriate curve in the Typical Operating
Characteristics section, and an estimate of LIR based
on the above discussion:
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input cur-
rent at the minimum input voltage (V
servation of energy and the expected efficiency at that
operating point (η
in the Typical Operating Characteristics :
Calculate the ripple current at that operating point and
the peak current required for the inductor:
The inductor’s saturation current rating and the
MAX8795A’s LX current limit (I
and the inductor’s DC current rating should exceed
I
less than 0.1Ω series resistance.
IN(DC,MAX)
MAIN(MAX)
L
=
I
I
RIPPLE
PEAK
⎛
⎜
⎝
. For good efficiency, choose an inductor with
), the expected efficiency (η
I
IN DCMAX
V
MAIN
V
(
IN
=
,
=
I
IN DCMAX
MIN
⎞
⎟
⎠
V
(
2
IN MIN
⎛
⎜
⎝
IN
) taken from the appropriate curve
)
(
I
MAIN MAX
,
=
), the maximum output current
L V
I
MAIN MAX
V
)
×
MAIN
V
)
×
(
IN MIN
+
(
MAIN
V
(
LIM
(
I
RIPPLE
MAIN
−
)
) should exceed I
2
×
V
)
×
IN
)
f
× η
OSC
×
f
−
OSC
IN(MIN)
V
V
MIN
MAIN
IN MIN
⎞
⎟
⎠
TYP
(
⎛
⎜
⎝
η
LIR
) using con-
) taken from
TYP
)
)
⎞
⎟
⎠
PEAK
,