Datasheet

Thermal-Overload Protection
When the junction temperature exceeds T
J
= +160°C
(typ), a thermal sensor activates a fault-protection latch,
which shuts down all outputs, allowing the IC to cool
down. All outputs remain off until the IC cools and the
input voltage is cycled below, then back above the IN
UVLO threshold.
The thermal-overload protection protects the IC in the
event of fault conditions. For continuous operation, 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
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 current
ripple and therefore reduce the peak current, which
decreases core losses in the inductor and I
2
R losses in
the entire power path. However, large inductor values
also require more energy storage and more turns of
wire, which increase physical size and can increase I
2
R
losses in the inductor. Low inductance values decrease
the physical 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 called LIR,
which is the ratio of the inductor peak-to-peak ripple
current to the 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 0.5. 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 resis-
tance 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 efficiency
improvements in typical operating regions.
In the typical operating circuit of Figure 1, the LCD’s
gate-on supply voltage is generated from an unregulat-
ed positive charge pump and the gate-off supply volt-
age is generated from a regulated negative charge
pump controlled by the gate-off linear regulator. Both
charge pumps are driven by the step-up regulator’s LX
node; therefore, the additional load on LX must be con-
sidered in the inductance and current calculations. The
effective maximum output current, I
MAIN (EFF)
becomes
the sum of the maximum load current of the step-up
regulator’s output plus the contributions from the posi-
tive and negative charge pumps:
where I
MAIN(MAX)
is the maximum step-up output cur-
rent, n
VN
is the number of negative charge-pump
stages, n
VP
is the number of positive charge-pump
stages, I
VN
is the negative charge-pump output cur-
rent, and I
VP
is the positive charge-pump output cur-
rent, assuming the initial pump source for I
VP
is V
MAIN
.
Calculate the approximate inductor value using the typ-
ical input voltage (V
IN
), the maximum output current
(I
MAIN(EFF)
), the expected efficiency (η
TYP
) taken from
an appropriate curve in the Typical Operating
Characteristics , the desired switching frequency (f
OSC
),
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
IN(MIN)
using con-
servation of energy and the expected efficiency at that
operating point (η
MIN
) taken from an appropriate curve
in the Typical Operating Characteristics :
Calculate the ripple current at that operating point and
the peak current required for the inductor:
II
I
PEAK IN DC MAX
RIPPLE
=+
(, )
2
I
VVV
LV f
RIPPLE
IN MIN MAIN IN MIN
MAIN O
=
×
()
××
() ()
-
SSC
I
IV
V
IN DC MAX
MAIN EFF MAIN
IN MIN MIN
(, )
()
()
=
×
×η
L
V
V
VV
If
IN
MAIN
MAIN IN
MAIN EFF OSC
=
⎛
⎝
⎜
⎞
⎠
⎟
×
⎛
⎝
2
-
()
⎜⎜
⎜
⎞
⎠
⎟
⎟
⎛
⎝
⎜
⎞
⎠
⎟
η
TYP
LIR
II nInI
MAIN EFF MAIN MAX VN VN VP VP() ( )
()=+×++×1
MAX17103/AUO-P1721.14
DC-DC Converter with Integrated Scan Driver,
VGL Controller, Op Amp, and LDO for TFT LCD
______________________________________________________________________________________ 23