MAX1542 Datasheet, PDF (15/20 Pages) Maxim Integrated Products – TFT LCD DC-to-DC Converter with Operational Amplifiers

TFT LCD DC-to-DC Converter with
Operational Amplifiers
IN
5µA
N
DEL
REF
CTL
MAX1542
MAX1543
2.5V
SRC
Q1 P
COM
P
Q2
1kΩ
Q3
N
P
DRN
Figure 4. Switch Control
MAX1543 ONLY
values also require more energy storage and more
turns of wire, which increase physical size and can
increase I²R losses in the inductor. Low inductance val-
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 output cur-
rent. The best trade-off between inductor size and cir-
cuit efficiency for step-up converters 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 inductor
resistance is relatively high, more ripple can be accept-
ed 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 that 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, and an estimate for LIR based on the
above paragraphs:
L ≅ V_IN2 x η_TYP x (V_MAIN − V_IN) /
(V_MAIN2 x LIR x I_MAIN(MAX) x f_OSC)
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
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