MAX8740 Datasheet, PDF (8/12 Pages) Maxim Integrated Products – TFT-LCD Step-Up DC-DC Converter

TFT-LCD Step-Up DC-DC Converter
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 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_OUT(MAX)), the expected efficiency (η_TYP) taken from
an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
L
=


V_IN
V_OUT


2



V_OUT − V_IN
I_OUT(MAX) × 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 an appropriate curve
in the Typical Operating Characteristics:
I_{IN(DC, MAX)}
=
I_OUT(MAX) × V_OUT
V_IN(MIN) × η_MIN
Calculate the ripple current at that operating point and
the peak current required for the inductor:
I_RIPPLE
=
V_IN(MIN) × (V_OUT − V_IN(MIN) )
L × V_OUT × f_OSC
I_PEAK = I_{IN(DC, MAX)} + ^IRIPPLE
2
The inductor’s saturation current rating and the
MAX8740’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_OUT(MAX)) is 900mA with a 13.5V output
and a 5V typical input voltage. Choosing an LIR of 0.35
and estimating efficiency of 85% at this operating point:
L
=
 5V  ²  13.5V − 5V 
 13.5V   0.9A × 1.2MHz
 0.85
 0.35
≈
2.7µH
Using the circuit’s minimum input voltage (4.5V) and
estimating efficiency of 85% at that operating point:
I_{IN(DC, MAX)}
=
0.9A ×
4.5V ×
3.5V
0.85
≈
3.2A
The ripple current and the peak current are:
I_RIPPLE
=
4.5V × (12.5V − 4.5V)
2.7µH × 13.5V × 1.2MHz
≈
0.93A
I_PEAK
=
3.2A +
0.93A
2
≈
3.7A
Output Capacitor Selection
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharg-
ing of the output capacitance, and the ohmic ripple due
to the capacitor’s equivalent series resistance (ESR):
V_RIPPLE = V_RIPPLE(C) + V_RIPPLE(ESR)
V_RIPPLE(C)
≈
I_OUT
C_OUT


V_OUT
V_OUT
− V_IN
f_OSC

^,
and
V_RIPPLE(ESR) ≈ I_PEAK R_ESR(COUT)
where I_PEAK is the peak inductor current (see the
Inductor Selection section). For ceramic capacitors,
the output voltage ripple is typically dominated by
V_RIPPLE(C). The voltage rating and temperature charac-
teristics of the output capacitor must also be considered.
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