Datasheet

Step-Up, Step-Down Regulator, Gate-On Charge Pump,
and Boost-Buck Regulator for TV TFT LCD Display
MAX17122
______________________________________________________________________________________ 29
Temperature-Compensated
Boost-Buck Regulator
Inductor Selection
The inductance value, peak current rating, and series
resistance are factors to consider when selecting the
inductor. These factors influence the converter’s efficien-
cy, 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 cur-
rent 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 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 cur-
rent 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 resis-
tance 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 efficiency
improvements in typical operating regions.
Calculate the approximate inductor value using the
typical input voltage (V
IN3
), the typical output voltage
(V
GOFF2
), the maximum output current (I
VOFF2(MAX)
),
the assumed efficiency (E
TYP
) of 85%, and an estimate
of LIR based on the above discussion:
IN3 GOFF2
TYP
3
VOFF2(MAX) SW IN3 GOFF2
V (-V )
L
I f (V - V ) LIR
η
=
Choose an available inductor value from an appropri-
ate inductor family. Calculate the maximum DC inductor
current at the minimum input voltage V
IN3(MIN)
and cold
temperature output voltage (V
GOFF2_COLD
) using con-
servation of energy and the expected efficiency at that
operating point (E
MIN
) taken from an appropriate curve
in the Typical Operating Characteristics:
GOFF2(MAX) GOFF2_COLD
L3(DC,MAX)
IN3(MIN) MIN
I (-V )
I
V
×
=
× η
Calculate the ripple current at that operating point and
the peak current required for the inductor:
( )
IN3 GOFF2_COLD
GOFF2_RIPPLE
3 IN3 GOFF2_COLD SW
-V V
I
L V V f
=
−
GOFF2_RIPPLE
GOFF2_PEAK L3(DC,MAX)
I
I I
2
= +
The inductor’s saturation current rating and the MAX17122’s
LX3 current limit should exceed I
GOFF2
_
PEAK
and the
inductor’s DC current rating should exceed I
L3(DC,MAX)
.
For good efficiency, choose an inductor with less than
0.1I series resistance.
Considering the typical operating circuit in Figure 1, the
maximum load current (I
GOFF2(MAX)
) is 450mA with a
-12V typical output and a typical input voltage of 12V.
The estimated efficiency is 85% at this operating point.
Because the inductor is large, so is the series resistance;
choose an LIR of 0.5 to minimize power loss:
3
12V 12V 85%
L 30 H
0.45A 750kHz (12V 12V) 0.5
×
= =
× × +
F
A 22FH inductor is used in the typical operating cir-
cuit (Figure 1). Using the circuit’s minimum input volt-
age (8V), cold-temperature output voltage (-20V), and
estimating efficiency of 85% at that operating point:
L3(DC,MAX)
450mA 20V
I 1.32A
8V 85%
×
= ≈
×
The ripple current and the peak current are:
GOFF2_RIPPLE
12V 20V
I 0.46A
22 H (12V 20V) 750kHz
×
= ≈
× + ×F
GOFF2_PEAK
0.46A
I 1.32A 1.55A
2
= + ≈