Datasheet

Step-Up, Step-Down Regulator, Gate-On Charge Pump,
and Boost-Buck Regulator for TV TFT LCD Display
MAX17122
______________________________________________________________________________________ 27
The amplitude of the capacitive soar is a function of the
load step, the output capacitor value, the inductor value,
and the output voltage:
2
2 OUTB
OUTB_SOAR
OUTB OUTB
L ( I )
V
2 C V
× ∆
=
× ×
Given the component values in the circuit of Figure 1,
during a full 2A step load transient, the voltage step due
to capacitor ESR is negligible. The voltage sag and soar
are 138mV and 129mV, respectively.
Rectifier Diode
The MAX17122’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommended
for most applications because of their fast recovery time
and low forward voltage. In general, a 3A Schottky diode
works well in the MAX17122’s step-down regulator.
Step-Up Regulator
Inductor Selection
The inductance value, peak current rating, and series resis-
tance are factors to consider when selecting the inductor.
These factors influence the converter’s efficiency, maxi-
mum 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 loss-
es 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 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 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
IN
), the maximum output cur-
rent (I
AVDD(MAX)
), the expected efficiency (E
TYP
) taken
from an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
2
IN AVDD IN TYP
1
AVDD AVDD(MAX) SW
V V - V
L
V I f LIR
 
 
η
 
=  
 
 
 
×
 
 
 
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input current
at the minimum input voltage V
IN(MIN)
using conserva-
tion of energy and the expected efficiency at that operat-
ing point (E
MIN
) taken from an appropriate curve in the
Typical Operating Characteristics:
AVDD(MAX) AVDD
IN(DC,MAX)
IN(MIN) MIN
I V
I
V
×
=
× η
Calculate the ripple current at that operating point and
the peak current required for the inductor:
( )
IN(MIN) AVDD IN(MIN)
AVDD_RIPPLE
AVDD AVDD SW
V V - V
I
L V f
×
=
× ×
AVDD_RIPPLE
AVDD_PEAK IN(DC,MAX)
I
I I
2
= +
The inductor’s saturation current rating and the MAX17122’s
LX1 current limit should exceed I
AVDD
_
PEAK
and the
inductor’s DC current rating should exceed I
IN(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
AVDD(MAX)
) is 2.2A with a 15V
output and a typical input voltage of 12V. Choosing an
LIR of 0.3 and estimating efficiency of 90% at this oper-
ating point:
2
1
12V 15V - 12V 90%
L 3.49 H
15V 2.2A 750kHz 0.3
    
= =
    
×
    
F