Rainbow Electronics MAX17117 - Manual (Page 18)

Internal-Switch Boost Regulator with Integrated
7-Channel Scan Driver, Op Amp, and LDO
MAX17117
18 _____________________________________________________________________________________
Design Procedure
Main Step-Up Regulator
Inductor Selection
The minimum inductance value, peak current rating, and
series resistance are factors to consider when select-
ing the inductor. These factors influence the converter’s
efficiency, 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 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 among 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 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.
In Figure 1, the LCD’s gate-on and gate-off supply
voltages are generated from two unregulated charge
pumps driven by the step-up regulator’s LX node. The
additional load on LX must therefore be considered 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 positive and nega-
tive charge pumps:
MAIN(EFF) MAIN(MAX) VN VN VP VP
I I n I (n 1) I = + Ч + + Ч
where I MAIN(MAX) is the maximum step-up output cur-
rent, n
VN
is the number of negative charge-pump stag-
es, n
VP
is the number of positive charge-pump stages,
I VN is the negative charge-pump output current, and I VP
is the positive charge-pump output current, assuming
the initial pump source for I
VP
is V
MAIN
.
Calculate the approximate inductor value using the
typical input voltage (V
IN ), the maximum output cur-
rent (I MAIN(EFF) ), the expected efficiency (E 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:
2
IN MAIN IN TYP
MAIN MAIN(EFF) OSC
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:
MAIN(EFF) MAIN
IN(DC,MAX)
IN(MIN) MIN
I V
I
V
Ч
=
Ч η
Calculate the ripple current at that operating point and
the peak current required for the inductor:
( )
IN(MIN) MAIN IN(MIN)
RIPPLE
MAIN OSC
V V V
I
L V f
Ч −
=
Ч Ч
RIPPLE
PEAK IN(DC,MAX)
I
I I
2
= +
The inductor’s saturation current rating and the
MAX17117 LX current limit 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
0.1I series resistance.
Considering the typical application circuit, the maximum
load current (I
MAIN(MAX) ) is 200mA, with an 8.5V output
and a typical input voltage of 3.3V. The effective full-load
step-up current is:
MAIN(EFF)
I 200mA 1 10mA (2 1) 25mA 285mA = + Ч + + Ч =