Rainbow Electronics MAX8715 - Manual (Page 11)

MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
______________________________________________________________________________________ 11
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 efficiency
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 of LIR based on the
above discussion:
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:
Calculate the ripple current at that operating point and
the peak current required for the inductor:
The inductor’s saturation current rating and the
MAX1790/MAX8715s’ 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 0.1 Ω series resistance.
Considering the application circuit in Figure 4, the maxi-
mum load current (I
MAIN(MAX)
) is 150mA with a 9V output
and a typical input voltage of 3.3V. Choosing an LIR of 0.5
and estimating efficiency of 85% at this operating point:
Using the circuit’s minimum input voltage (3V) and esti-
mating efficiency of 80% at that operating point:
The ripple current and the peak current are:
Diode Selection
The output diode should be rated to handle the output
voltage and the peak switch current. Make sure that the
diode’s peak current rating is at least I
PK
and that its
breakdown voltage exceeds V OUT . Schottky diodes are
recommended.
Input and Output Capacitor Selection
Low-ESR capacitors are recommended for input
bypassing and output filtering. Low-ESR tantalum
capacitors are a good compromise between cost and
performance. Ceramic capacitors are also a good
choice. Avoid standard aluminum electrolytic capaci-
tors. A simple equation to estimate input and output-
capacitor values for a given voltage ripple is as follows:
where V RIPPLE is the peak-to-peak ripple voltage on the
capacitor.
Output Voltage
The MAX1790/MAX8715 operate with an adjustable
output from V
IN to 13V. Connect a resistor voltage-
divider to FB (see the Typical Operating Circuit) from
the output to GND. Select the resistor values as follows:
where V FB , the boost-regulator feedback set point, is
1.24V. Since the input bias current into FB is typically 0,
R R
V
V
OUT
FB
1 2 1
= −
⎛
⎝⎜ ⎞
⎠⎟
C
0.5 L I
V V
PK
2
RIPPLE OUT
≥
Ч Ч ⎛⎝
⎞
⎠
Ч
I A
A
A
PEAK
= + ≈ 0 6
0 25
2
0 725 .
.
.
I
V V V
H V MHz
A
RIPPLE
=
Ч −
Ч Ч
≈
3 9 3
6 8 9 1 2
0 25
( )
. .
.
µ
I
A V
V
A
IN DC MAX ( , )
.
.
.
=
Ч
Ч
≈
0 15 9
3 0 8
0 6
L
V
V
V V
A MHz
H = ⎛
⎝⎜
⎞
⎠⎟ −
×
⎛
⎝⎜ ⎞
⎠⎟ ⎛
⎝⎜ ⎞
⎠⎟
≈
3 3
9
9 3 3
0 15 1 2
0 85
0 5
6 8
2
. .
. .
.
.
. µ
I I
I
PEAK IN DC MAX
RIPPLE = +
( , )
2
I
V V V
L V f
RIPPLE
IN MIN MAIN IN MIN
MAIN OSC
=
Ч −
Ч Ч
( ) ( )
( )
I
I V
V
IN DC MAX
MAIN MAX MAIN
IN MIN MIN
( , )
( )
( )
=
Ч
Ч η
L
V
V
V V
I f LIR
IN
MAIN
MAIN IN
MAIN MAX OSC
TYP =
⎛
⎝⎜ ⎞
⎠⎟ −
×
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝⎜ ⎞
⎠⎟
2
( )
η