Low-Noise Step-Up DC-DC Converters
ratio of inductor resistance to other power path resis-
2
3.3V
9V
9V − 3.3V
0.15A ×1.2MHz 0.5
0.85
⎛
⎞ ⎛
⎞⎛
⎟⎜
⎠⎝
⎞
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.
L =
≈ 6.8μH
⎜
⎝
⎟ ⎜
⎠ ⎝
⎟
⎠
Using the circuit’s minimum input voltage (3V) and esti-
mating efficiency of 80% at that operating point:
0.15A × 9V
3V × 0.8
I
=
≈ 0.6A
IN(DC,MAX)
The ripple current and the peak current are:
Once a physical inductor is chosen, higher and lower
values of the inductor should be evaluated for efficiency
improvements in typical operating regions.
3V × (9V − 3V)
6.8μH× 9V ×1.2MHz
I
=
≈ 0.25A
RIPPLE
Calculate the approximate inductor value using the typ-
ical input voltage (V ), the maximum output current
IN
0.25A
(I
), the expected efficiency (η
) taken from
MAIN(MAX)
TYP
I
= 0.6A +
≈ 0.725A
PEAK
an appropriate curve in the Typical Operating
Characteristics, and an estimate of LIR based on the
above discussion:
2
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
breakdown voltage exceeds V
recommended.
2
⎛
⎞
⎛
⎞
V
V
− V
× f
η
⎛
TYP
⎝ LIR ⎠
⎞
IN
MAIN
IN
L =
⎜
⎟
⎜
⎟
⎜
⎟
and that its
PK
V
I
⎝
⎠
MAIN
MAIN(MAX) OSC
⎝
⎠
. Schottky diodes are
OUT
Choose an available inductor value from an appropriate
inductor family. Calculate the maximum DC input cur-
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:
rent at the minimum input voltage V
using con-
IN(MIN)
servation of energy and the expected efficiency at that
operating point (η ) taken from an appropriate curve
MIN
in the Typical Operating Characteristics:
I
× V
MAIN(MAX)
MAIN
I
=
IN(DC,MAX)
V
× η
MIN
IN(MIN)
Calculate the ripple current at that operating point and
the peak current required for the inductor:
2
⎛
⎞
0.5 × L ×
I
PK
⎝
⎠
C ≥
V
× (V
− V
)
IN(MIN)
MAIN
IN(MIN)
V
× V
OUT
RIPPLE
I
=
RIPPLE
L × V
× f
MAIN OSC
where V
is the peak-to-peak ripple voltage on the
RIPPLE
capacitor.
I
RIPPLE
I
= I
+
PEAK
IN(DC,MAX)
2
Output Voltage
The inductor’s saturation current rating and the
MAX1790/MAX8715s’ LX current limit (I ) should
The MAX1790/MAX8715 operate with an adjustable
LIM
output from V to 13V. Connect a resistor voltage-
IN
exceed I
and the inductor’s DC current rating should
PEAK
divider to FB (see the Typical Operating Circuit) from
exceed I
. For good efficiency, choose an
IN(DC,MAX)
the output to GND. Select the resistor values as follows:
inductor with less than 0.1Ω series resistance.
⎛
⎞
Considering the application circuit in Figure 4, the maxi-
V
V
OUT
R1= R2
− 1
⎟
⎜
mum load current (I
) is 150mA with a 9V output
MAIN(MAX)
⎝
⎠
FB
and a typical input voltage of 3.3V. Choosing an LIR of 0.5
and estimating efficiency of 85% at this operating point:
where V , the boost-regulator feedback set point, is
FB
1.24V. Since the input bias current into FB is typically 0,
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