Datasheet

MAX17103/AUO-P1721.14
The inductor’s saturation current rating and the
MAX17103 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
0.1Ω series resistance.
Considering the typical operating circuit, the maximum
load current (I
MAIN(MAX)
) is 250mA, with an 8.5V output
and a typical input voltage of 3.3V. The effective full-
load step-up current is:
Choosing an LIR of 0.2 and estimating efficiency of
85% at this operating point:
A 10µH inductor is chosen. Then, using the circuit’s
minimum input voltage (3.0V) and estimating efficiency
of 80% at that operating point:
The ripple current and the peak current at that input
voltage are:
Peak Inductor Current-Limit Setting
The inductor peak current limit can be adjusted up to
2A (max) by choosing the appropriate R
ENA
resistor as
configured in Figure 1 by the following:
The above threshold set by R
ENA
varies depending on
the step-up converter’s input voltage, output voltage,
and duty cycle.
Output Capacitor Selection
The total output-voltage ripple has two components: the
capacitive ripple caused by the charging and discharg-
ing of the output capacitance, and the ohmic ripple due
to the capacitor’s equivalent series resistance (ESR):
and:
where I
PEAK
is the peak inductor current (see the Main
Step-Up Regulator Inductor Selection
section). For
ceramic capacitors, the output-voltage ripple is typically
dominated by V
RIPPLE(C)
. The voltage rating and tem-
perature characteristics of the output capacitor must
also be considered.
Input Capacitor Selection
The input capacitor (C3) reduces the current peaks
drawn from the input supply and reduces noise injec-
tion into the IC. A 10µF ceramic capacitor is used in the
typical operating circuit (Figure 1) because of the high
source impedance seen in typical lab setups. Actual
applications usually have much lower source imped-
ance since the step-up regulator often runs directly
from the output of another regulated supply.
Rectifier Diode
The MAX17103 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 1A Schottky
diode complements the internal MOSFET well.
Output-Voltage Selection
The output voltage of the main step-up regulator is
adjusted by connecting a resistive voltage-divider from
the output (V
MAIN
) to AGND with the center tap con-
nected to FB (see Figure 1). Select R8 in the 10kΩ to
50kΩ range. Calculate R7 using the following equation:
Place R1 and R2 close to the IC.
RR
V
V
MAIN
12
124
1=×
⎛
⎝
⎜
⎞
⎠
⎟
.
-
VIR
RIPPLE ESR PEAK ESR COUT() ( )
≈
V
I
C
VV
Vf
RIPPLE C
MAIN
OUT
MAIN IN
MAIN OSC
()
≈
⎛
⎝
⎜
⎞
⎠
-
⎟⎟
VV V
RIPPLE RIPPLE C RIPPLE ESR
=+
() ( )
R
VV
I
ENA
LDOO
OCP
≈
(.)()-1 25 80000
IA
A
A
PEAK
=+ =103
0 162
2
111.
.
.
I
VVV
μH V MHz
A
RIPPLE
=
×
()
××
≈
3853
10 85 12
0 162
.
..
.
-
I
AV
V
A
IN DC MAX(, )
..
.
.=
×
×
≈
029 85
308
103
L
V
V
VV
AMHz
=
⎛
⎝
⎜
⎞
⎠
⎟
×
⎛
⎝
⎜
⎞
⎠
33
85
85 33
029 12
2
.
.
..
..
-
⎟⎟
⎛
⎝
⎜
⎞
⎠
⎟
≈
085
02
96
.
.
.μH
ImAmAmAmA
MAIN EFF()
()=+×++×=250 1 10 2 1 10 290
DC-DC Converter with Integrated Scan Driver,
VGL Controller, Op Amp, and LDO for TFT LCD
24 ______________________________________________________________________________________