MAX8753ETI-T Maxim Integrated, MAX8753ETI-T Datasheet - Page 16

• Manufacturer Part Number: MAX8753ETI-T
• Description: LCD Drivers
• Manufacturer: Maxim Integrated

TFT LCD DC-DC Converter with
Integrated Charge Pumps
plus the contributions from the positive and negative
charge pumps:
where I
is the number of negative charge-pump stages, I
the negative charge-pump output current, and I
the positive charge-pump output current.
Calculate the approximate inductor value using the typ-
ical input voltage (V
(I
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
servation of energy and the expected efficiency at that
operating point (η
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
MAX8753’s LX current limit (I
I
exceed I
inductor with less than 0.1Ω series resistance.
Considering the typical operating circuit, the maximum
load current (I
and a typical input voltage of 3.3V:
Choosing an LIR of 0.45 and estimating efficiency of
80% at this operating point:
16
PEAK
MAIN(MAX)
I
MAIN EFF
______________________________________________________________________________________
and the inductor’s DC current rating should
MAIN(MAX)
I
MAIN EFF
(
I
IN(DC,MAX)
RIPPLE
L
=
), the expected efficiency (η
I
IN DCMAX
(
I
)



PEAK
(
V
=
MAIN
MAIN(MAX)
V
140
)
IN
=
=
,
I
MIN
is the maximum output current, n
MAIN MAX
V
=
mA



IN MIN
I
2
IN DCMAX
. For good efficiency, choose an
IN



(
) taken from an appropriate curve
)
(
(
I
MAIN EFF
=
+ ×
), the maximum output current
L V
2 10
I
V
MAIN EFF
)
×
,
MAIN
) is 140mA with a 9V output
V
)
×
(
+
IN MIN
(
MAIN
n
V
(
NEG
(
mA
MAIN
)
)
−
LX(MAX)
×
+
V
IN
f
OSC
I
)
RIPPLE
×
)
×
+ ×
× η
I
×
NEG
f
−
3 10
OSC
2
V



IN(MIN)
V
MIN



MAIN
IN MIN
) should exceed
η
+ ×
LIR
TYP
(
TYP
3
mA
I



) taken from
POS
)
using con-
=
)
190
NEG
POS
mA
NEG
is
is
Using the circuit’s minimum input voltage (2.6V) and
estimating efficiency of 70% at that operating point:
The ripple current and the peak current are:
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):
where I
Inductor Selection section). For ceramic capacitors,
the output voltage ripple is typically dominated by
V
teristics of the output capacitor must also be considered.
The input capacitor (C
current peaks drawn from the input supply and reduces
noise injection 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 impedance since the step-up regulator often
runs directly from the output of another regulated sup-
ply. Typically, C
used in the typical operating circuit. Ensure a low noise
supply at IN by using adequate C
RIPPLE(C)
V
RIPPLE C
L
PEAK
=
I



. The voltage rating and temperature charac-
RIPPLE
V
3 3
RIPPLE
I
9
IN DC MAX
( )
.
V
I
V
PEAK
RIPPLE ESR
is the peak inductor current (see the
(
V
≈



IN
2
C
=
I
,
MAIN



=
OUT
6 8
can be reduced below the values
2 6
=
0 19
(
.
V
.
.
0 94
RIPPLE C
)
9
µ
.
V
IN
V
=
H



A
)
×
−
V
) (see Figure 1) reduces the
0 19
×
V
2 6
A
≈
Output Capacitor Selection
×
(
MAIN
.
MAIN OSC
3 3
.
9
9
I
+
PEAK ESR COUT
1
Input Capacitor Selection
.
( )
V
V
V
MHz
0 27
A
V
−
×
×
.
+
×
f
2 6
1
2
−
0 7
R
MHz
.
V
IN
9
.
V



A
RIPPLE ESR
V
IN



V
.
0 80
0 45
)
=
≈



.
.
1 08
(
0 94
=
and
.
.
0 27
(

 ≈

.
A
A
6 8
)
A
)
. µ
H