Max8758 – Rainbow Electronics MAX8758 User Manual
Page 16
MAX8758
In
Figure
1’s Typical Operating Circuit, the LCD’s gate-
on and gate-off voltages are generated from two unreg-
ulated charge pumps driven by the step-up regulator’s
LX node. The additional load on LX must therefore be
considered in the inductance calculation. The effective
maximum output current I
MAIN(EFF)
becomes the sum
of the maximum load current on the step-up regulator’s
output plus the contributions from the positive and neg-
ative charge pumps:
I
MAIN(EFF)
= I
MAIN(MAX)
+ n
NEG
x I
NEG
+ (n
POS
+ 1) x I
POS
where I
MAIN(MAX)
is the maximum output current, n
NEG
is the number of negative charge-pump stages, n
POS
is
the number of positive charge-pump stages, I
NEG
is
the negative charge-pump output current, and I
POS
is
the positive charge-pump output current, assuming the
pump source for I
POS
is V
MAIN
.
The required inductance can then be calculated as
follows:
where V
IN
is the typical input voltage and η
TYP
is the
expected efficiency obtained from the appropriate
curve in the Typical Operating Characteristics.
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 guaran-
teed minimum value of the MAX8758’s 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 0.1Ω series resistance.
Considering the Typical Operating Circuit, the maxi-
mum load current (I
MAIN(MAX)
) is 300mA for the step-
up regulator, 20mA for the two-stage positive charge
pump, and 20mA for the one-stage negative charge
pump. Altogether, the effective maximum output cur-
rent, I
MAIN(EFF)
is 360mA with an 8.5V output and a
typical input voltage of 3.3V. The switching frequency is
set to 1.2MHz. Choosing an LIR of 0.4 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:
The peak-inductor current does not exceed the guaran-
teed minimum value of the LX current limit in the
Electrical Characteristics
table
.
Step-Up Regulator 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):
V
RIPPLE
= V
RIPPLE(C)
+ V
ARIPPLE(ESR)
and
V
RIPPLE(ESR)
≈ I
PEAK
x R
ESR
where I
PEAK
is the peak inductor current (see the Step-
Up Regulator Inductor Selection section). For ceramic
capacitors, the output voltage ripple is typically dominat-
ed by V
RIPPLE(C)
. The voltage rating and temperature
characteristics of the output capacitor must also be con-
sidered.
V
I
C
V
V
V
f
RIPPLE C
MAIN
MAIN
MAIN
IN
MAIN
SW
( )
≈
Ч
Ч
−
I
A
A
A
PEAK
.
.
.
=
+
≈
1 28
0 4
2
1 48
I
V
V
V
H
V
MHz
A
RIPPLE
( .
)
.
.
.
.
=
Ч
Ч
Ч
≈
−
3
8 5
3
4 2
8 5
1 2
0 4
µ
I
A
V
V
A
IN DC MAX
(
,
)
.
.
.
.
=
Ч
Ч
≈
0 36
8 5
3
0 8
1 28
L
V
V
V
V
A
MHz
H
.
.
.
.
.
.
.
.
.
=
Ч
−
Ч
Ч
≈
3 3
8 5
8 5
3 3
0 36
1 2
0 85
0 4
4 2
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 DCMAX
MAIN EFF
MAIN
IN MIN
MIN
(
,
)
(
)
(
)
=
Ч
Ч η
L
V
V
V
V
I
f
LIR
IN
MAIN
MAIN
IN
MAIN EFF
OSC
TYP
(
)
=
Ч
Ч
Ч
−
2
η
Step-Up Regulator with Switch Control and
Operational Amplifier for TFT LCD
16
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