MAX8758 Maxim Integrated Products, MAX8758 Datasheet - Page 16

• Manufacturer Part Number: MAX8758
• Description: Step-Up Regulator
• Manufacturer: Maxim Integrated Products

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
of the maximum load current on the step-up regulator’s
output plus the contributions from the positive and neg-
ative charge pumps:
where I
is the number of negative charge-pump stages, n
the number of positive charge-pump stages, I
the negative charge-pump output current, and I
the positive charge-pump output current, assuming the
pump source for I
The required inductance can then be calculated as
follows:
where V
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
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 guaran-
teed minimum value of the MAX8758’s LX current limit
(I
rating should exceed I
choose an inductor with less than 0.1Ω series resistance.
Step-Up Regulator with Switch Control
and Operational Amplifier for TFT LCD
16
LIM
L
) should exceed I
______________________________________________________________________________________
=
I
RIPPLE
MAIN(MAX)



IN
I
V
I
MAIN(EFF)
IN DCMAX
MAIN
V
I
is the typical input voltage and η
IN
(
PEAK
,
=



2
POS
V
MIN
is the maximum output current, n
=
×
IN MIN
)
= I
(
+ (n



I
PEAK
=
) taken from an appropriate curve
IN DC MAX
MAIN(MAX)
is V
I
MAIN EFF
IN(DC,MAX)
L
(
I
MAIN EFF
)
POS
×
V
MAIN
V
MAIN(EFF)
×
MAIN
,
and the inductor’s DC current
IN MIN
(
V
(
(
MAIN
(
+ 1) x I
V
.
MAIN
)
)
+ n
−
)
×
)
+
. For good efficiency,
V
× η
×
IN
×
NEG
f
POS
I
OSC
RIPPLE
IN(MIN)
−
becomes the sum
f
V
OSC
MIN
MAIN
V
2
x I
IN MIN

 ×

(
NEG
using con-



TYP
)
η
)
LIR
TYP
NEG
POS
POS
is the
NEG



is
is
is
Considering the Typical Operating Circuit, the maxi-
mum load current (I
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
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.
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
Up Regulator Inductor Selection section). For ceramic
capacitors, the output voltage ripple is typically dominat-
ed by V
characteristics of the output capacitor must also be con-
sidered.
L
=
I
RIPPLE



MAIN(EFF)
Step-Up Regulator Output Capacitor Selection
3 3
8 5
I
V
PEAK
IN DC MAX
.
.
RIPPLE C
RIPPLE(C)
V
V
V
(
I
RIPPLE
PEAK



2
,
=
is the peak inductor current (see the Step-
V
×
( )
RIPPLE(ESR)
4 2



is 360mA with an 8.5V output and a
=
= V
)
.
0 36
. The voltage rating and temperature
.
≈
µ
8 5
3
=
1 28
MAIN(MAX)
H
.
RIPPLE(C)
V
.
A
C
V
I
MAIN
0 36
×
×
MAIN
×
.
A
−
3
8 5
( .
1 2
8 5
V
+
3 3
.
≈ I
.
A
.
MHz
V
×
V
PEAK
V
0 4
×
×
+ V
) is 300mA for the step-
.
×
2
0 8
−



8 5
.

 ×

A
1 2
.
V
ARIPPLE(ESR)
V
3
.
MAIN
x R
MAIN
V
V
≈
MHz
)



ESR
0 85
≈
1 48
0 4
.
.
.
×
1 28
−

 ≈

.
≈
A
f
V
SW
IN
0 4
A
.
4 2



.
A
µ
H