MAX8798AETX+T Maxim Integrated Products, MAX8798AETX+T Datasheet - Page 23

• Manufacturer Part Number: MAX8798AETX+T
• Description: Display Drivers Internal-Switch Boos t Regulator with Int
• Manufacturer: Maxim Integrated Products

Specifications of MAX8798AETX+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Choosing an LIR of 0.5 and estimating efficiency of
85% at this operating point:
A 2.6µH inductor is chosen. Then, using the circuit’s
minimum input voltage (3V) and estimating efficiency of
80% at that operating point:
The ripple current and the peak current at that input
voltage 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):
and:
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
drawn from the input supply and reduces noise injec-
tion into the IC. A 10µF ceramic capacitor is used in
RIPPLE(C)
Integrated 3-Channel Scan Driver for TFT LCDs
L
=
PEAK
⎛
⎜
⎝
I
RIPPLE
. The voltage rating and temperature charac-
3 3
V
8
V
RIPPLE
.
V
I
V
RIPPLE C
IN DCMAX
RIPPLE ESR
I
V
PEAK
is the peak inductor current (see the
(
⎞
⎟
⎠
2
=
⎛
⎜
⎝
,
______________________________________________________________________________________
=
0 4
2 6
( )
=
(
.
.
V
1 33
8
RIPPLE C
.
A
μ
)
3
≈
V
H
V
=
)
IN
×
C
−
I
MAIN
A
≈
×
×
1 2
0 4
Internal-Switch Boost Regulator with
OUT
) reduces the current peaks
3 3
3
Output Capacitor Selection
.
I
.
(
8
+
PEAK ESR COUT
.
V
8
V
( )
MHz
Input Capacitor Selection
A
V
0 6
V
×
×
.
×
−
2
⎛
⎜
⎝
0 8
1 2
+
V
A
R
8
.
3
V
.
V
MAIN
⎞
⎟
⎠
V
V
MAIN OSC
RIPPLE ESR
MHz
⎛
⎜
⎝
=
)
≈
0 85
1 53
0 5
.
1 33
.
(
.
.
f
−
≈
A
V
⎞
⎟ ≈
⎠
(
0 6
IN
A
.
)
2 8
⎞
⎟
⎠
A
. μ
)
H
Figure 2 because of the high source impedance seen
in typical lab setups. Actual applications usually have
much lower source impedance since the step-up regu-
lator often runs directly from the output of another regu-
lated supply. Typically, C
values used in Figure 2. Ensure a low noise supply at
IN by using adequate C
variation can be tolerated on C
from C
The MAX8798’s high switching frequency demands a
high-speed rectifier. Schottky diodes are recommend-
ed for most applications because of their fast recovery
time and low forward voltage. In general, a 2A Schottky
diode complements the internal MOSFET well.
The output voltage of the main step-up regulator is
adjusted by connecting a resistive voltage-divider from
the output (V
nected to FB (see Figure 2). Select R2 in the 10kΩ to
50kΩ range. Calculate R1 with the following equation:
where V
is 1.235V (typ). Place R1 and R2 close to the IC.
Choose R
gain for fast transient response. Choose C
the integrator zero to maintain loop stability.
For low-ESR output capacitors, use the following equa-
tions to obtain stable performance and good transient
response:
To further optimize transient response, vary R
20% steps and C
transient response waveforms.
IN
REF
using an RC lowpass filter (seen in Figure 2).
R
COMP
C
, the step-up regulator’s feedback set point,
COMP
COMP
MAIN
to set the high-frequency integrator
R
≈
COMP
) to AGND with the center tap con-
1
≈
1000
=
10
R
2
×
IN
×
I
in 50% steps while observing
×
MAIN MAX
L I
IN
. Alternatively, greater voltage
V
⎛
⎜
⎝
V
×
OUT
V
IN
can be reduced below the
V
Output Voltage Selection
MAIN
MAIN MAX
REF
(
×
V
×
OUT
IN
C
Loop Compensation
(
−
)
OUT
1
×
if IN is decoupled
⎞
⎟
⎠
R
×
)
COMP
C
Rectifier Diode
OUT
COMP
COMP
to set
23
in