MAX8795AGCJ/V+T Maxim Integrated Products, MAX8795AGCJ/V+T Datasheet - Page 21

• Manufacturer Part Number: MAX8795AGCJ/V+T
• Description: Display Drivers TFT-LCD DC-DC Conver ter with Operational
• Manufacturer: Maxim Integrated Products

Specifications of MAX8795AGCJ/V+T

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

a typical input voltage of 5V. Choosing an LIR of 0.5 and
estimating efficiency of 85% at this operating point:
Using the circuit’s minimum input voltage (4.5V) and
estimating efficiency of 80% 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 discharging
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
drawn from the input supply and reduces noise injection
into the IC. A 22μF ceramic capacitor is used in the typi-
cal applications 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 supply. Typically, C
be reduced below the values used in the typical applica-
tions circuit. Ensure a low-noise supply at IN by using
adequate C
and
RIPPLE(C)
:
L
RIPPLE
=
I
I
RIPPLE
PEAK
⎛
⎜
⎝
. The voltage rating and temperature charac-
14
5
V
IN
V
I
V
V
IN DCMAX
RIPPLE
V
RIPPLE C
. Alternately, greater voltage variation can
RIPPLE ESR
(
⎞
⎟
⎠
=
is the RIPPLE inductor current (see the
2
=
1 94
⎛
⎜
⎝
.
,
______________________________________________________________________________________
3 3
0 5
.
.
4 5
( )
=
(
A
.
14
μ ×
A
V
)
+
H
V
RIPPLE C
≈
×
=
V
IN
0 77
)
×
1 2
C
I
−
0 5
.
MAIN
14
) reduces the current peaks
4 5
≈
.
(
Output-Capacitor Selection
2
OUT
.
14
5
.
I
PEAK ESR COUT
MHz
V
V
Input-Capacitor Selection
A
A
V
V
( )
×
×
×
−
≈
⎛
⎜
⎝
1 2
14
0 8
4 5
2 33
+
.
V
R
⎞
⎟
⎠
V
.
.
.
⎛
⎜
⎝
MAIN
V
V
MAIN OSC
MHz
0 85
RIPPLE ESR
V
0 5
≈
.
A
)
.
1 94
(
.
f
−
⎞
⎟ ≈
⎠
≈
V
TFT-LCD DC-DC Converter with
0 77
(
A
IN
.
3 3
)
.
⎞
⎟
⎠
A
μ
)
H
IN
can
be tolerated on C
RC lowpass filter (see R10 and C13 in Figure 1).
The MAX8795A’s 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 2A Schottky
diode complements the internal MOSFET well.
The output voltage of the main step-up regulator can be
adjusted by connecting a resistive voltage-divider from the
output (V
FB (see Figure 1). Select R2 in the 10kΩ to 50kΩ range.
Calculate R1 with the following equation:
where V
is 1.233V. 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.
For highest efficiency, always choose the lowest num-
ber of charge-pump stages that meet the output
requirement. Figures 8 and 9 show the positive and
negative charge-pump output voltages for a given
V
Operational Amplifiers
MAIN
Selecting the Number of Charge-Pump Stages
for one-, two-, and three-stage charge pumps.
FB
MAIN
R
C
, the step-up regulator’s feedback set point,
COMP
COMP
COMP
) to AGND with the center tap connected to
IN
≈
to set the high-frequency integrator
≈
R
COMP
253
10
1
if IN is decoupled from C
=
×
R
×
I
2
L I
MAIN MAX
V
V
in 50% steps while observing
×
×
IN
OUT
⎛
⎜
⎝
MAIN MAX
V
×
Output-Voltage Selection
(
MAIN
V
V
FB
×
OUT
C
(
)
Loop Compensation
OUT
×
−
×
1
R
Charge Pumps
⎞
⎟
⎠
)
C
COMP
OUT
Rectifier Diode
COMP
IN
using an
COMP
to set
21
in