MAX8740ETB+T Maxim Integrated Products, MAX8740ETB+T Datasheet - Page 9

• Manufacturer Part Number: MAX8740ETB+T
• Description: Display Drivers Step-Up DC-DC Conver p-Up DC-DC Converter
• Manufacturer: Maxim Integrated Products

Specifications of MAX8740ETB+T

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

The input capacitor (C
drawn from the input supply and reduces noise injection
into the IC. A 10µF ceramic capacitor is used in the typi-
cal operating circuit (Figure 1) because of the high
source impedance seen in typical lab setups. Actual
applications usually have much lower source imped-
ance since the step-up regulator often runs directly from
the output of another regulated supply. Typically, C
can be reduced below the values used in the typical
operating circuit. Ensure a low noise supply at IN by
using adequate C
tion can be tolerated on C
using an RC lowpass filter (see R3 and C3 in Figure 1).
The MAX8740’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. The diode should be
rated to handle the output voltage and the peak switch
current. Make sure that the diode’s peak current rating
is at least I
section and that its breakdown voltage exceeds the
output voltage.
The MAX8740 operates with an adjustable output from
V
output (V
FB (see Figure 1). Select R2 in the 10kΩ to 50kΩ range.
Calculate R1 with the following equation:
where V
is 1.28V (typ). Place R1 and R2 close to the IC.
The voltage feedback loop needs proper compensation
to prevent excessive output ripple and poor efficiency
caused by instability. This is done by connecting a
resistor (R
COMP to GND, and another capacitor (C
COMP to GND. R
quency integrator gain for fast transient response, while
C
loop stability. The second capacitor, C
IN
COMP
to 28V. Connect a resistive voltage-divider from the
FB
is chosen to set the integrator zero to maintain
OUT
COMP
, the step-up regulator’s feedback set point,
PEAK
) to GND with the center tap connected to
R
1
) and capacitor (C
IN
calculated in the Inductor Selection
=
COMP
_______________________________________________________________________________________
. Alternatively, greater voltage varia-
R
2
IN
×
is chosen to set the high-fre-
) reduces the current peaks
IN



Input Capacitor Selection
if IN is decoupled from C
V
Output Voltage Selection
Rectifier Diode Selection
V
OUT
Loop Compensation
FB
TFT-LCD Step-Up DC-DC Converter
COMP
−
1



COMP2
) in series from
COMP2
, is cho-
) from
IN
IN
sen to cancel the zero introduced by output-capaci-
tance ESR. For optimal performance, choose the com-
ponents using the following equations:
For the ceramic output capacitor, where ESR is small,
C
compensation is by inspecting the transient response
of the MAX8740. Adjust R
sary to obtain optimal transient performance.
The soft-start capacitor should be large enough that it
does not reach final value before the output has
reached regulation. Calculate C
where C
any bypass capacitor on the output bus, V
maximum output voltage, I
current allowed, I
during power-up, and V
The load must wait for the soft-start cycle to finish
before drawing a significant amount of load current.
The duration after which the load can begin to draw
maximum load current is:
COMP2
C
COMP
R
C
COMP
OUT
COMP
is optional. The best gauge of correct loop
C


 

2
V
SS
IN
≈
is the total output capacitance including
≈
t
×
>
0 0036
≈
MAX
V
.
315
OUT
OUT
I
21
INRUSH
10
= 6.77 x 10
×
2
×
×
is the maximum output current
IN
×
10
−
L
I
V
V
OUT MAX
COMP
is the minimum input voltage.
IN
OUT
R
−
×
V
−
V
ESR
6
INRUSH
IN
IN
×
(
I
Soft-Start Capacitor
I
OUT MAX
OUT
×
×
×
×
SS
V
5
×
and C
OUT
V
C
V
x C
(
C
)
OUT
OUT
to be:
OUT
L
OUT
×
×
is the peak inrush
SS
×
×
R
V
)
COMP
OUT
COMP
I
OUT MAX
×
C
OUT


 

(
OUT
as neces-
)
is the
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