Tft-lcd step-up dc-dc converter – Rainbow Electronics MAX17062 User Manual
Page 10
MAX17062
Input Capacitor Selection
The input capacitor (C
IN
) reduces the current peaks
drawn from the input supply and reduces noise injection
into the IC. Two 4.7μF ceramic capacitors are used in
the
Typical Operating 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
IN
can be reduced below the values used in
the typical operating circuit. Ensure a low-noise supply
at IN by using adequate C
IN
. Alternatively, greater volt-
age variation can be tolerated on C
IN
if IN is decoupled
from C
IN
using an RC lowpass filter (see R1 and C3 in
Figure 1).
Rectifier Diode Selection
The MAX17062’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
PEAK
calculated in the
Inductor Selection
section and that its breakdown voltage exceeds the
output voltage.
Output Voltage Selection
The MAX17062 operates with an adjustable output from
V
IN
to 20V. Connect a resistive voltage-divider from the
output (V
MAIN
) to AGND with the center tap connected
to FB (see Figure 1). Select R2 in the 10k
Ω to 50kΩ
range. Calculate R1 with the following equation:
where V
FB
, the step-up regulator’s feedback set point,
is 1.24V (typ). Place R1 and R2 close to the IC.
Loop Compensation
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
) and capacitor (C
COMP
) in series from
COMP to AGND, and another capacitor (C
COMP2
) from
COMP to AGND. R
COMP
is chosen to set the high-
frequency integrator gain for fast transient response,
while C
COMP
is chosen to set the integrator zero to
maintain loop stability. The second capacitor, C
COMP2
,
is chosen to cancel the zero introduced by output-
capacitance ESR. For optimal performance, choose the
components using the following equations:
For the ceramic output capacitor, where ESR is small,
C
COMP2
is optional. The best gauge of correct loop
compensation is by inspecting the transient response
of the MAX17062. Adjust R
COMP
and C
COMP
as neces-
sary to obtain optimal transient performance.
Soft-Start Capacitor
The soft-start capacitor should be large enough that it
does not reach final value before the output has
reached regulation. Calculate C
SS
to be:
where C
OUT
is the total output capacitance including
any bypass capacitor on the output bus, V
OUT
is the
maximum output voltage, I
INRUSH
is the peak inrush
current allowed, I
OUT
is the maximum output current
during power-up, and V
IN
is the minimum input voltage.
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:
t
MAX
= 2.4 x 10
5
x C
SS
C
C
V
V
V
V
I
I
V
SS
OUT
OUT
IN
OUT
IN
INRUSH
OUT
OUT
>
Ч
Ч
Ч
−
Ч
Ч
−
Ч
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
−
21
10
6
2
R
V
V
C
L
I
C
V
C
I
R
C
R
L
I
V
V
COMP
IN
OUT
OUT
MAIN MAX
COMP
OUT
OUT
MAIN MAX
COMP
COMP
ESR
MAIN MAX
IN
OUT
.
(
)
(
)
(
)
≈
Ч
Ч
Ч
Ч
≈
Ч
Ч
Ч
≈
Ч
Ч
Ч
Ч
315
10
0 0036
2
R
R
V
V
MAIN
FB
1
2
1
=
×
−
⎛
⎝
⎜
⎞
⎠
⎟
TFT-LCD Step-Up DC-DC Converter
10
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