Chip information – Rainbow Electronics MAX1579 User Manual
Page 11
MAX1578/MAX1579
Complete Bias and White LED Power Supplies
for Small TFT Displays
______________________________________________________________________________________
11
Component Selection
Use only ceramic capacitors with an X5R, X7R, or bet-
ter dielectric. See Table 1 for a list of recommended
components.
Capacitor Selection
Use low-ESR ceramic capacitors. Recommended val-
ues for the capacitors are shown in Table 1. To ensure
stability over a wide temperature range, ceramic
capacitors with an X5R or X7R dielectric are recom-
mended. Place these capacitors as close to the IC as
possible.
Inductor Selection
Recommended inductor values range from 10µH to
47µH. A 22µH inductor optimizes the efficiency for most
applications while maintaining low 15mV
P-P
input ripple.
With input voltages near 5V, a larger value of inductance
can be more efficient. To prevent core saturation, ensure
that the inductor-saturation current rating exceeds the
peak inductor current for the application. Calculate the
peak inductor current with the following formula:
Schottky Diode Selection
The MAX1578/MAX1579 require a high-speed rectifica-
tion diode (D1) for optimum performance. A Schottky
diode is recommended due to its fast recovery time
and low forward-voltage drop. Ensure that the diode’s
average and peak current ratings exceed the average
output current and the peak inductor current, respec-
tively. In addition, the diode’s reverse breakdown volt-
age must exceed V
OUT
. The RMS diode current is
calculated as:
PC Board Layout and Routing
Due to fast switching waveforms, careful PC board lay-
out is required. An evaluation kit (MAX1578EVKIT) is
available to speed design. When laying out a board,
minimize trace lengths between the IC and R1, the
inductor, the diode, the input capacitor, and the output
capacitor. Keep traces short, direct, and wide. Keep
noisy traces, such as the LX node trace, away from CS.
The IN bypass capacitor (C
IN
) should be placed as
close to the IC as possible. The transfer capacitors for
the charge pumps should be located as close as possi-
ble to the IC. PGND and GND should be connected
directly to the exposed paddle underneath the IC. The
ground connections of C
IN
and C
OUT
should be as
close together as possible. The traces from IN to the
inductor and from the Schottky diode to the LEDs may
be longer. The MAX1579 evaluation kit contains a sam-
ple layout to speed designs.
Chip Information
TRANSISTOR COUNT: 3801
PROCESS: BiCMOS
I
I
I
DIODE RMS
OUT
PEAK
(
)
=
×
I
V
I
V
V
s
L
PEAK
OUT MAX
LED MAX
IN MIN
IN MIN
=
Ч
Ч
+
Ч
Ч
(
)
(
)
(
)
(
)
.
.
0 8
0 8
2
µ