Rainbow Electronics MAX763A User Manual
Page 10
MAX748A/MAX763A
3.3V, Step-Down,
Current-Mode PWM DC-DC Converters
10
______________________________________________________________________________________
The capacitor’s ESR should be less than 0.25
Ω
to keep
the output ripple less than 50mV
p-p
over the entire cur-
rent range (using a 22µH inductor). Capacitor ESR
usually rises at low temperatures, but OS-CON capaci-
tors provide very low ESR below 0°C. Table 3 lists
capacitor suppliers.
Other Components
The catch diode should be a Schottky or high-speed
silicon rectifier with a peak current rating of at least
1.0A for full-load (500mA) operation. The 1N5817 is a
good choice. The 330pF outer-loop compensation
capacitor provides the widest input voltage range and
best transient characteristics.
Printed Circuit Layouts
A good layout is essential for stable, low-noise opera-
tion. The layouts and component placement diagrams
MAX748A
MAX763A
SHDN
D1
1N5817
C5
330pF
L1
22
µ
H
C4*
220
µ
F
(10V)
OUTPUT
3.3V
C2
1.0
µ
F
C3*
150
µ
F
(16V)
8
1
LX
V+
OUT
CC
SS
REF
GND
3
6
2
C6
1000pF
C1
0.047
µ
F
INPUT
MAX748A 3.3V TO 16.0V
MAX763A 3.3V TO 11.0V
VIN
7
5
4
*OS-CON Series (very low ESR)
MAX748A
MAX763A
SHDN
D1
1N5817
C5
330pF
L1
22
µ
H
C4*
100
µ
F
(6.3V)
OUTPUT
3.3V
C2
1.0
µ
F
C3*
68
µ
F
(16V)
8
1
LX
V+
OUT
CC
SS
REF
GND
3
6
2
C6
1000pF
C1
0.047
µ
F
INPUT
MAX748A 3.3V TO 16.0V
MAX763A 3.3V TO 11.0V
VIN
7
5
4
*Sprague 293D or 595D Series-16V. See Table 3
for alternative suppliers.
in Figures 6-9 have been tested successfully over a
wide range of operating conditions.
The 1µF input
bypass capacitor must be positioned as close to
the V+ and GND pins as possible.
Also, place the
output capacitor as close to the OUT and GND pins as
possible. The traces connecting ground to the input
and output filter capacitors and to the catch diode
must be short to reduce inductance. Use an uninter-
rupted ground plane if possible.
Output-Ripple Filtering
A simple lowpass pi-filter (Figure 3) can be added to
the output to reduce output ripple to about 5mV
p-p
.
The cutoff frequency shown is 21kHz. Since the filter
inductor is in series with the circuit output, minimize the
filter inductor’s resistance so the voltage drop across it
is not excessive.
Figure 4. Standard 3.3V Step-Down Application Circuit Using
Through-Hole Components (all temperature ranges)
Figure 5. Standard 3.3V Step-Down Application Circuit Using
Surface-Mount Components (Commercial and Extended
Industrial Temperature Ranges)