Rainbow Electronics MAX763A - Manual (Page 9)

MAX748A/MAX763A
3.3V, Step-Down,
Current-Mode PWM DC-DC Converters
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____________Applications Information
Fixed +3.3V Step-Down
Converter Application
Figure 3 shows the standard 3.3V step-down circuit with
components shown for commercial temperature range
applications. Figures 4, 5, and Table 2 suggest external
component values for both SO and through-hole wide
temperature range applications. These circuits are use-
ful in systems that require high current and high efficien-
cy and are powered by an unregulated supply, such as
a battery or wall-plug AC-DC adapter.
The MAX748A delivers a guaranteed 300mA for input
voltages of 4V to 16V, and a guaranteed 500mA for
input voltages of 4.75V to 16V with 800mA typical out-
put currents. The MAX763A delivers a guaranteed
300mA for input voltages of 4V to 11V, a guaranteed
500mA for input voltages of 4.75V to 11V, and has
700mA typical output currents. The MAX748A/
MAX763A operate from an input down to 3V (the upper
limit of undervoltage lockout), but with some reduction
in output voltage and maximum output current.
Inductor Selection
The MAX748A/MAX763A require no inductor design
because they are tested in-circuit, and are guaranteed
to deliver the power specified in the
Electrical
Characteristics with high efficiency using a single
22µH inductor. The 22µH inductor’s incremental satu-
ration current rating should be greater than 1A for
500mA load operation. Table 3 lists inductor types and
suppliers for various applications. The surface-mount
inductors have nearly equivalent efficiencies to the
larger through-hole inductors.
Output Filter Capacitor Selection
The primary criterion for selecting the output filter
capacitor is low effective series resistance (ESR). The
product of the inductor-current variation and the output
capacitor’s ESR determines the amplitude of the saw-
tooth ripple seen on the output voltage. Minimize the
output filter capacitor’s ESR to maintain AC stability.
Table 2. Component Table for
Wide Temperature Applications
C1(µF) C2(µF) C3(µF) C4(µF) C5(pF) C6(pF) L1(µH)
Through-
Hole
0.047 1.0 150* 220* 330 1000 22
SO 0.047 1.0 68** 100*** 330 1000 22
* Sanyo OS-CON Series (very low ESR)
** 16V or greater maximum voltage rating.
*** 6.3V or greater maximum voltage rating.
L2
25 µ H
FILTER
OUTPUT
OUTPUT
C7
2.2 µ F
OPTIONAL 21kHz LOWPASS OUTPUT FILTER
MAX748A
MAX763A
SHDN
D1
1N5817
C5
330pF
L1
22
µ H
C4
150
µ F
OUTPUT
3.3V
C2
1.0
µ F
C3
150
µ F
8
1
LX
V+
OUT
CC SS
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
REF
Figure 3. Standard 3.3V Step-Down Application Circuit Using Through-Hole Components (commercial temperture range)