5V, Step-Down,
Current-Mode PWM DC-DC Converters
MAX730A/MAX738A/MAX744A
Table 1. Typical Soft-Start Times
MAX730A CIRCUIT CONDITIONS
R1 (kΩ)
510
510
510
510
510
510
510
None
None
None
None
None
None
V+ (V)
6
9
11
9
9
9
9
6
9
11
9
9
9
I
OUT
(mA)
0
0
0
150
300
150
150
0
0
0
150
150
150
C4 (µF)
100
100
100
100
100
390
680
100
100
100
100
390
680
C1 = 0.01
2
1
1
1
1
3
4
16
10
8
34
39
40
SOFT-START TIME (ms) vs. C1 (µF)
C1 = 0.047
6
4
2
4
5
6
6
34
22
18
134
147
152
C1 = 0.1
11
6
4
8
9
9
9
51
34
28
270
280
285
C1 = 0.47
28
15
11
21
27
23
24
125
82
66
1263
1275
1280
MAX738A/MAX744A CIRCUIT CONDITIONS
R1 (kΩ)
510
510
510
510
510
None
None
None
None
V+ (V)
7
12
16
12
12
7
12
16
12
I
OUT
(mA)
0
0
0
300
750
0
0
0
300
C4 (µF)
100
100
100
100
100
100
100
100
100
C1 = 0.01
1
1
1
1
1
12
7
6
27
SOFT-START TIME (ms) vs. C1 (µF)
C1 = 0.047
4
2
1
3
5
27
16
13
112
C1 = 0.1
6
3
2
5
8
40
25
20
215
C1 = 0.47
18
8
6
3
21
100
54
68
1114
Internal Reference
The +1.23V bandgap reference supplies up to 100µA
at REF. Connect a 0.01µF bypass capacitor from REF
to GND.
____________Applications Information
Figure 3 shows the standard 5V step-down application
circuits. Table 3 lists the components for the desired
operating temperature range. These circuits are useful
in systems that require high current at high efficiency
and are powered by an unregulated supply, such as a
battery or wall-plug AC-DC transformer. These circuits
operate over the entire line, load, and temperature
ranges using the single set of component values shown
in Figure 3 and listed in Table 3.
Oscillator
The internal oscillator of the MAX730A typically oper-
ates at 170kHz (160kHz for the MAX738A and 185kHz
for the MAX744A). The MAX744A is guaranteed to
operate at a minimum of 159kHz and a maximum of
212.5kHz over the operating voltage and temperature
range, making it ideal for use in portable communica-
tions systems. The
Typical Operating Characteristics
graphs indicate oscillator frequency stability over tem-
perature and supply voltage.
Inductor Selection
The MAX730A/MAX738A/MAX744A 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
9
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