MAX750A/MAX758A
Adjustable, Step-Down,
Current-Mode PWM Regulators
10 ______________________________________________________________________________________
Table 1. Typical Soft-Start Times
MAX750A CIRCUIT CONDITIONS SOFT-START TIME (ms) vs. C1 (µF)
R1 (kΩ) V+ (V) I
OUT
(mA) C4 (µF) C1 = 0.01 C1 = 0.047 C1 = 0.1 C1 = 0.47
510 6 0 100 2 6 11 28
510 9 0 100 1 4 6 15
510 11 0 100 1 2 4 11
510 9 150 100 1 4 8 21
510 9 300 100 1 5 9 27
510 9 150 390 3 6 9 23
510 9 150 680 4 6 9 24
None 6 0 100 16 34 51 125
None 9 0 100 10 22 34 82
None 11 0 100 8 18 28 66
None 9 150 100 34 134 270 1263
None 9 150 390 39 147 280 1275
None 9 150 680 40 152 285 1280
MAX750A CIRCUIT CONDITIONS SOFT-START TIME (ms) vs. C1 (µF)
R1 (kΩ) V+ (V) I
OUT
(mA) C4 (µF) C1 = 0.01 C1 = 0.047 C1 = 0.1 C1 = 0.47
510 7 0 100 1 4 6 18
510 12 0 100 1 2 3 8
510 16 0 100 1 1 2 6
510 12 300 100 1 3 5 3
510 12 750 100 1 5 8 21
None 7 0 100 12 27 40 100
None 12 0 100 7 16 25 54
None 16 0 100 6 13 20 68
None 12 300 100 27 112 215 1114
as the temperature falls, and excessive ESR is the most
likely cause of trouble at temperatures below 0°C.
Sanyo OS-CON series through-hole and surface-mount
tantalum capacitors exhibit low ESR at temperatures
below 0°C. Refer to Table 2 for recommended capaci-
tor values and suggested capacitor suppliers.
Other Components
The catch diode should be a Schottky or high-speed
silicon rectifier with a peak current rating of about 1A
for full-load (750mA) operation. The 1N5817 is a good
choice. The 330pF outer-loop compensation capacitor
provides the widest input voltage range and best tran-
sient characteristics. For low-current applications, the
510kΩ resistor may be omitted (see the Maximum
Output Current vs. Supply Voltage, R1 Removed graph
in the
Typical Operating Characteristics
).
Printed Circuit Layouts
A good layout is essential for clean, stable operation.
The layouts and component placement diagrams given
in Figures 4-7 have been successfully tested over a
wide range of operating conditions. The 1µF bypass
capacitor (C2) must be positioned as close to the V+
