High Voltage Step Down Regulator

A4447

7

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

RTSET Selection. Correct selection of RTSET values will

ensure that minimum on time of the switcher is not violated

defined by the curve in the RTEST Value Selection graph below.

Note. The curve represents the minimum RTSET value. When

tolerance should also be considered, so that under all operating

conditions the resistance on the TSET pin remains as close to the

curve as possible.

based on common operating conditions. For other operating con-

ditions, refer to the RTSET Value Selection graph.

FB Resistor Selection. The impedance of the FB network

should be kept low to improve noise immunity. Large value resis-

tors can pick up noise generated by the inductor, which can affect

voltage regulation of the switcher.

RTSET Value Selection*

Selection Graph

Recommended Common Values

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

0

15

30

45

60

75

90

105

120

135

150

165

180

195

210

(V)

(V)

(kΩ)

42

5

8.4

37.4

42

3.3

12.7

54.9

42

1.8

23.3

90.9

42

1.5

28

105

24

5

4.8

20.0

24

3.3

7.3

32.4

24

1.8

13.3

54.9

24

1.5

16

66.5

12

5

2.4

7.68

12

3.3

3.6

13.7

12

1.8

6.6

30.1

12

1.5

8

37.4

the switcher.

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R1

1.30 kΩ

2.55 kΩ

2.87 kΩ

6.34 kΩ

R2

1.47 kΩ

2.00 kΩ

0.910 kΩ

1.20 kΩ