DC-DC Converter
QHL750 Series
Application Notes
Input Fusing and Safety Considerations
The QHL750 series converters have no internal fuse. In
order to achieve maximum safety and system
protection, always use an input line fuse. We
recommended a 30 A fast acting fuse. It is also
recommended that the circuit has a transient voltage
suppressor diode (TVS) across the input terminals to
protect the unit against surge or spike voltages and
input reverse voltage (as shown). A suitable part would
be SMCJ78A.
Fuse
2
1
5-7
+Vin
-Vin
+Vout
+Sense
Trim
Trim up Ru
12
13
+
-
TVS
Load
Vin
Trim down Rd
11
-Sense
-Vout
8-10
Output Voltage Adjustment
The Trim input permits the user to adjust the output
voltage up by 10% or down by 40%. This is
accomplished by connecting an external resistor
between the Trim pin and either the Vout (+) pin or the
Vout(-) pin (COM pin).
To Trim Down
To Trim Up
Connecting an external resistor (Rd) between the Trim pin and the
-Sense pin decreases the output voltage. The following equations
determine the required external resistor value to obtain a percentage
output voltage change of Δ%.
Connecting an external resistor (Ru) between the +Vout pin and the
+Sense pin increases the output voltage. The following equations
determine the required external resistor value to obtain a
percentage output voltage change of Δ%.
12 V
15 V
24 V
Rtrim_down (kΩ)
21.26
20.80
20.34
19.89
19.43
18.97
18.51
18.06
17.60
28 V
48 V
12 V
15 V
24 V
Rtrim_up (kΩ)
22.17
28 V
48 V
Trim Down
%
Trim Up %
1
10.63
10.40
10.17
9.943
9.714
9.486
9.257
9.029
8.800
8.571
8.343
8.114
7.886
7.657
7.429
7.200
6.971
6.743
6.514
6.286
6.057
5.829
5.600
5.371
5.143
4.914
4.686
4.457
4.229
4.000
3.771
3.543
3.314
3.086
2.857
2.629
2.400
2.171
1.943
1.714
13.29
13.00
12.71
12.43
12.14
11.86
11.57
11.29
11.00
10.71
10.43
10.14
9.857
9.571
9.286
9.000
8.714
8.429
8.143
7.857
7.571
7.286
7.000
6.714
6.429
6.143
5.857
5.571
5.286
5.000
4.714
4.429
4.143
3.857
3.571
3.286
3.000
2.714
2.429
2.143
24.80
24.27
23.73
23.20
22.67
22.13
21.60
21.07
20.53
20.00
19.47
18.93
18.40
17.87
42.51
41.60
40.69
39.77
38.86
37.94
37.03
36.11
35.20
34.29
33.37
32.46
31.54
30.63
29.71
28.80
27.89
26.97
26.06
25.14
24.23
23.31
22.40
21.49
20.57
19.66
18.74
17.83
16.91
16.00
15.09
14.17
13.26
12.34
11.43
10.51
9.600
8.686
7.771
6.857
1
2
11.09
11.31
11.54
11.77
12.00
12.23
12.46
12.69
12.91
13.14
13.86
14.14
14.43
14.71
15.00
15.29
15.57
15.86
16.14
16.43
25.87
26.40
26.93
27.47
28.00
28.53
29.07
29.60
30.13
30.67
44.34
45.26
46.17
47.09
48.00
48.91
49.83
50.74
51.66
52.57
2
22.63
3
3
23.09
4
4
23.54
5
5
24.00
6
6
24.46
7
7
24.91
8
8
25.37
9
9
25.83
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
17.14
10
26.29
16.69
16.23
15.77
15.31
14.86
14.40
13.94
13.49
13.03
12.57
12.11
Power Good Signal
17.33
Open collector output, referenced to -Sense Pin. Output is pulled low if
DC/DC is operating normally and floating if DC/DC is remotely turned
off or operating abnormally.
16.80
16.27
15.73
15.20
14.67
14.13
13.60
13.07
12.53
12.00
11.47
10.93
10.40
9.867
9.333
8.800
8.267
7.733
7.200
6.667
6.133
5.600
5.067
4.533
4.000
11.66
11.20
10.74
10.29
9.829
9.371
8.914
8.457
8.000
7.543
7.086
6.629
6.171
5.714
5.257
4.800
4.343
3.886
3.429
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