A4447
High Voltage Step Down Regulator
conditions the resistance on the TSET pin remains as close to the
curve as possible.
The RTEST Selection table shows recommended R
TSET
values
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.
Recommended Common Values
V
IN
(V)
42
42
42
42
24
24
24
24
12
12
12
12
V
OUT
(V)
5
3.3
1.8
1.5
5
3.3
1.8
1.5
5
3.3
1.8
1.5
V
IN
/ V
OUT
8.4
12.7
23.3
28
4.8
7.3
13.3
16
2.4
3.6
6.6
8
R
TSET
Value
(kΩ)
37.4
54.9
90.9
105
20.0
32.4
54.9
66.5
7.68
13.7
30.1
37.4
RTSET Selection.
Correct selection of RTSET values will
ensure that minimum on time of the switcher is not violated
and prevent the switcher from cycle skipping. For a given V
IN
to V
OUT
ratio, R
TSET
must be greater than or equal to the value
defined by the curve in the RTEST Value Selection graph below.
Note. The curve represents the minimum RTSET value. When
calculating R
TSET
, be sure to use V
IN
(max) / V
OUT
(min). Resistor
tolerance should also be considered, so that under all operating
RTSET Value Selection*
Selection Graph
70
65
60
55
50
V
IN
/ V
OUT
45
40
35
30
25
20
15
10
5
0
0
15
30
45
60
75
90
105
120
135
150
165
180
195
210
R
TSET
(kΩ)
*The R
TSET
resistor should be not smaller than 7.65 kΩ ±2% to prevent very short off-times from violating the minimum on-time of
the switcher.
Recommended Components
Component
L1
D1
CBYP
CBOOT
CIN
Sumida 68
μH
NIEC Schottky Barrier Diode 60 V TO-252AA
Ceramic X7A 0.22
μF
100 V
Ceramic X7A 0.01
μF
100 V
Electrolytic 100
μF
50 V; must be able to handle worst case ripple curent
Ceramic X7A 0.22
μF
50 V
United Chemi-Con PXA 220
μF
16 V Low ESR
COUT
Rubycon ZL 220
μF
25 V Low ESR
(Option 1)
V
OUT
1.5 V
R1
R2
1.30 kΩ
1.47 kΩ
1.8 V
2.55 kΩ
2.00 kΩ
3.3 V
2.87 kΩ
0.910 kΩ
5V
6.34 kΩ
1.20 kΩ
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Description
Part Number
RCH1216BNP-680K
NSQ03A06
Generic
Generic
Generic
Generic
PXA16VC221MJ12TP
25ZL220M8x11.5
EEUFM1E221
Panasonic FM 220
μF
25 V Low ESR (Option 2)
7
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
