a8447 Allegro MicroSystems, Inc., a8447 Datasheet - Page 7

• Manufacturer Part Number: a8447
• Description: A8447 High-voltage Step-down Regulator
• Manufacturer: Allegro MicroSystems, Inc.

A8447
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
to V
defined by the curve in the RTEST Value Selection graph below.
Note. The curve represents the minimum RTSET value. When
calculating R
tolerance should also be considered, so that under all operating
RTSET Value Selection
OUT
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
0
ratio, R
15
TSET
TSET
30
, be sure to use V
Recommended Components
L1
D1
CBYP
CBOOT
CIN
COUT
R1
R2
Component
45
must be greater than or equal to the value
60
Selection Graph
75
90
R
TSET
IN
105
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
Rubycon ZL 220 μF 25 V Low ESR
Panasonic FM 220 μF 25 V Low ESR (Option 2)
(max) / V
(k )
120
135
2.55 kΩ
2 kΩ
1.8 V
OUT
150
(min). Resistor
165
180
Description
195
IN
210
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
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.
2.87 kΩ
0.910 kΩ
3.3 V
V
(V)
42
42
42
24
24
24
12
12
12
V
(Option 1)
IN
OUT
Recommended Common Values
RCH1216BNP-680K
NSQ03A06
Generic
Generic
Generic
Generic
PXA16VC221MJ12TP
25ZL220M8x11.5
EEUFM1E221
V
(V)
5
3.3
1.8
5
3.3
1.8
5
3.3
1.8
OUT
115 Northeast Cutoff, Box 15036
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 [phone]
www.allegromicro.com
Part Number
6.3 kΩ
1.2 kΩ
5 V
V
12.7
23.3
13.3
IN
8.4
4.8
7.3
2.4
3.6
6.6
/ V
OUT
TSET
R
TSET
37.4
54.9
90.9
20.0
32.4
54.9
13.7
30.1
(kΩ)
7.68
values
Value
7