A4447 Просмотр технического описания (PDF) - Allegro MicroSystems
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A4447 Datasheet PDF : 9 Pages
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A4447
High Voltage Step Down Regulator
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 VIN
to VOUT ratio, RTSET 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 RTSET , be sure to use VIN(max) / VOUT(min). Resistor
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.
The RTEST Selection table shows recommended RTSET 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.
RTSET Value Selection*
Selection Graph
Recommended Common Values
70
65
60
55
50
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
RTSET (kΩ)
VIN
(V)
42
42
42
42
24
24
24
24
12
12
12
12
VOUT
(V)
5
3.3
1.8
1.5
5
3.3
1.8
1.5
5
3.3
1.8
1.5
VIN / VOUT
8.4
12.7
23.3
28
4.8
7.3
13.3
16
2.4
3.6
6.6
8
RTSET 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
*The RTSET 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
Description
L1
Sumida 68 μH
D1
NIEC Schottky Barrier Diode 60 V TO-252AA
CBYP
Ceramic X7A 0.22 μF 100 V
CBOOT
Ceramic X7A 0.01 μF 100 V
Electrolytic 100 μF 50 V; must be able to handle worst case ripple curent
CIN
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)
Panasonic FM 220 μF 25 V Low ESR (Option 2)
1.5 V
1.8 V
VOUT
3.3 V
R1
1.30 kΩ
2.55 kΩ
2.87 kΩ
R2
1.47 kΩ
2.00 kΩ
0.910 kΩ
Part Number
RCH1216BNP-680K
NSQ03A06
Generic
Generic
Generic
Generic
PXA16VC221MJ12TP
25ZL220M8x11.5
EEUFM1E221
5V
6.34 kΩ
1.20 kΩ
Allegro MicroSystems, Inc.
7
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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