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SP7601_EB Datasheet(PDF) 7 Page - Exar Corporation

Part # SP7601_EB
Description  29V High Voltage LED Driver
PDF  9 Pages
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Manufacturer  EXAR [Exar Corporation]
Direct Link  http://www.exar.com
Logo EXAR - Exar Corporation

SP7601_EB Datasheet(HTML) 7 Page - Exar Corporation

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Sept25-07 Rev D
SP7601 29V High Voltage LED Driver
© 2007 Exar Corporation
Page 7
Note that in applications where duty cycle is
low, Schottky losses comprise a larger
percentage of converter losses. In order to
improve the efficiency in these applications
choose a Schottky that meets the calculated
current rating and has a lower VF.
Feedback resistor RFB
RFB is part of the SP7601 loop compensation
network. Use a 30kΩ RFB for VIN of 20V and
larger. Use RFB of 60KΩ for VIN less than 20V.
Capacitor C1
This is the decoupling capacitor for the power
supply to the internal driver. Use a 0.1μF
capacitor and place it as closely to the VDR
and VIN pins as possible.
Design example- Design a drive circuit for a
0.35A LED with a 12V input voltage. Nominal
LED voltage is 3.3V.
1. Calculate RSET from equation (1):
Ohm
A
V
Rset
57
.
0
35
.
0
2
.
0
=
=
2. Calculate inductor value L1 from (3):
()
uH
A
MHz
V
V
V
V
L
57
035
.
0
2
.
1
12
3
.
3
12
3
.
3
1
=
×
×
−
×
=
A COILTRONICS LD-560 inductor rated at
56μH, 0.5ARMS and 0.57ASAT can be used.
3. Select input capacitor:
A 2.2μF CIN is needed as shown in table 1.
From equation (4), the ripple current rating of
CIN is a fraction of 0.35A. A 2.2uF, 16V ceramic
capacitor easily meets this requirement and
offers low ESR and ESL.
4. RDS(ON) is calculated from equation (5):
Ohm
A
mV
ON
RDS
42
.
0
35
.
0
05
.
1
5
.
1
5
.
1
350
)
(
=
⎟
⎠
⎞
⎜
⎝
⎛
×
×
×
≤
BVDSS should be at least 30V and QGD should
be less than 10nC. An ANPEC APM4427K with
RDS(ON)=0.14
Ohm,
BVDSS=30V
and
QGD=1.1nC can be used.
5. Schottky current rating
IF can be calculated
from (6):
A
A
V
V
If
3
.
0
35
.
0
12
3
.
3
1
=
×
⎟
⎠
⎞
⎜
⎝
⎛ −
≥
Voltage rating should be 30V. An SL03 rated at
30V/1.1A or equivalent can be used.
6. Calculate Rs from equation (2):
(
)
Ohm
uA
Ohm
A
V
Rs
7098
33
14
.
0
525
.
0
05
.
1
5
.
1
35
.
0
=
×
×
×
−
=
Use the maximum recommended Rs of 3kΩ
since the calculated value is larger.
Vin
FB
VDR
GND
SP7601
LX
Gate
LED1
1
60k
3
6
4
5
2
0.57 Ohm
C1
0.1uF
3k
1N4148
VIN
Dimming and
Power sequencing
GND
12V
2.2uF
D1, Vishay SL03
30V, 1.1A
L1, COILTRONICS LD1-560
56uH, 0.5Arms
M1, ANPEC APM4427K
30V, 0.14 Ohm
Figure 4. Circuit schematic for design example
GENERAL OVERVIEW



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