sp7600 Exar Corporation, sp7600 Datasheet - Page 9

• Manufacturer Part Number: sp7600
• Description: 2amp, 29v Buck Converter For Led Driver Applications
• Manufacturer: Exar Corporation

Schottky Rectifier selection
Select the Schottky D1 for Voltage rating V
voltage rating for 12V and 24V applications is 30V and 40V respectively. Current rating can be
calculated from:
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
R2 is part of SP7600 loop compensation network. Use a 30k R2 for Vin of 20V and larger. Use R2 of
60K for Vin less than 20V.
Capacitor C5
This is the decoupling capacitor for the power supply for the internal driver. Use a 0.1uF and place
as closely to VDR and SVIN pins as possible.
Design example - Design a drive circuit for a 1.5A LED with a 12V input voltage. Nominal LED
voltage is 3.6V.
1. Calculate RFB from equation (1):
A standard value of 0.12ohm 0805 size is selected.
An inductor of 4.7uH, rated at 3Arms and 3A Isat can be used.
3. Select input capacitor
A 10uF CIN (C1) is needed as shown in table 1. From (4) the ripple current rating of CIN is a fraction
of 1.5A. A 10uF, 16V ceramic capacitor easily meets this requirement and offers low ESR and ESL.
If
RFB
L
2. Calculate inductor value L1 from (3):
1
12/17/07 rev: A
EXAR RESERVES THE RIGHT TO MAKE CHANGES TO THIS DATASHEET. CALL FOR UPDATES: 1-510-668-7000
≥
=
12
=
⎛
⎜
⎝
3
V
1
1
0
6 .
5 .
2 .
−
V
×
V
Vin
A
1
Vo
×
3 .
=
(
12
MHz
⎞
⎟
⎠
. 0
×
V
13
Io
−
Ohm
×
3
…………………………… (5)
. 0
6 .
45
V
)
A
=
4
3 .
uH
R
and current rating I
SP7600
9
f
. Recommended schottky diode
©Copyright 2007 Exar Corporation
SP7600