sp7600 Exar Corporation, sp7600 Datasheet - Page 8

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

Overcurrent Programming
Resistor Rs can be used to program Overcurrent Protection (OCP). Use the following equation for
calculating the Rs value.
Where Iocp is the programmed overcurrent and is generally set 50% above nominal output current,
and Rds(on) =
Maximum value of Rs that can be used for programming OCP is 3k.
Inductor Selection
Select the inductor L1 for inductance, Irms and Isat. Calculate inductance from
Where:
Vin is converter input voltage
Vo is converter output voltage. Since voltage across Rset is small, Vo approximately equals Vf (for a
string of series connected LEDs Vo equals total Vf)
Inductor Isat and Irms must allow sufficient safeguard over output current Io. As a rule of thumb
these parameters should be 50% higher than Io. Where high efficiency is required a low DCR
inductor should be used.
Input capacitor selection
Select the input capacitor for capacitance and ripple current rating. Use the capacitances listed in
table 1 as a starting point and if needed increase Cin. Calculate the ripple current requirement from:
Where D is converter duty cycle:
Ceramic capacitors are recommended for input filtering due to their low Equivalent Series
Resistance (ESR), Equivalent Series inductance (ESL) and small form factor.
Rs
L
I Δ
Irip
D =
0.71 to 1.2
=
L
12/17/07 rev: A
EXAR RESERVES THE RIGHT TO MAKE CHANGES TO THIS DATASHEET. CALL FOR UPDATES: 1-510-668-7000
Table1- Cin selection
=
is inductor current ripple (nominally set to 30% of Io)
Vo
Io (A)
=
Vin
< 0.7
> 1.2
Vo
Vin
. 0
Io
×
35
×
(
×
Vin
V
f
−
×
D
−
95mohms.
(
1
Δ
(
1
Vo
5 .
Cin (uF)
I
2 x 4.7
−
L
×
)
2.2
4.7
D
………………………. (3)
. 1
)
33
05
………………. (4)
uA
×
Iocp
×
Rds
(
on
)
)
………… (2)
SP7600
8
©Copyright 2007 Exar Corporation
SP7600