Datasheet

LT3695 Series
16
3695fa
Low ESR is important, so choose one that is intended for
use in switching regulators. The ESR should be specifi ed
by the supplier, and should be 0.05 or less. Such a
capacitor will be larger than a ceramic capacitor and will
have a larger capacitance, because the capacitor must be
large to achieve low ESR. Table 3 lists several capacitor
vendors.
Diode Selection
The catch diode (D1 from Block Diagram) conducts cur-
rent only during switch off time. Average forward current
in normal operation can be calculated from:
I
D(AVG)
= I
OUT
• (1 – DC)
where DC is the duty cycle. The only reason to consider a
diode with larger current rating than necessary for nominal
operation is for the case of shorted or overloaded output
conditions. For the worst case of shorted output the diode
average current will then increase to a value that depends
on the following internal parameters: switch current limit,
catch diode (DA pin) current threshold and minimum
on-time. The worst case (taking maximum values for the
above mentioned parameters) is given by the following
expression:
IA
V
L
ns
D AVG M AX
IN
()
••=+2
1
2
150
Peak reverse voltage is equal to the regulator input voltage
if it is below the overvoltage protection threshold. This
feature keeps the switch off for V
IN
> V
OVLO
(39.9V maxi-
mum). For inputs up to the maximum operating voltage
of 36V, use a diode with a reverse voltage rating greater
APPLICATIONS INFORMATION
Table 3. Capacitor Vendors
VENDOR PHONE URL PART SERIES COMMANDS
Panasonic [phone] www.panasonic.com Ceramic, Polymer, Tantalum EEF Series
Kemet [phone] www.kemet.com Ceramic, Tantalum T494, T495
Sanyo [phone] www.sanyovideo.com Ceramic, Polymer, Tantalum POSCAP
Murata [phone] www.murata.com Ceramic
AVX www.avxcorp.com Ceramic, Tantalum TPS Series
Taiyo Yuden [phone] www.taiyo-yuden.com Ceramic
Table 4. Schottky Diodes
PART NUMBER V
R
(V) I
AVE
(A) V
F
at 1A (mV) V
F
at 2A (mV)
On-Semiconducor
MBR0520L 20 0.5
MBR0540 40 0.5 620
MBRM120E 20 1 530 595
MBRM140 40 1 550
Diodes Inc.
B0530W 30 0.5
B0540W 40 0.5 620
B120 20 1 500
B130 30 1 500
B140 40 1 500
B220 20 2 500
B230 30 2 500
B140HB 40 1 530
DFLS240L 40 2 500
DFLS140 40 1.1 510
B240 40 2 500
Central Semiconductor
CMSH1-40M 40 1 500
CMSH1-40ML 40 1 400
CMSH2-40M 40 2 550
CMSH2-40L 40 2 400
CMSH2-40 40 2 500
than the input voltage. If transients at the input of up to
60V are expected, use a diode with a reverse voltage rat-
ing of 40V. Table 4 lists several Schottky diodes and their
manufacturers. If operating at high ambient temperatures,
consider using a Schottky with low reverse leakage.