lt3517 Linear Technology Corporation, lt3517 Datasheet - Page 10

• Manufacturer Part Number: lt3517
• Description: Full-featured Led Driver With 1.5a Switch Current
• Manufacturer: Linear Technology Corporation

LT3517
Table 1 provides some recommended inductor vendors.
Table 1. Inductor Manufacturers
VENDOR
Sumida
Toko
Cooper
Vishay
Input Capacitor Selection
For proper operation, it is necessary to place a bypass
capacitor to GND close to the V
1μF or greater capacitor with low ESR should be used. A
ceramic capacitor is usually the best choice.
In the buck mode confi guration, the capacitor at the input
to the power converter has large pulsed currents due to
the current returned though the Schottky diode when the
switch is off. For best reliability, this capacitor should have
low ESR and ESL and have an adequate ripple current
rating. The RMS input current is:
where D is the switch duty cycle. A 2.2μF ceramic type
capacitor is usually suffi cient.
Output Capacitor Selection
The selection of output capacitor depends on the load
and converter confi guration, i.e., step-up or step-down.
For LED applications, the equivalent resistance of the LED
is typically low, and the output fi lter capacitor should be
sized to attenuate the current ripple.
APPLICATIONS INFORMATION
10
I
IN(RMS)
= I
LED
• (1– D) • D
PHONE
[phone]
[phone]
[phone]
[phone]
IN
pin of the LT3517. A
WEB
www.sumida.com
www.toko.com
www.cooperet.com
www.vishay.com
(8)
To achieve the same LED ripple current, the required fi lter
capacitor value is larger in the boost and buck-boost mode
applications than that in the buck mode applications. For
LED buck mode applications, a 1μF ceramic capacitor
is usually suffi cient. For the LED boost and buck-boost
mode applications, a 2.2μF ceramic capacitor is usually
suffi cient. Very high performance PWM dimming appli-
cations may require a larger capacitor value to support
the LED voltage during PWM transitions.
Use only ceramic capacitors with X7R, X5R or better dielec-
tric as they are best for temperature and DC bias stability
of the capacitor value. All ceramic capacitors exhibit loss
of capacitance value with increasing DC voltage bias, so
it may be necessary to choose a higher value capacitor
to get the required capacitance at the operation voltage.
Always check that the voltage rating of the capacitor is
suffi cient. Table 2 shows some recommended capacitor
vendors.
Table 2. Ceramic Capacitor Manufacturers
VENDOR
Taiyo Yuden
AVX
Murata
TDK
Loop Compensation
The LT3517 uses an internal transconductance error
amplifi er whose V
The external inductor, output capacitor, and the compen-
sation resistor and capacitor determine the loop stability.
The inductor and output capacitor are chosen based on
C
PHONE
[phone]
[phone]
[phone]
[phone]
output compensates the control loop.
WEB
www.t-yuden.com
www.avxcorp.com
www.murata.com
www.tdk.com
3517f