LT3517
APPLICATIONS INFORMATION
Table 1 provides some recommended inductor vendors.
Table 1. Inductor Manufacturers
ToachievethesameLEDripplecurrent, therequiredfilter
capacitorvalueislargerintheboostandbuck-boostmode
applications than that in the buck mode applications. For
LED buck mode applications, a 1μF ceramic capacitor
is usually sufficient. For the LED boost and buck-boost
mode applications, a 2.2μF ceramic capacitor is usually
sufficient. Very high performance PWM dimming appli-
cations may require a larger capacitor value to support
the LED voltage during PWM transitions.
VENDOR
Sumida
Toko
PHONE
WEB
(408) 321-9660
(408) 432-8281
(561) 998-4100
(402) 563-6866
www.sumida.com
www.toko.com
www.cooperet.com
www.vishay.com
Cooper
Vishay
Input Capacitor Selection
UseonlyceramiccapacitorswithX7R,X5Rorbetterdielec-
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
sufficient. Table 2 shows some recommended capacitor
vendors.
For proper operation, it is necessary to place a bypass
capacitor to GND close to the V pin of the LT3517. A
IN
1μF or greater capacitor with low ESR should be used. A
ceramic capacitor is usually the best choice.
In the buck mode configuration, the capacitor at the input
to the power converter has large pulsed currents due to
the current returned though the Schottky diode when the
switchisoff. Forbestreliability, thiscapacitorshouldhave
low ESR and ESL and have an adequate ripple current
rating. The RMS input current is:
Table 2. Ceramic Capacitor Manufacturers
VENDOR
Taiyo Yuden
AVX
PHONE
WEB
(408) 573-4150
(843) 448-9411
(770) 436-1300
(847) 803-6100
www.t-yuden.com
www.avxcorp.com
www.murata.com
www.tdk.com
IIN(RMS) = ILED • (1–D)•D
(8)
where D is the switch duty cycle. A 2.2μF ceramic type
capacitor is usually sufficient.
Murata
TDK
Output Capacitor Selection
Loop Compensation
The selection of output capacitor depends on the load
and converter configuration, i.e., step-up or step-down.
For LED applications, the equivalent resistance of the LED
is typically low, and the output filter capacitor should be
sized to attenuate the current ripple.
The LT3517 uses an internal transconductance error
amplifier whose V output compensates the control loop.
C
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
3517fa
10
Linear [ Linear ]
