Datasheet

7
LT3491
3491fa
APPLICATIO S I FOR ATIO
WUUU
INDUCTOR SELECTION
A 10µH inductor is recommended for most LT3491 appli-
cations. Although small size and high efficiency are major
concerns, the inductor should have low core losses at
2.3MHz and low DCR (copper wire resistance). Some
small inductors in this category are listed in Table 1. The
efficiency comparison of different inductors is shown in
Figure 3.
Table 1. Recommended Inductors
CURRENT
L DCR RATING
PART (µH) (Ω) (mA) VENDOR
LQH32CN100K53 10 0.3 450 Murata
LQH2MCN100K02 10 1.2 225 www.murata.com
SD3112-100 10 0.446 550 Cooper
www.cooperet.com
1001AS-100M 10 0.48 460 Toko
(TYPE D312C) www.toko.com
CDRH2D11 10 0.5375 280 Sumida
CDRH2D14 10 0.294 700 www.sumida.com
Table 2 shows a list of several ceramic capacitor manufac-
turers. Consult the manufacturers for detailed information
on their entire selection of ceramic parts.
Table 2. Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden [phone]
www.t-yuden.com
AVX [phone]
www.avxcorp.com
Murata [phone]
www.murata.com
OVERVOLTAGE PROTECTION
The LT3491 has an internal open-circuit protection circuit.
In the cases of output open circuit, when the LEDs are
disconnected from the circuit or the LEDs fail open circuit,
V
CAP
is clamped at 27V (typ). The LT3491 will then switch
at a very low frequency to minimize input current. The V
CAP
and input current during output open circuit are shown in
the Typical Performance Characteristics. Figure 4 shows
the transient response when the LEDs are disconnected.
Figure 3. Efficiency Comparison of Different Inductors
CAPACITOR SELECTION
The small size of ceramic capacitors make them ideal for
LT3491 applications. Use only X5R and X7R types be-
cause they retain their capacitance over wider temperature
ranges than other types such as Y5V or Z5U. A 1µF input
capacitor and a 1µF output capacitor are sufficient for
most applications.
INRUSH CURRENT
The LT3491 has a built-in Schottky diode. When supply
voltage is applied to the V
IN
pin, an inrush current flows
through the inductor and the Schottky diode and charges
up the CAP voltage. The Schottky diode inside the LT3491
can sustain a maximum current of 1A.
Figure 4. Output Open-Circuit Waveform
V
CAP
10V/DIV
I
L
200mA/DIV
V
IN
= 3.6V
CIRCUIT OF
FRONT PAGE
APPLICATION
LEDs DISCONNECTED
AT THIS INSTANT
500µs/DIV
3491 F04
LED CURRENT (mA)
0
30
EFFICIENCY (%)
35
45
50
55
80
65
5
10
3491 F03
40
70
75
60
15
20
MURATA LQH2MCN100K02
MURATA LQH32CN100K53
TOKO 10001AS-100M
SUMIDA CDRH2D11
SUMIDA CDRH2D14
V
IN
= 3.6V
4 LEDs
FRONT PAGE
APPLICATION CIRCUIT