LT3497
APPLICATIONS INFORMATION
DUTY CYCLE
The duty cycle for a step-up converter is given by:
V
+
V –V
D
=
OUT D IN
V
OUT
+
V
D
– V
CESAT
EFFICIENCY (%)
80
75
70
65
60
55
50
45
0
5
10
LED CURRENT (mA)
15
20
3497 F03
15µH MURATA LQH32CN150K53
15µH MURATA LQH2MCN150K02
15µH COOPER SD3112-150
15µH TOKO 1001AS-150M TYPE D312C
15µH SUMIDA CDRH2D11/HP
where:
V
OUT
= Output voltage
V
D
= Schottky forward voltage drop
V
CESAT
= Saturation voltage of the switch
V
IN
= Input voltage
The maximum duty cycle achievable for LT3497 is 88%
when running at 2.3MHz switching frequency. Always
ensure that the converter is not duty-cycle limited when
powering the LEDs at a given frequency.
INDUCTOR SELECTION
A 15µH inductor is recommended for most LT3497 ap-
plications. 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
inductors in this category with small size are listed in
Table 1. The efficiency comparison of different inductors
is shown in Figure 3.
Table 1: Recommended Inductors
L
(µH)
15
15
10
10
15
15
15
MAX
DCR
(Ω)
0.58
1.6
0.3
1.2
0.654
0.80
0.739
CURRENT
RATING
(mA)
300
200
450
225
440
360
410
Figure 3. Efficiency Comparison of Different Inductors
and a 1µF output capacitor are sufficient for most applica-
tions. Table 2 shows a list of several ceramic capacitor
manufacturers. Consult the manufacturers for detailed
information on their entire selection of ceramic parts.
Table 2: Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden
AVX
Murata
(800) 368-2496
www.t-yuden.com
(803) 448-9411
www.avxcorp.com
(714) 852-2001
www.murata.com
PART
LQH32CN150K53
LQH2MCN150K02
LQH32CN100K53
LQH2MCN100K02
SD3112-150
1001AS-150M
(TYPE D312C)
CDRH2D11/HP
VENDOR
Murata
www.murata.com
OVERVOLTAGE PROTECTION
The LT3497 has an internal open-circuit protection
circuit for both converters. In the cases of output open
circuit, when the LEDs are disconnected from the circuit
or the LEDs fail open circuit, the converter V
CAP
voltage
is clamped at 32V (typ). Figure 4a shows the transient
response of the front page application step-up converter
with LED1 disconnected. With LED1 disconnected, the
converter starts switching at the peak inductor current
limit. The converter output starts ramping up and finally
gets clamped at 32V (typ). The converter will then switch
at low inductor current to regulate the converter output
at the clamp voltage. The V
CAP
and input current during
output open circuit are shown in the Typical Performance
Characteristics.
3497f
Cooper
www.cooperet.com
Toko
www.toko.com
Sumida
www.sumida.com
CAPACITOR SELECTION
The small size of ceramic capacitors make them ideal for
LT3497 applications. Use only X5R and X7R types because
they retain their capacitance over wider temperature ranges
than other types such as Y5V or Z5U. A 1µF input capacitor
9
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
