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LT3491EDC#TRMPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
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 (800) 368-2496
www.t-yuden.com
AVX (803) 448-9411
www.avxcorp.com
Murata (714) 852-2001
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
8
LT3491
3491fa
For low DCR inductors, which is usually the case for this
application, the peak inrush current can be simplified as
follows:
I
V
L
r
L
LC
PK
IN
=
⎛
⎝
⎜
⎞
⎠
⎟
=
=
–.
•
• exp – •
•
•
–
06
2
2
1
ω
α
ω
π
α
ω
rr
L
2
2
4•
where L is the inductance, r is the DCR of the inductor and
C is the output capacitance.
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Currents
V
IN
(V) r (Ω)L (µH) C
OUT
(µF) I
P
(A)
4.2 0.3 10 1.0 1.06
4.2 1.2 10 1.0 0.86
4.2 0.58 15 1.0 0.83
4.2 1.6 15 1.0 0.68
PROGRAMMING LED CURRENT
The feedback resistor (R
SENSE
) and the sense voltage
(V
CAP
– V
LED
) control the LED current.
The CTRL pin controls the sense reference voltage as
shown in the Typical Performance Characteristics. For
CTRL higher than 1.5V, the sense reference is 200mV,
which results in full LED current. In order to have accurate
LED current, precision resistors are preferred (1% is
recommended). The formula and table for R
SENSE
selec-
tion are shown below.
R
mV
I
SENSE
LED
=
200
Table 4. R
SENSE
Value Selection for 200mV Sense
I
LED
(mA) R
SENSE
(Ω)
540
10 20
15 13.3
20 10
DIMMING CONTROL
There are three different types of dimming control circuits.
The LED current can be set by modulating the CTRL pin
with a DC voltage, a filtered PWM signal or directly with a
PWM signal.
Using a DC Voltage
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
The CTRL pin voltage can be modulated to set the dimming
of the LED string. As the voltage on the CTRL pin increases
from 0V to 1.5V, the LED current increases from 0 to I
LED
.
As the CTRL pin voltage increases beyond 1.5V, it has no
effect on the LED current.
The LED current can be set by:
I
mV
R
when V V
I
V
LED
SENSE
CTRL
LED
CTRL
≈ >
≈
200
15
6
,.
.
225
125
•
,.
R
when V V
SENSE
CTRL
<
Feedback voltage variation versus control voltage is given
in the Typical Performance Characteristics.
APPLICATIO S I FOR ATIO
WUUU
9
LT3491
3491fa
I
L
200mA/DIV
I
LED
20mA/DIV
PWM
5V/DIV
V
IN
= 3V
4 LEDs
2ms/DIV
3491 F07
Using a Filtered PWM Signal
A filtered PWM signal can be used to control the bright-
ness of the LED string. The PWM signal is filtered (Figure
5) by a RC network and fed to the CTRL pin.
The corner frequency of R1, C1 should be much lower than
the frequency of the PWM signal. R1 needs to be much
smaller than the internal impedance of the CTRL pin which
is 10MΩ (typ).
APPLICATIO S I FOR ATIO
WUUU
LT3491
CTRL
C1
0.1µF
PWM
10kHz TYP
3491 F05
R1
100k
Figure 5. Dimming Control Using a Filtered PWM Signal
Direct PWM Dimming
Changing the forward current flowing in the LEDs not only
changes the intensity of the LEDs, it also changes the
color. The chromaticity of the LEDs changes with the
change in forward current. Many applications cannot
tolerate any shift in the color of the LEDs. Controlling the
intensity of the LEDs with a direct PWM signal allows
dimming of the LEDs without changing the color. In
addition, direct PWM dimming offers a wider dimming
range to the user.
Dimming the LEDs via a PWM signal essentially involves
turning the LEDs on and off at the PWM frequency. The
typical human eye has a limit of ~60 frames per second. By
increasing the PWM frequency to ~80Hz or higher, the eye
will interpret that the pulsed light source is continuously on.
Additionally, by modulating the duty cycle (amount of “on-
time”), the intensity of the LEDs can be controlled. The color
of the LEDs remains unchanged in this scheme since the
LED current value is either zero or a constant value.
Figure 6 shows a Li-Ion powered driver for four white
LEDs. Direct PWM dimming method requires an external
NMOS tied between the cathode of the lowest LED in the
string and ground as shown in Figure 6. A simple logic
level Si2302 MOSFET can be used since its source is
connected to ground. The PWM signal is applied to the
CTRL pin of the LT3491 and the gate of the MOSFET. The
PWM signal should traverse between 0V to 2.5V, to ensure
proper turn on and off of the driver and the NMOS
transistor Q1. When the PWM signal goes high, the LEDs
are connected to ground and a current of I
LED
= 200mV/
R
SENSE
flows through the LEDs. When the PWM signal
goes low, the LEDs are disconnected and turn off. The
MOSFET ensures that the LEDs quickly turn off without
discharging the output capacitor which in turn allows the
LEDs to turn on faster. Figure 7 shows the PWM dimming
waveforms for the circuit in Figure 6.
CTRL
PWM
FREQ
V
IN
L1
10µH
V
IN
3V TO 5V
R
SENSE
10Ω
3491 F06
LT3491
SW
CAP
LED
100k
GND
0V
2.5V
C2
1µF
Q1
Si2302
C1
1µF
Figure 6. Li-Ion to Four White LEDs with Direct PWM Dimming
Figure 7. Direct PWM Dimming Waveforms
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LT3491EDC#TRMPBF

Mfr. #:
Buy LT3491EDC#TRMPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LED Lighting Drivers White LED Driver w/ Integrated Schottky Diode in DFN
Lifecycle:
New from this manufacturer.
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