Datasheet

LT3478/LT3478-1
12
34781f
CTRL1 (V)
0
LED CURRENT (mA)
1.40
3478 F04
0.35 0.70 1.05
1400
1050
700
350
0
T
A
= 25°C
CTRL2 = V
REF
(FOR LT3478 SCALE
BY 0.1Ω/R
SENSE
)
LT3478-1
V
REF
Programming Maximum LED current
Maximum LED current can be programmed using the CTRL1
pin with CTRL2 tied to the V
REF
pin (see Figures 4 and 5).
The maximum allowed LED current is defi ned as:
(LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps
()
(,.)•
.
LT Max LED Current
Min CTRL
R
S
3478
1105
01
=
EENSE
Amps
LED current vs CTRL1 is linear for approximately
0.1V < CTRL1 < 0.95V
For maximum possible LED current, connect CTRL1 and
CTRL2 to the V
REF
pin.
Without the ability to back off LED current as temperature
increases, many LED drivers are limited to driving the
LED(s) at only 50% or less of their maximum rated currents.
This limitation requires more LEDs to obtain the intended
brightness for the application. The LT3478/LT3478-1 al-
low the output LED(s) to be programmed for maximum
allowable current while still protecting the LED(s) from
excessive currents at high temperature. This is achieved
by programming a voltage at the CTRL2 pin with a nega-
tive temperature coeffi cient using a resistor divider with
temperature dependent resistance (Figures 7 and 8).
CTRL2 voltage is programmed higher than CTRL1 voltage.
This allows initial LED current to be defi ned by CTRL1.
As temperature increases, CTRL2 voltage will fall below
CTRL1 voltage causing LED currents to be controlled by
CTRL2 pin voltage. The choice of resistor ratios and use
of temperature dependent resistance in the divider for the
CTRL2 pin will defi ne the LED current curve breakpoint
and slope versus temperature (Figure 8).
A variety of resistor networks and NTC resistors with differ-
ent temperature coeffi cients can be used for programming
APPLICATIO S I FOR ATIO
WUU
U
Figure 6. LED Current Derating Curve vs Ambient Temperature
maximum allowed LED current versus temperature to
warn against exceeding this current limit and damaging
the LED (Figure 6).
Figure 5. Programming LED Current
Figure 4. LED Current vs CTRL1 Voltage
LT3478/LT3478-1
(LT3478)
V
REF
CTRL2
CTRL1
V
OUT
LED
10
13
12
R2 R
SENSE
R1
3478 F05
T
A
AMBIENT TEMPERATURE (°C)
0
I
f
FORWARD CURRENT (mA)
900
LUXEON V EMITTER
(GREEN, CYAN, BLUE, ROYAL BLUE)
θ
JA
= 20°C/W
700
800
0
500
400
300
200
100
600
25
3478 F06
50 75 100
EXAMPLE
LT3478-1
PROGRAMMED LED
CURRENT DERATING CURVE
LUXEON V EMITTER
CURRENT DERATING
CURVE
Programming LED Current Derating vs Temperature
A useful feature of the LT3478/LT3478-1 is the ability
to program a derating curve for maximum LED current
versus temperature. LED data sheets provide curves of
Luxeon V (Maximum) and LT3478-1
(Programmed) Current Derating
Curves vs Temperature