Datasheet

LT3478/LT3478-1
15
34781f
The value for R4 can now be solved using equation (a)
where,
R4 = 0.77 (R
NTC
(25°C) + R
Y
) = 0.77 (22k + 7.5k)
R4 = 22.7k (choose 22.6k)
I
LED
slope can now be calculated from,
I
LED
slope = [CTRL2(50°C) – CTRL2(25°C)]/25°C
where CTRL2 (50°C) = 1.24/(1 + 22.6/(7.9 + 7.5)) =
503mV
and CTRL2 (25°C) = 1.24/(1 + 39.2/(22 + 28.7)) =
699mV
giving I
LED
slope (from 25°C to 50°C)
= 503mV – 699mV/25°C
= –196mV/25°C => I
LED
slope = –196mA/25°C
Using the Murata simulation tool for the resistor network
and values in the above example shows a CTRL2 volt-
age curve that fl attens out as temperatures approach
100°C ambient. The fi nal resistor network chosen for the
derating curve in Figure 6 used option C network with
R4 = 19.3k, R
NTC
= 22k (NCP15XW223J0SRC) and R
Y
= 3.01k. Although the CTRL2 downward slope is greater
than –200mA/25°C initially, the slope is required to avoid
exceeding maximum allowed LED currents at high ambient
temperatures (see Figure 6).
PWM Dimming
Many LED applications require an accurate control of the
brightness of the LED(s). In addition, being able to main-
tain a constant color over the entire dimming range can
be just as critical. For constant color LED dimming, the
LT3478/LT3478-1 provide a PWM pin and special internal
circuitry to allow up to a 3000:1 wide PWM dimming
range. With an N-channel MOSFET connected between
the LED(s) and ground and a PWM signal connected to
the gate of the MOSFET and the PWM pin (Figure 9), it
is possible to control the brightness of the LED(s) based
on PWM signal duty cycle only. This form of dimming is
superior to dimming control using an analog input voltage
(reducing CTRL1 voltage) because it allows constant color
to be maintained during dimming. The maximum current
for the output LED(s) is programmed for a given bright-
ness/color and “chopped” over a PWM duty cycle range
(Figure 10) from 100% to as low as 0.033%.
APPLICATIO S I FOR ATIO
WUU
U
Figure 9. PWM Dimming Control Using the LT3478/LT3478-1
Figure 10. PWM Dimming Waveforms Using the
LT3478/LT3478-1
LT3478/
LT3478-1
SHDN
V
IN
V
REF
V
OUT
LED
CTRL2
OVPSET
PWMV
C
V
S
LSW
PWM DIMMING
CONTROL
3478 F09
CTRL1
R
T
R
SENSE
C
OUT
D1
D2
(LT3478)
PWM
INDUCTOR
CURRENT
3478 F10
LED
CURRENT
MAX I
LED
T
PWM
TON
PWM
(= 1/f
PWM
)
Some general guidelines for LED Current Dimming using
the PWM pin (see Figure 10):
(1) PWM Dimming Ratio (PDR) = 1/(PWM duty cycle) =
1/(TON
PWM
• f
PWM
)
(2) Lower f
PWM
allows higher PWM Dimming Ratios
(use minimum f
PWM
= 100Hz to avoid visible fl icker and
to maximize PDR)
(3) Higher f
OSC
value improves PDR (allows lower TON
PWM
)
but will reduce effi ciency and increase internal heating. In
general, minimum operational TON
PWM
= 3 • (1/f
OSC
).
(4) Lower inductor value improves PDR