LT3760
17
3760fc
applicaTions inForMaTion
turn LED currents on/off as quickly as possible. For PWM
low, the LT3760 turns off the boost converter, turns off
all LED channel currents and disconnects the V
C
pin and
internal V
OUT
resistor divider connected to the OVP error
amplifier. This allows the part to quickly return to the last
state of operation when the PWM pin is returned high.
Some general guidelines for LED current dimming using
the PWM pin (see Figure 5):
(1) PWM Dimming Ratio (PDR) = 1/(PWM Duty Cycle) =
1/T
ON(PWM)
• f
PWM
(2) Lower PWM frequency (f
PWM
) allows higher PWM
dimming ratios (Typically choose 100Hz to maximize PDR
and to avoid visible flicker which can occur for display
systems with refresh rates at frequencies below 80Hz)
(3) Higher f
OSC
value improves PDR (allows lower T
ON(PWM)
)
but will reduce efficiency and increase internal heating. In
general, minimum operational T
ON(PWM)
= 3 • (1/f
OSC
)
(4) Lower inductor value improves PDR
(5) Higher output capacitor value improves PDR
(6) Choose the Schottky diode for the LT3760 boost con-
verter for minimum reverse leakage current.
Programming LED Current Derating (Breakpoint and
Slope) versus LED Ambient Temperature (CTRL Pin)
LED data sheets provide curves of maximum allowed
LED
current versus
ambient temperature to warn against
damaging of the LED (Figure 6). The LT3760 LED driver
improves the utilization and reliability of the LED(s) by al-
PWM
CURRENT
3760 F05
LED
CURRENT
MAX I
LED
PWM
T
ON(PWM)
(= 1/f
PWM
)
Figure 5. PWM Dimming Waveforms
Analog Dimming
The LT3760 allows for LED dimming (brightness reduction)
by analog dimming or by PWM dimming. Analog dimming
uses the CTRL pin voltage below 1V to reduce LED
brightness by reducing LED current. For CTRL pin voltage
below 1V, the current in each LED pin is given by:
I LED
X
( )
≈ CTRL •
R
ISET
0.04 < CTRL < 1V
( )
For CTRL pin voltages below 40mV (greater than 25:1
dimming) the LED current will approach zero current. The
CTRL pin voltage can be derived from a resistor divider
from V
REF
pin to ground or generated from an external
source. If analog dimming is not required, the pin can be
directly connected to the V
REF
pin. The only drawback of
analog dimming is that reducing LED current to reduce
the brightness of the LED also changes the perceived
color of the LED.
PWM Dimming
Many applications require an accurate control of the bright-
ness of the LED(s). In addition, being able to maintain a
constant color over the entire dimming range can be just
as critical. For constant color LED dimming the LT3760
provides a PWM pin and special internal circuitry to achieve
up to a 3000:1 wide PWM dimming range. This is achieved
by operating the LED at it’s programmed current and then
controlling the on time of that LED current. The duty cycle
of the PWM pin controls the on time of each LED pin
current source (Figure 5). For maximum PWM dimming
ratios (low PWM duty cycles) it is important to
be able to
Figure 6. LED Current Derating vs LED Ambient Temperature
T
A
-TEMPERATURE (°C)
0
F
- FORWARD CURRENT (mA)
90
80
70
60
50
40
30
20
10
0
40 706030 50
3760 F06
802010
RESISTOR
OPTION A
LT3760
PROGRAMMED LED
CURRENT DERATING
CURVE