LT3518 LINER [Linear Technology], LT3518 Datasheet - Page 10

• Manufacturer Part Number: LT3518
• Description: Full-Featured LED Driver with 2.3A Switch Current
• Manufacturer: LINER [Linear Technology]

LT3518
APPLICATIONS INFORMATION
Dimming Control
There are two methods to control the current source for
dimming using the LT3518. The fi rst method uses the
PWM pin to modulate the current source between zero
and full current to achieve a precisely programmed aver-
age current. To make this method of current control more
accurate, the switch demand current is stored on the V
node during the quiescent phase. This feature minimizes
recovery time when the PWM signal goes high. To further
improve the recovery time, a disconnect switch is used in
the LED current path to prevent the output capacitor from
discharging in the PWM signal low phase. The minimum
PWM on or off time will depend on the choice of operating
frequency through R
the SYNC function, the SYNC and PWM signals must have
the aligned rising edges to achieve the optimized high PWM
dimming ratio. For best current accuracy, the minimum
PWM low or high time should be at least six switching
cycles (3μs for f
determined by the system and is unlikely to be longer than
12ms. The maximum PWM dimming ratio (PWM
be calculated from the maximum PWM period (t
the minimum PWM pulse width (t
Example:
t
PWM
The second method of dimming control uses the CTRL
pin to linearly adjust the current sense threshold during
the PWM high state. When the CTRL pin voltage is less
than 1V, the LED current is:
10
MAX
PWM
I
LED
= 9ms, t
RATIO
=
RATIO
10 • R
= 9ms/3μs = 3000:1
V
MIN
CTRL
=
SENSE
SW
t
t
MAX
= 3μs (f
MIN
= 2MHz). Maximum PWM period is
T
input pin or SYNC pin. When using
SW
= 2MHz)
MIN
) as follows:
RATIO
MAX
) and
) can
(1)
(2)
C
When V
to be:
The LED current programming feature possibly increases
total dimming range by a factor of ten.
The CTRL pin should not be left open (tie to V
used). The CTRL pin can also be used in conjunction with
a PTC thermistor to provide overtemperature protection
for the LED load.
Setting Output Voltage
For a boost application, the output voltage can be set by
selecting the values of R1 and R2 (see Figure 3) according
to the following equation:
V
I
LED
OUT
CTRL
=
=
R
100mV
SENSE
R2
R1
is higher than 1V, the LED current is clamped
V
+ 1 • 1.01V
REF
LT3518
45.3k
5k
PTC
Figure 2
Figure 3
FB
2V
V
OUT
R1
R2
3518 F03
3518 F02
49.9k
CTRL
REF
if not
3518fb
(3)
(4)