LT3518HFE(RevB) View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LT3518HFE Datasheet PDF : 20 Pages
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LT3518
APPLICATIONS INFORMATION
Dimming Control
There are two methods to control the current source for
dimming using the LT3518. The first 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 VC
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 RT input pin or SYNC pin. When using
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 fSW = 2MHz). Maximum PWM period is
determined by the system and is unlikely to be longer than
12ms. The maximum PWM dimming ratio (PWMRATIO) can
be calculated from the maximum PWM period (tMAX) and
the minimum PWM pulse width (tMIN) as follows:
PWMRATIO
=
tMAX
tMIN
(1)
Example:
tMAX = 9ms, tMIN = 3μs (fSW = 2MHz)
PWMRATIO = 9ms/3μs = 3000:1
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:
ILED
=
10
VCTRL
• RSENSE
(2)
When VCTRL is higher than 1V, the LED current is clamped
to be:
ILED
=
100mV
RSENSE
(3)
The LED current programming feature possibly increases
2V
VREF
45.3k
49.9k
CTRL
5k
PTC
3518 F02
Figure 2
total dimming range by a factor of ten.
The CTRL pin should not be left open (tie to VREF if not
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:
VOUT
=
R1
R2
+
1
•
1.01V
(4)
VOUT
LT3518
R1
FB
R2
3518 F03
Figure 3
3518fb
10
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