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LT3519/LT3519-1/LT3519-2
APPLICATIONS INFORMATION
Dimming Control
When V CTRL is higher than 1.2V, the LED current is clamped
There are two methods to control the current source for
dimming using the LT3519/LT3519-1/LT3519-2. The ?rst
method, PWM Dimming, uses the PWM pin to modulate
the current source between zero and full current to achieve
to be:
I LED =
250mV
R SENSE
PWM RATIO =
V OUT = ?
+ 1 ? ? 1.22V
a precisely programmed average current. To make this
method of current control more accurate, the switch de-
mand current is stored on the internal V C node during the
quiescent phase when PWM is low. This feature minimizes
recovery time when the PWM signal goes high. To obtain
best PWM dimming performance, it is necessary to use
an external disconnect switch in the LED current path to
prevent the output capacitor from discharging during the
PWM signal low phase. For best product of analog and PWM
dimming, the minimum PWM low or high time should be at
least six switching cycles (3μs for f SW = 2MHz). Maximum
PWM period is determined by the system. The maximum
PWM dimming ratio (PWM RATIO ) can be calculated from
the maximum PWM period (t MAX ) and the minimum PWM
pulse width (t MIN ) as follows:
t MAX
t MIN
Example:
t MAX = 9ms, t MIN = 3μs (f SW = 2MHz)
When V CTRL is more than 1V but less than 1.2V, the
LED current is in the nonlinear region of V ISP -V ISN
Threshold vs V CTRL as shown in the Typical Performance
Characteristics.
The LED current programming feature through the CTRL
pin possibly increases the total dimming range by a factor
of ten. In order to have the accurate LED current, precision
resistors are preferred (1% is recommended). The CTRL
pin should not be left open. Tie to V REF if not used.
Programming Output Voltage (Constant Voltage
Regulation) or Open LED/Overvoltage Threshold
For a boost application, the output voltage can be set by
selecting the values of R1 and R2 (see Figure 1) according
to the following equation:
? R1 ?
? R2 ?
V OUT
PWM RATIO =
9ms
3μs
= 3000:1
LT3519/
LT3519-1/ FB
LT3519-2
R1
The second method of dimming control, Analog Dim-
ming, 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 but more than 100mV,
the LED current is:
R2
3519 F01
Figure 1. FB Resistor Divider for Boost LED Driver
I LED =
V CTRL – 100mV
4 ? R SENSE
3519fa
9
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