LT3518 Просмотр технического описания (PDF) - Analog Devices

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LT3518

Full-Featured LED Driver with 2.3A Switch Current

Analog Devices 

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 us-

ing 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

=

VCTRL

10 • 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

total dimming range by a factor of ten.

VREF

2V

45.3k

5k

PTC

49.9k

CTRL

3518 F02

Figure 2

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

3518ff

For more information www.linear.com/LT3518

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