LT3593 datasheet(7/16 Pages) LINER

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LT3593

3593f

OPERATION

The LT3593 uses a constant frequency, current mode

control boost scheme to provide excellent line and load

regulation. Operation can be best understood by referring

to the Block Diagram in Figure 1.

Output Disconnect

An internal switch connects between the CAP pin and the

LED pin. The top of the LED string is connected to the

LED pin. The string can be terminated to ground or to

VIN if a low number of LEDs are driven. When the part is

in shutdown, the switch is off; allowing no current to run

through the LEDs.

Current Programming

The LED current can be set to 32 different values

(625μA – 20mA) by strobing the CTRL pin. A 5-bit

counter is decremented by 1 on a CTRL pin rising edge,

reducing the programmed current by 625μA each time.

The desired current can be calculated by:

ILED = 20mA – (N – 1) • 625μA

where N is the number of rising edges. When the desired

current setting is reached, the CTRL pin must stay high.

128μs after the last rising edge on the CTRL pin, the

regulator will enable the output and start to regulate the

LED current to the programmed value. Figure 2 shows

an example of how the part can be programmed.

If a different current is desired, the CTRL pin can be strobed

again. The ﬁrst falling edge will reset the internal register

to the 20mA setting. While the new current is being pro-

grammed, the device will continue to regulate the previously

programmed value until the CTRL pin has remained high

for at least 128μs, at which time the regulated current will

slew to the newly programmed value. Figure 3 shows how

to program a new LED current level.

If the CTRL pin is held low for longer than 128μs, the part

will go into the shutdown mode.

Figure 2. Current Programming

CTRL

12.5mA

3593 F02

CTRL

15mA

CTRL

17.5mA

CTRL

FULL CURRENT

20mA

CTRL

LED

CURRENT

128μs

20mA

17.5mA

SHDN

3593 F03

128μs

Figure 3. Current Programming and Shutdown Timing

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