LTC3675

29

3675f

Table 13. LED Driver Regulator Program Register 1 Bit Format

Bit7

x

Unused

Bit6

Bit5

Mode1

Mode0

Mode1 = Mode0 = 0 is default; both LED pins are regulated.

Mode1 = 0 Mode0 = 1; Only LED1 is regulated. (Single string application).

Mode1 = 1 Mode0 = 0; LED driver is configured as a high voltage boost regulator.

Mode1 = Mode0 = 1; Both LED pins are regulated, but boost is not powered up. In this mode an external

voltage is needed to drive the LED’s.

Bit4

Slow Edge

This bit controls the slew rate of the switch node. Default is ‘0’ which enables the switch node to slew at a

faster rate than if the bit were programmed a ‘1.’

Bit3

2xFS

This bit doubles the full-scale programmed LED current. Default is ‘1.’

Bit2(GRAD2)

Bit1(GRAD1)

Bit0(GRAD0)

DAC Control

LED current gradation timing bits. Default is ‘111.’ See Table 14.

APPLICATIONS INFORMATION

These times are shown in Table 14. The default state of

000 in GRAD[2:0] results in a very fast ramp time that

cannot be visually perceived.

Table 14. LED Gradation Bits

GRAD2, GRAD1, GRAD0

GRADATION STEP TIME

000

0.056 ms

001

0.912 ms

010

1.824 ms

011

3.648 ms

100

7.296 ms

101

14.592 ms

110

29.184 ms

111 (Default)

58.368 ms

The LED DAC register is at sub-address 08h. All 8 bits in

this register are used to control LED current. The default

state of this register is 00h which disables the LED driver.

See Table 1.

Operating the LED Driver As a High Voltage Boost

Regulator

The LED driver may be configured as a high voltage boost

regulator capable of producing an output voltage up to

this mode, the LED_OV pin serves as the feedback pin.

The feedback resistors are selected as discussed in the

Switching Regulator Output voltage and Feedback Network

section. The LED_FS pins must be tied to the input supply

in this mode. When configured as a high voltage boost,

the LED DAC register is ignored.

To maintain stability, the average inductor current must

be maintained below 750mA. This limits the deliverable

output current at low input supply voltages. Figure 8 gives

an example of the LED driver configured as a high voltage

boost regulator.

Input and Output Decoupling Capacitor Selection

The LTC3675 has multiple input supply pins and output

pins. Each of these pins must be decoupled with low ESR

capacitors to GND. These capacitors must be placed as

close to the pins as possible. Ceramic dielectric capacitors

are a good compromise between high dielectric constant

and stability versus temperature and DC bias. Note that the

capacitance of a capacitor deteriorates at higher DC bias.

It is important to consult manufacturer data sheets and

obtain the true capacitance of a capacitor at the DC bias

voltage it will be operated at. For this reason, avoid the

use of Y5V dielectric capacitors. The X5R/X7R dielectric

capacitors offer good overall performance.

The input supply voltage pins 6, 7, 10 and 40 all need

to be decoupled with at least 10μF capacitors. The input

supply pins 31 and 34 and the DVCC pin 41 need to be

decoupled with 2.2μF capacitors. The outputs of the 1A

buck regulators need 22μF capacitors, while the outputs

of the 500mA buck regulators need 10μF capacitors. The

buck-boost output regulator needs a 22μF decoupling

capacitor. The boost regulator needs two 22μF output

decoupling capacitors. The LED driver output pin should

be decoupled with a 4.7μF capacitor.