LTC3577 datasheet(18/38 Pages) LINER

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LTC3675

3675fa

OPERATION

output voltage to 1.2V. The FB2 pin is tied to VIN, which

conﬁgures buck regulator 2 as the slave. The SW1 and

SW2 pins must be tied together. The register contents of

the master program the combined buck regulator’s behavior

and the register contents of the slave are ignored. The slave

buck control circuitry draws no current. The enable of the

master buck (EN1) controls the operation of the combined

bucks, the enable of the slave regulator (EN2) is ignored.

Buck regulators 2 and 3 may be conﬁgured as combined

buck regulators capable of delivering up to 1.5A load

current with buck regulator 2 being the master. Buck

regulators 3 and 4 may be conﬁgured as combined buck

regulators capable of delivering up to 1A load current with

buck regulator 3 being the master.

BOOST SWITCHING REGULATOR

The boost regulator is capable of delivering up to 1A load

current for a programmed output voltage of up to 5V. The

boost regulator may be enabled only via I2C. The mode

of operation, feedback regulation voltage and switch slew

rate can all be controlled via I2C.

The boost regulator can operate in either PWM mode or

in Burst Mode operation. In PWM operating mode, the

regulator operates at a constant frequency of 2.25MHz

and provides a low noise solution. For light loads, Burst

Mode operation offers improved efﬁciency. The boost

regulator has forward and reverse current limiting, soft-

start to limit inrush current during start-up, short-circuit

protection and slew rate control for lower radiated EMI.

The boost regulator also features true output disconnect

when in shutdown. In shutdown, an internal 10k resistor

pulls the output to GND.

BUCK-BOOST SWITCHING REGULATOR

The buck-boost regulator is a 2.25MHz voltage mode

regulator. The buck-boost regulator is capable of delivering

up to 1A load current for a programmed output voltage of

3.3V. The regulator can be enabled via its enable pin or via

I2C. The mode of operation, feedback regulation voltage

and switch slew rate can all be controlled via I2C.

The buck-boost regulator can operate in either PWM

mode or in Burst Mode operation. The PWM operating

mode provides a low noise solution. For light loads, Burst

Mode operation offers improved efﬁciency. The buck-boost

regulator has forward current limiting, soft-start to limit

inrush current during start-up, short-circuit protection

and slew rate control for lower radiated EMI.

When the output voltage is below 2.65V (typical) during

start-up, Burst Mode operation is disabled and switch D is

turned off. The forward current is carried by the switch D

well diode and there is no reverse current ﬂowing in this

condition. In shutdown, an internal 10k resistor pulls the

output to GND.

LED DRIVER

The LED driver uses a constant frequency, current mode

boost converter to supply power to up to two strings of 10

series LEDs. The series string of LEDs is connected from

the output of the boost converter to an LED pin. The LED

pin is a programmable constant current sink. The boost

converter will regulate its output to force the LED pin to

300mV. The percentage of full-scale current sunk by the

LED pin is programmed via I2C.

The LED boost converter is designed for very high duty

cycle operation and can boost from below 3V to 40V out

at up to 55mA. The LED boost also features an overvolt-

age protection feature to limit the output voltage in case

of an open circuit in an LED string. The boost converter

will operate in either continuous conduction mode, dis-

continuous conduction mode or pulse-skipping mode

depending on the inductor current required for regulation.

The boost converter may also be conﬁgured to operate

as an independent high voltage boost regulator via I2C.

The LED driver may also be conﬁgured as a single string

LED driver. When driving a single string, LED1 and LED2

should be tied together.

The LED driver features a fully automatic gradation circuit.

This circuit allows the current to ramp up or down at a

controlled rate between any two current levels. On power-up

the LED DAC register is set to 0. To enable the LED driver

a non-zero value must be programmed into this register.

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