LTC3675 Datasheet, PDF (18/36 Pages) Linear Technology – 7-Channel Confi gurable High Power PMIC

LTC3675
OPERATION
output voltage to 1.2V. The FB2 pin is tied to V_IN, 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 be-
havior 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.
The gradation circuit will then ramp the current to the
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