LM3537_13 Datasheet, PDF (13/33 Pages) Texas Instruments – 8-Channel WLED Driver with Four Integrated LDOs

LM3537
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Circuit Description
SNVS634B – JUNE 2011 – REVISED MAY 2013
OVERVIEW
The LM3537 is a white LED driver system based upon an adaptive 3/2× - 1× CMOS charge pump capable of
supplying up to 180 mA of total output current. With two separately controlled groups of constant current sinks,
the LM3537 is an ideal solution for platforms requiring a single white LED driver for main display and sub display
(or keypad). The tightly matched current sinks ensure uniform brightness from the LEDs across the entire small-
format display.
Each LED is configured in a common anode configuration, with the peak drive current set to 25 mA. An I²C-
compatible interface is used to enable the device and vary the brightness within the individual current sink
groups. For group A, 128 brightness control levels are available (user defined linear or exponential dimming
curve). Group B has 8 linearly-spaced analog brightness levels.
The LM3537 provides an input for an Ambient Light Sensor to adaptively adjust the diode current based on
ambient conditions, and a PWM pin to allow the diode current to be pulse width modulated to work with a display
driver utilizing dynamic or content adjusted backlight control (DBC or CABC). Additionally, the device provides 20
mA power supply output for the sensor. The GPOs can also be configured to serve as a gain control interface for
sensors with HW-controlled gain.
The LM3537 also integrates three 150-mA LDO and one 300-mA LDO voltage regulators, which can be turned
on/off using separate enable bits on each LDO. Each LDO operates with a power rail input voltage range
between 1.8 V and 5.5V allowing them to be supplied from the battery or a step-down converter. Furthermore,
the regulated output voltages can be adjusted through the serial bus.
CIRCUIT COMPONENTS
Charge Pump
The input to the 3/2× - 1× charge pump is connected to the V_{IN_A} pin, and the regulated output of the charge
pump is connected to the V_OUT pin. The operating input voltage range of the LM3537 is 2.7V to 5.5V. The
device’s regulated charge pump has both open-loop and closed-loop modes of operation. When the device is in
open loop, the voltage at V_OUT is equal to the gain times the voltage at the input. When the device is in closed
loop, the voltage at V_OUT is regulated to 4.2V (typ.). The charge pump gain transitions are actively selected to
maintain regulation based on LED forward voltage and load requirements.
Diode Current Sinks
The matched current outputs are generated with a precision current mirror that is biased off the charge pump
output. Matched currents are ensured with the use of tightly matched internal devices and internal mismatch
cancellation circuitry. There are eight regulated current sinks configurable into 2 different lighting regions.
Ambient Light Sensing (ALS) and Interrupt
The LM3537 provides an Ambient Light Sensing input for use with ambient backlight control. Connecting the
anode of a photo diode to this pin and configuring the appropriate ALS resistor, the LM3537 can be configured to
adjust the LED current to five unique settings corresponding to four adjustable light region trip points.
Additionally, when the LM3537 determines that an ambient condition has changed, the interrupt pin, when
connected to a pullup resistor will toggle to a '0' alerting the controller. Available resistor values are shown in
Table 1 below.
Copyright © 2011–2013, Texas Instruments Incorporated
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