LM3538

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SNVS674A – JUNE 2011 – REVISED MAY 2013

CIRCUIT DESCRIPTION

OVERVIEW

The LM3538 is a white LED driver system based upon an adaptive 3/2× - 1× CMOS charge pump capable of

supplying up to 150mA of total output current. With two separately controlled groups of constant current sinks,

the LM3538 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 25mA. 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 LM3538 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

20mA 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 LM3538 also integrates three 150-mA LDOs and one 300-mA LDO voltage regulator which can be turned

on/off using separate enable bits on each LDO. Each LDO operates with a power rail input voltage range

between 1.8V 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 VIN_A pin, and the regulated output of the charge

pump is connected to the VOUT pin. The operating input voltage range of the LM3538 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 VOUT is equal to the gain times the voltage at the input. When the device is in closed

loop, the voltage at VOUT 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 six regulated current sinks configurable into two different lighting regions.

Ambient Light Sensing (ALS) and Interrupt

The LM3538 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 LM3538 can be configured to

adjust the LED current to five unique settings corresponding to four adjustable light region trip points.

Additionally, when the LM3538 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.

Table 1. ALS Resistor Values

r_als[4]

r_als[3]

r_als[2]

r_als[1]

r_als[0]

RALS (typ) Value

Unit

1

1

1

1

1

0.651

k

Ω

1

1

1

1

0

0.672

k

Ω

1

1

1

0

1

0.695

k

Ω

1

1

1

0

0

0.720

k

Ω

1

1

0

1

1

0.747

k

Ω

1

1

0

1

0

0.776

k

Ω

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