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LM3535

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SNVS598B – AUGUST 2010 – REVISED MARCH 2018

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7.3 Feature Description

7.3.1 Charge Pump

The input to the 3/2× or 1× charge pump is connected to the VIN pin, and the regulated output of the charge

pump is connected to the VOUT pin. The recommended input voltage range of the LM3535 is 2.7 V to 5.5 V. The

device 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 4.3 V (typical). The charge pump gain transitions are actively selected to

maintain regulation based on LED forward voltage and load requirements.

7.3.2 Diode Current Sinks

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 3 different lighting regions.

7.3.3 Ambient Light Sensing (ALS) And Interrupt

The LM3535 provides an ambient light sensing input for use with ambient backlight control. By connecting the

anode of a photo diode / sensor to the sensor input pins, and configuring the appropriate ALS resistors, the

LM3535 can be configured to adjust the diode current to five unique settings, corresponding to four adjustable

light region trip points. Additionally, when the LM3535 determines that an ambient condition has changed, the

interrupt pin, when connected to a pullup resistor toggles to a 0 alerting the controller. See I2C Compatible

Interface for more details regarding the register configurations.

7.3.4 Dynamic Backlight Control Input (PWM Pin)

The pulse width modulation (PWM) pin allows a display driver utilizing dynamic backlight control (DBC) to adjust

the LED brightness based on the content. The PWM input can be turned on or off (Acknowledge or Ignore), and

the polarity can be flipped (active high or active low) through the I2C interface. The current sinks of the LM3535

require approximately 15 µs to reach steady-state target current. This turnon time sets the minimum usable PWM

pulse width for DBC/CABC.

7.3.5 LED Forward Voltage Monitoring

The LM3535 has the ability to switch gains (1× or 3/2×) based on the forward voltage of the LED load. This

ability to switch gains maximizes efficiency for a given load. Forward voltage monitoring occurs on all diode pins.

At higher input voltages, the LM3535 operates in pass mode, allowing the VOUT voltage to track the input voltage.

As the input voltage drops, the voltage on the Dxx pins also drops (VDXX = VVOUT – VLEDx). Once any of the active

Dxx pins reaches a voltage approximately equal to 130 mV, the charge pump will switch to the gain of 3/2. This

switchover ensures that the current through the LEDs never becomes pinched off due to a lack of headroom

across the current sinks. Once a gain transition occurs, the LM3535 remains in the gain of 3/2 until an I2C write

to the part occurs. At that time, the LM3535 re-evaluates the LED conditions and selects the appropriate gain.

Only active Dxx pins are monitored.

7.3.6 Configurable Gain Transition Delay

To optimize efficiency, the LM3535 has a user selectable gain transition delay that allows the part to ignore short

duration input voltage drops. By default, the LM3535 does not change gains if the input voltage dip is shorter

than 3 to 6 milliseconds. There are three selectable gain transition delay ranges available on the LM3535. All

delay ranges are set within the VF Monitor Delay Register. See Internal Registers of LM3535 for more

information regarding the delay ranges.

7.3.7 Hardware Enable (HWEN)

The LM3535 has a hardware enable/reset pin (HWEN) that allows the device to be disabled by an external

controller without requiring an I2C write command. Under normal operation, hold the HWEN pin high (logic 1) to

prevent an unwanted reset. When the HWEN is driven low (logic 0), all internal control registers reset to the

default states, and the device becomes disabled. See the Electrical Characteristics section of the data sheet for

required voltage thresholds.