LM3538

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SNVS674A – JUNE 2011 – REVISED MAY 2013

PWM Input

A PWM (Pulse Width Modulation) pin is provided on the LM3538 to allow 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 LM3538 require approximately 15µs to reach steady-state target current. This turn-on

time sets the minimum usable PWM pulse width for DBC. The external PWM input is effective for Group A LEDs

only.

LED Forward Voltage Monitoring

The LM3538 has the ability to switch gains (1x or 3/2x) 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 LM3538 will operate in pass mode, allowing the VOUT voltage to track the input voltage.

As the input voltage drops, the voltage on the Dx pins will also drop (VDX = VVOUT – VLEDx). Once any of the

active Dx pins reaches a voltage approximately equal to 150mV, the charge pump will switch to the gain of 3/2.

This switch-over 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 LM3538 will remain in the gain of 3/2

until an I2C write to the part occurs. At that time, the LM3538 will re-evaluate the LED conditions and

select the appropriate gain.

Only active Dx pins will be monitored.

Configurable Gain Transition Delay

To optimize efficiency, the LM3538 has a user-selectable gain transition delay that allows the part to ignore short

duration input voltage drops. By default, the LM3538 will not change gains if the input voltage dip is shorter than

3 to 6 milliseconds. There are four selectable gain transition delay ranges available on the LM3538.

Hardware Enable (HWEN)

The LM3538 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, the HWEN pin should be held 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 part becomes disabled. Please see the Charge Pump and LED Drivers

Electrical Characteristics(1)(2) section of the datasheet for required voltage thresholds.

Low Dropout Voltage Regulators

The four low dropout voltage regulators are designed to operate with small-size ceramic input and output

capacitors. They can operate with power rail voltages down to 1.8V. The LDOs 2, 3 and 4 offer a typical dropout

voltage of 100 mV at 150mA output current. The single, higher-current LDO 1 offers a typical dropout voltage of

220mV at 300mA output current. The LDOs are enabled by the EN_LDO1, EN_LDO2, EN_LDO3 and EN_LDO4

bits (see Table 5 for details). Table 4 summarizes the supported output voltages. At startup, the LDOs are off but

are preset to 1.8V (for LDO2 and LDO3) and 2.8V (for LDO1 and LDO4).

Table 4. Regulator Voltage Options

LDOX_VOUT[4]

LDOX_VOUT[3]

LDOX_VOUT[2]

LDOX_VOUT[1]

LDOX_VOUT[0]

Output Voltage

(typ.)

1

1

1

1

1

3.30V

1

1

1

1

0

3.20V

1

1

1

0

1

3.10V

1

1

1

0

0

3.00V

1

1

0

1

1

2.95V

1

1

0

1

0

2.90V

1

1

0

0

1

2.85V

1

1

0

0

0

2.80V

(1)

All voltages are with respect to the potential at the GND pins.

(2)

Min and Max limits are specified by design, test, or statistical analysis. Typical numbers are not ensured, but do represent the most

likely norm.

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