ZONE0

ZONE1

ZONE2

ZONE3

ZONE4

5

10

15

20

25

30

35

40

45

50

55

60

THE END OF THE

COUNTDOWN PERIOD

TIMER PERIOD

STARTED

TIMER PERIOD

STARTED

TIMER PERIOD

STARTED

TIMER PERIOD

STARTED

ELAPSED TIME (s)

THE END OF THE

COUNTDOWN

PERIOD

LM3537

www.ti.com

SNVS634B – JUNE 2011 – REVISED MAY 2013

Solid line shows the ALS operation when the timer is disabled. Dashed line shows the operation when the 10s timer

is enabled. Dotted line shows the operation when the device sticks to the highest zone.

Figure 5. Countdown Timer Principle

PWM Input

A PWM (Pulse Width Modulation) pin is provided on the LM3537 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 LM3537 require approximately 15 µs 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 LM3537 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 LM3537 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 150 mV, 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 LM3537 will remain in the gain of 3/2

until an I2C write to the part occurs. At that time, the LM3537 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 LM3537 has a user-selectable gain transition delay that allows the part to ignore short

duration input voltage drops. By default, the LM3537 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 LM3537.

Hardware Enable (HWEN)

The LM3537 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 Electrical Characteristics section of

the datasheet for required voltage thresholds.

Copyright © 2011–2013, Texas Instruments Incorporated

Submit Documentation Feedback

17

Product Folder Links: LM3537