LM3537_13 Datasheet, PDF (17/33 Pages) Texas Instruments – 8-Channel WLED Driver with Four Integrated LDOs

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LM3537
SNVS634B – JUNE 2011 – REVISED MAY 2013
ZONE4
ZONE3
TIMER PERIOD TIMER PERIOD
STARTED
STARTED
TIMER PERIOD
STARTED
ZONE2
ZONE1
ZONE0
TIMER PERIOD
STARTED
THE END OF THE
COUNTDOWN PERIOD
THE END OF THE
COUNTDOWN
PERIOD
5
10 15 20 25 30 35 40 45 ^{50 55 60}
ELAPSED TIME (s)
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 V_OUT voltage to track the input voltage.
As the input voltage drops, the voltage on the Dx pins will also drop (V_DX = V_VOUT – V_LEDx). 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.
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