LM3697_15 Datasheet, PDF (12/43 Pages) Texas Instruments – High-Efficiency Three-String White LED Driver

LM3697
SNOSCS2C – NOVEMBER 2013 – REVISED OCTOBER 2015
www.ti.com
The PWM input accepts a logic level voltage and internally filters it to an analog control voltage. This results in a
linear response of duty cycle to current, where 100% duty cycle corresponds to the programmed brightness code
multiplied by the Full-Scale Current setting.
PWM Input
polarity
Analog Domain
LPF
Digital Domain
Backlight Digital LED Control Block
Full-Scale Current Select
Brightness Setting
Exponential or Linear Mapping
Startup/Shutdown Ramp Generator
Runtime Ramp Generator
Full-Scale
Current
Control
DAC
To Assigned
High Voltage
Current Sinks
Figure 9. PWM Input Architecture
7.3.5.1 PWM Input Frequency Range
The usable input frequency range for the PWM input is governed on the low end by the cutoff frequency of the
internal low-pass filter (540 Hz, Q = 0.33) and on the high end by the propagation delays through the internal
logic. For frequencies below 2 kHz the current ripple begins to become a larger portion of the DC LED current.
Additionally, at lower PWM frequencies the boost output voltage ripple increases, causing a non-linear response
from the PWM duty cycle to the average LED current due to the response time of the boost. For the best
response of current vs. duty cycle, the PWM input frequency must be kept between 2 kHz and 100 kHz.
7.3.5.2 PWM Input Polarity
The PWM Input can be set for active low polarity, where the LED current is a function of the negative duty cycle.
This is set via the PWM Configuration register (see Table 14).
7.3.5.3 PWM Zero Detection
The LM3697 incorporates a feature to detect when the PWM input is near zero. After the near zero pulse width
has been detected the PWM pulse must be greater than t_PWM to affect the HVLED output current (see Electrical
Characteristics ). Bit 3 in the PWM Configuration register is used to disable this feature.
7.3.6 Start-up/Shutdown Ramp
The high-voltage LED start-up and shutdown ramp times are independently configurable in the start-up/shutdown
transition time Register (see Table 6). There are 16 different start-up and 16 different shutdown times. The start-
up times can be programmed independently from the shutdown times, but each Control bank is not
independently configurable.
The start-up ramp time is from when the Control Bank is enabled to when the LED current reaches its initial set
point. The shutdown ramp time is from when the Control Bank is disabled to when the LED current reaches 0.
7.3.7 Run-Time Ramp
Current ramping from one brightness level to the next is programmed via the Control A and B Run-Time Ramp
Time Register (see Table 7). There are 16 different ramp-up times and 16 different ramp-down times. The ramp-
up time can be programmed independently from the ramp-down time, but each Control Bank cannot be
independently programmed. For example, programming a ramp-up or ramp-down time is a global setting for all
high-voltage LED Control Banks.
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