To Assigned

High Voltage

Current Sinks

PWM Input

LPF

Backlight Digital LED Control Block

Full-Scale Current Select

Brightness Setting

Exponential or Linear Mapping

Startup/Shutdown Ramp Generator

Runtime Ramp Generator

Analog Domain

Digital Domain

DAC

Full-Scale

Current

Control

polarity

12

LM3697

SNOSCS2D – NOVEMBER 2013 – REVISED MARCH 2019

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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.

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 tPWM 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.