LM3697_15 Datasheet, PDF (13/43 Pages) Texas Instruments – High-Efficiency Three-String White LED Driver

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LM3697
SNOSCS2C – NOVEMBER 2013 – REVISED OCTOBER 2015
7.3.8 High-Voltage Control A and B Ramp Select
The LM3697 provides three options for Control A and B ramp times (see Table 8). When the Run-time Ramp
Select bits are set to 00, the control bank uses both the Start-up/Shutdown and Run-time ramp times. When the
Run-time Ramp Select bits are set to 01, the control bank uses the Start-up/Shutdown ramp times for both start-
up/shutdown and run-time. When the Run-time Ramp Select bits are set to 1x the control bank uses a zero µsec
run-time ramp.
7.4 Device Functional Modes
7.4.1 LED Current Mapping Modes
All control banks can be programmed for either exponential or linear mapping modes (see Figure 10 and
Figure 11). These modes determine the transfer characteristic of backlight code to LED current. Independent
mapping of Control Banks A and B is not allowed: both banks uses the same mapping mode.
7.4.1.1 Exponential Mapping
In Exponential Mapping Mode the current ramp (either up or down) appears to the human eye as a more uniform
transition then the linear ramp. This is due to the logarithmic response of the eye.
7.4.1.1.1 8-Bit Code Calculation
In Exponential Mapping Mode the brightness code to backlight current transfer function is given by the equation:
( ) _{ILED = ILED_FULLSCALE x 0.85}
44
-
Code + 1
5.8181818
x D_PWM
(1)
Where I_{LED_FULLSCALE} is the full-scale LED current setting (see Table 10), Code is the 8-bit backlight code in the
Control Brightness MSB register and D_PWM is the PWM Duty Cycle.
7.4.1.1.2 11-Bit Code Calculation
In Exponential Mapping Mode the brightness code to backlight current transfer function is given by the equation:
( ) _{ILED = ILED_FULLSCALE x 0.85}
44 -
Code
8
+
1
5.8181818
x D_PWM
(2)
Where I_{LED_FULLSCALE} is the full-scale LED current setting (see Table 10), Code is the 11-bit backlight code in the
Control Brightness MSB and LSB registers and D_PWM is the PWM Duty Cycle.
7.4.1.2 Linear Mapping
In Linear Mapping Mode the brightness code to backlight current has a linear relationship.
7.4.1.2.1 8-Bit Code Calculation
The 8-bit linear mapping follows the equation:
I_LED = I_{LED_FULLSCALE} x
1
255
x Code x D_PWM
(3)
Where I_{LED_FULLSCALE} is the full-scale LED current setting, Code is the 8-bit backlight code in the Control
Brightness MSB register and D_PWM is the PWM Duty Cycle.
7.4.1.2.2 11-Bit Code Calculation
The 11-bit linear mapping follows the equation:
I_LED = I_{LED_FULLSCALE} x
1
2047
x Code x D_PWM
(4)
Where I_{LED_FULLSCALE} is the full-scale LED current setting, Code is the 11-bit backlight code in the Control
Brightness MSB and LSB registers and D_PWM is the PWM Duty Cycle.
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