LM3697_15 Datasheet, PDF (11/43 Pages) Texas Instruments – High-Efficiency Three-String White LED Driver

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Feature Descriptions (continued)
LM3697
SNOSCS2C – NOVEMBER 2013 – REVISED OCTOBER 2015
1Mhz Eff - 500Khz Eff, 4.7uH, Three String
(Negative values when 500Khz more efficient)
3.0%
1Mhz LED Eff > 500Khz LED Eff
2.0%
1.0%
0.0%
-1.0%
-2.0%
Auto-Frequency
Threshold = 0x6C
-3.0%
-4.0%
500Khz LED Eff > 1Mhz LED Eff
-5.0%
-6.0%
BRIGHTNESS CODE
C002
Figure 8. Auto-Frequency Boost Efficiency Improvement Illustration
Table 1 summarizes the general recommendations for Auto-Frequency Threshold setting vs Inductance values
and LED string configurations. These are general recommendations — the optimum Auto-Frequency Threshold
setting must be evaluated for each application.
INDUCTOR
4.7 µH
10 µH
22 µH
Table 1. Auto-Frequency Threshold Settings
THREE STRING
AUTO-FREQUENCY PEAK EFFICIENCY
THRESHOLD
IMPROVEMENT
6C
2.2 %
74
1.7 %
7C
0.7 %
PEAK
CONFIGURATION
3p5s
3p4s
3p3s
AUTO-
FREQUENCY
THRESHOLD
AC
B4
BC
TWO STRING
PEAK EFFICIENCY
PEAK
IMPROVEMENT CONFIGURATION
1.1 %
1.3 %
0.7 %
2p6s
2p5s
2p4s
7.3.4 Brightness Register Current Control
The LM3697 features Brightness Register Current Control for simple user-adjustable current control set by
writing directly to the appropriate Control Bank Brightness Registers. The current for Control Banks A and B is a
function of the full-scale LED current, the 11-bit code in the respective brightness register, and the PWM input
duty cycle (if PWM is enabled). The Control A/B brightness must always be written with LSB's first and MSB's
last.
7.3.4.1 8-Bit Control (Preferred)
The preferred operating mode is to control the high-voltage LED brightness by setting the Control Bank LSB
register (3 LSB's) to zero and using only the Control Bank MSB register (8 MSB's). In this mode the LM3697
controls the 3 LSB's to ramp the high-voltage LED current using all 11-bits.
7.3.4.2 11-Bit Control
In this mode of operation, both Control Bank LSB and MSB registers must be written whenever a change in
Brightness is required. The high-voltage LED current will not change until the Control Bank MSB register is
written. If the brightness change affects only the 3 LSB's, the Control Bank MSB register (8 MSB's) must be re-
written to change the high-voltage LED current.
7.3.5 PWM Control
The LM3697's PWM input can be enabled for Control Banks A or B (see Table 14). Once enabled, the LED
current becomes a function of the code in the Control Bank Brightness Configuration Register and the PWM
input-duty cycle.
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