LM3697_15 Datasheet, PDF (14/43 Pages) Texas Instruments – High-Efficiency Three-String White LED Driver

LM3697
SNOSCS2C – NOVEMBER 2013 – REVISED OCTOBER 2015
Device Functional Modes (continued)
21.00
18.00
Exponential
Linear
15.00
12.00
9.00
6.00
3.00
0.00
BRIGHTNESS CODE
C001
Figure 10. LED Current Mapping Modes (8-Bit)
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21
Exponential
Linear
18
15
12
9
6
3
0
BRIGHTNESS CODE
C001
Figure 11. LED Current Mapping Modes (11-Bit)
7.4.2 Fault Flags/Protection Features
The LM3697 contains both LED-open and LED-short fault detection. These fault detections are designed to be
used in production level testing and not normal operation. For the fault flags to operate, they must be enabled via
the LED Fault Enable Register (see Table 22). The following sections detail the proper procedure for reading
back open and short faults in the high-voltage LED strings.
7.4.2.1 Open LED String (HVLED)
An open LED string is detected when the voltage at the input to any active high-voltage current sink has fallen
below 200 mV, and the boost output voltage has hit the OVP threshold. This test assumes that the HVLED string
that is being detected for an open is connected to the LM3697 device's boost output (COUT+) (see Table 20).
For an HVLED string not connected to the LM3697's boost output voltage, but connected to another voltage
source, the boost output will not trigger the OVP flag. In this case an open LED string is not detected.
The procedure for detecting an open fault in the HVLED current sinks (provided they are connected to the boost
output voltage) is:
• Apply power to the LM3697
• Enable Open Fault (Register 0xB4, bit [0] = 1)
• Assign HVLED1, HVLED2 and HVLED3 to Bank A (Register 0x10, Bits [2:0] = (0, 0, 0)
• Set the start-up ramp times to the fastest setting (Register 0x11 = 0x00)
• Set Bank A full-scale current to 20.2 mA (Register 0x17 = 0x13)
• Configure HVLED1, HVLED2 and HVLED3 for LED string anode connected to COUT (Register 0x19, bits[2:0]
= (1,1,1))
• Set Control A Brightness MSB to max (Register 0x21 = 0xFF)
• Enable Bank A (Register 0x24 Bit[0] = 1
• Wait 4 ms
• Read back bits[2:0] of register 0xB0. Bit [0] = 1 (HVLED1 open). Bit [1] = 1 (HVLED2 open). Bit [2] = 1
(HVLED3 open)
• Disable all banks (Register 0x24 = 0x00)
7.4.2.2 Shorted LED String (HVLED)
The LM3697 features an LED short fault flag indicating one or more of the HVLED strings have experienced a
short. The method for detecting a shorted HVLED strings is if the current sink is enabled and the string voltage
(V_OUT - V_HVLED1/2/3) falls to below (V_IN - 1 V) . This test must be performed on one HVLED string at a time.
Performing the test with more than one current sink enabled can result in a faulty reading.
The procedure for detecting a short in an HVLED string is:
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