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 (VOUT - VHVLED1/2/3) falls to below (VIN - 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:
- Apply power to the LM3697
- Enable Short Fault (Register 0xB4, bit [1] = 1)
- Assign HVLED1 to Bank A (Register 0x10, Bits [2:0] = (1, 1, 0)
- Set the startup ramp times to the fastest setting (Register 0x11 = 0x00)
- Set Bank A full-scale current to 20.2 mA (Register 0x17 = 0x13)
- Enable Feedback on the HVLED Current Sinks (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[0] of register 0xB2. 1 = HVLED1 short.
- Disable all banks (Register 0x24 = 0x00)
- Repeat the procedure for the HVLED2 and HVLED3 strings