Datasheet

15
MSL2021 [DATASHEET]
42062A–LED–02/2013
10. Fault Conditions
The MSL2021 detects fault conditions, and takes corrective action when faults are verified.
String open circuit and LED short circuit conditions of the color-adjust string are monitored. When one of these faults
occurs, FLTB pulls low to indicate a fault condition and the color-adjust LEDs turn off. Read Fault Status register 0x23 to
determine the fault type. Clear these faults by toggling EN low then high. Faults that persist re-establish the fault
response. Mask string faults using Fault Disable register 0x22.
For the main LED string, when an open LED occurs, the voltage of the AC/DC or DC/DC input power supply reaches the
maximum allowed.
Over-temperature protection puts the device to sleep when the die temperature is above 133

C. The device turns back
on when the die temperature falls below 118

C, and normal operation resumes. While asleep, the I
2
C interface remains
active; see “Fault Disable register (FAULT, 0x22)” and “Fault Status register (FAULTSTAT, 0x23), Read Only” on page
29 for more information about thermal shutdown.
Table 10-1. Fault Conditions, Response and Recovery
11. Applications Information
11.1 Turn-On Sequence
The MSL2021 waits for 250ms after power is applied to allow the AC/DC or DC/DC input supply to establish the default
voltage. Then the MSL2021 starts to optimize the LED string voltage (V
LED
), and then starts to drive the LED strings. It is
critical that the AC/DC or DC/DC converter that powers the LED strings reaches its nominal output voltage in less than
250ms after power is applied. When the 250ms start-up delay is complete, the efficiency optimizer adjusts the LED
voltage to the proper level to drive the main string. After the voltage is set, normal PWM operation begins for both the
main and color-adjust strings. This turn-on sequence allows the light to come up at the proper color and intensity without
flashing or flicker.
11.2 Setting the Main String Current with R
S
The Main string LED on-current regulates by monitoring the voltage at the S pin, the main string MOSFET source resistor
connection. The default feedback voltage at the S pin is 200mV. Choose the string current sense resistor R
S
using:
where I
LED
is the main string regulation current. The main string reference voltage (MREF) register 0x20 sets the
feedback voltage, to 200mV, at 2mV per LSB. The regulation voltage, V
S(FB)
, is:
Fault Response Recovery action
Die Temperature > 133

C Asleep (I
2
C still active)
When die temperature falls below 118

C operation
resumes as if EN is pulled high
Color-adjust string has
shorted LEDs
Color-adjust string turns off, FLTB pulls
low, and bit 0 of the Fault Status register
0x23 sets high
Correct the short condition in LED string. Toggle EN low to
high to resume operation
Color-adjust string is open
circuit
Color-adjust string turns off, FLTB pulls
low, and bit 1 of the Fault Status register
0x23 sets high
Correct the open condition in LED string. Toggle EN low to
high resume operation
R
S
0.2
I
LED
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