Datasheet

17
Atmel MSL3080 Datasheet
8 String 60mA LED Driver with Integrated Boost Controller
9.0 Application Information
9.1 Bypassing VIN and PVIN
Bypass VIN with a capacitor of at least 1µF. Bypass PVIN to PGND with at least 2µF. Place all bypass capacitors close to the MSL3080.
9.2 Setting the LED Current
Set the on-current for all LED strings with a resistor from ILED to GND. Choose the resistor using:
where I
LED
is the LED full-scale current in Amps.
The maximum LED current per-string is 60mA. Driving all eight strings with 60mA at high duty cycles and elevated ambient temperatures
requires proper thermal management to avoid over-temperature shutdown. Connect the exposed pad (EP) to a large copper ground
plane for best thermal and electrical performance.
9.3 Fault Monitoring and Automatic Fault Handling
The MSL3080 monitors the LED strings and boost regulator output voltage to detect LED short-circuit, LED string open-circuit and Boost
Over-voltage faults. String faults latch the open drain fault output FLTB low. A boost over-voltage fault pulls FLTB low but does not latch.
When shorted LEDs are detected in a string, the driver disables and the Efciency Optimizer stops monitoring it. The MSL3080 ags
these strings as faults in registers 0x05 through 0x08, pulls FLTB low and recalibrates the LED power supply voltage. Set the short circuit
voltage threshold with a resistor between SCTH and GND, as explained in the section “Setting the LED Short-Circuit Threshold”
beginning on page 17. For information about the fault registers and the I
2
C compatible serial interface see the
“MSL3040/50/60/80/86/87/88 Programming Guide”.
When an open circuit occurs, the Efciency Optimizer detects a loss of current regulation which must persist for greater than 2 µs to be
detected. The minimum on-time for the strings is 2 µs. In this case the Efciency Optimizer increases the LED voltage (boost regulator
output voltage), in an attempt to bring the LED current back in to regulation. This continues until the voltage is at the maximum level.
The MSL3080 then determines that any LED strings that are not regulating current are open circuit. It disables those strings, ags them
as faults in registers 0x05 through 0x08, pulls FLTB low and recalibrates the LED power supply voltage. When the boost regulator is at
its maximum value ctitious LED short circuit faults can occur. Toggle EN low and then high to clear all faults and return the MSL3080
to controlling and monitoring all LED strings. Fault conditions that persist re-establish fault responses. For information about the fault
registers and the I
2
C compatible serial interface see the “MSL3040/50/60/80/86/87/88 Programming Guide”.
9.4 Setting the LED Short-Circuit Threshold
When a given string, STRn, is sinking LED string current, the fault detection circuit monitors the STRn voltage. Typical optimized STRn
on-voltage is 0.5V. When one or more LED’s of a string are shorted, the STRn voltage increases above the nominal. When the voltage
is above the Short-Circuit Threshold the fault circuit generates an LED short circuit fault. In most cases, two LEDs in a string must be
shorted to cause a short circuit fault, but because LED V
F
differs for different LEDs, the number of shorted LEDs required to generate a
fault varies.Set the LED short-circuit threshold with a resistor from SCTH to GND using:
Table 9.1 Short Circuit Threshold Resistor
R
SCTH
Threshold Voltage
1.0kΩ (or GND) 4.9V
27kΩ 5.8V
68kΩ 6.8V
330kΩ (or open) 7.6V
R
SCTH
is measured only at power up, and when EN is taken high, to set the threshold level. Additionally, register 0x04 holds the Short
Circuit Threshold level, changeable through the I
2
C compatible serial interface. For information about the Short Circuit Threshold register
and the serial interface see the “MSL3040/50/60/80/86/87/88 Programming Guide”.