Zxld1322 – Diodes ZXLD1322 User Manual
Page 9
ZXLD1322
ZXLD1322
Document number: DS32166 Rev. 3 - 2
9 of 17
April 2010
© Diodes Incorporated
A Product Line of
Diodes Incorporated
ADJ pin
The ADJ pin is connected to the internal 500mV reference (V
REF
) via a 100k resistor. This biases the ADJ pin to the
reference voltage and defines nominal 100% LED current.
The ADJ pin can be overdriven with an external dc voltage between 50mV and 500mV to reduce the LED current
proportionally between 10% and 100% of the nominal value.
LED current can also be adjusted by applying a low frequency PWM signal to the ADJ pin to turn the device On and Off.
This will produce an average output current proportional to the duty cycle of the control signal.
The device can be shut down by shorting the ADJ pin to ground, or pulling it to a voltage below 28mV with a suitable open
collector NPN or open drain NMOS transistor. In the shutdown state, most of the circuitry inside the device is switched off
and residual quiescent current will be typically 12
μA.
Thermal control of LED current
The 'Thermal compensation current' circuit produces a sourcing current (Itc) which is zero for voltages above 75mV on T
ADJ
and increases to 100
μA when T
ADJ
falls to 50mV. This current is summed into the control node and subtracted from the
demand current, causing LED current to reduce from 100% down to zero over this input range. The potential divider,
consisting of a fixed resistor Rt and an NTC Thermistor Rth between V
REF
and ground, defines the voltage on T
ADJ
and sets
the threshold temperature. Further details are given in the application notes.
The Thermal Control feature can be disabled by leaving the T
ADJ
pin floating, or by connecting it to VREF.
Over-temperature shutdown
The ZXLD1322 incorporates an over-temperature shutdown circuit to protect the device against damage caused by excess
die temperature, resulting from excessive power dissipation in the switch. The output of the 'Over-temp Shutdown' circuit will
go high when the die temperature exceeds 150°C (nominal). This will turn off the drive to the switch during normal operation.
Operation will resume when the device has cooled to a safe level.