ZXLD1321

Issue 1 - January 2008

10

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© Zetex Semiconductors plc 2008

Low voltage operation (start-up mode)

For supply voltages below 2V, the normal control loop will have insufficient headroom to operate

reliably. This condition is detected by the 'under-voltage comparator', which compares a fraction

of the internal supply voltage (Vcc) against VREF. When the comparator output is active

(Vcc<1.8V), the output of the normal switch drive circuit is disabled and an alternative 'Start-up

oscillator and driver' enabled. The start-up oscillator provides a nominal 50kHz fixed frequency

drive signal to the base of the switch transistor, which is independent of VADJ and the voltage on

CFB. Under low voltage conditions, the peak current in the coil ramps to approximately 25% of

the normal value and the "OFF" time is fixed.

The low voltage start-up mode allows the device to operate down to 1.2V nominal. This allows

the chip to work from a single cell in boost mode and by configuring the device in Bootstrap

mode, normal operation of the control loop will occur once the output has risen above 2.2V.

Details of Bootstrap-Boost mode are given in the application notes.

ADJ pin

The ADJ pin is connected to the internal 500mV reference (VREF) 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 12µA.

Thermal control of LED current

The 'Thermal compensation current' circuit produces a sourcing current (Itc) which is zero for

voltages above 75mV on TADJ and increases to 100µA when TADJ 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 VREF and ground, defines the voltage on TADJ

and sets the threshold temperature. Further details are given in the application notes.

The Thermal Control feature can be disabled by leaving the TADJ pin floating, or by connecting

it to VREF.

Over-temperature shutdown

The ZXLD1321 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.