TS19450

AC/DC WLED Driver with External MOSFET

Universal High Brightness

5/8

Version: A10

Application Information (Continue)

The input current drawn from the VIN pin is a sum of the 1.0mA current drawn by the internal circuit and the current

drawn by the GATE driver (which in turn depends on the switching frequency and the GATE charge of the external

obtained from the datasheet of the MOSFET).

Current Sense

The current sense input of the TS19450 goes to the non-inverting inputs of two comparators. The inverting terminal of

one comparator is tied to an internal 250mV reference whereas the inverting terminal of the other comparator is

connected to the LD pin. The outputs of both these comparators are fed into an OR GATE and the output of the OR

GATE is fed into the reset pin of the flip-flop. Thus, the comparator which has the lowest voltage at the inverting

terminal determines when the GATE output is turned off.

The outputs of the comparators also include a 150~280ns blanking time which prevents spurious turn-offs of the exter-

nal MOSFET due to the turn-on spike normally present in peak current mode control. In rare cases, this internal

blanking might not be enough to filter out the turn-on spike. In these cases, an external RC filter needs to be added

Please note that the comparators are fast (with a typical 80ns response time). Hence these comparators are more

susceptible to be triggered by noise than the comparators of the TS19450. A proper layout minimizing external induc-

tances will prevent false triggering of these comparators.

Oscillator

The oscillator in the TS19450 is controlled by a single resistor connected at the RT pin. The equation governing the

If the resistor is connected between RT and GND, TS19450 operates in a constant frequency mode and the above

equation determines the time-period. If the resistor is connected between RT and GATE, the TS19450 operates in a

constant off-time mode and the above equation determines the off-time.

Linear Dimming

The Linear Dimming pin is used to control the LED current. There are two cases when it may be necessary to use the

Linear Dimming pin.

internal 250mV is used. In these cases, an external voltage divider from the VDD pin can be connected to the LD pin

* Linear dimming may be desired to adjust the current level to reduce the intensity of the LEDs. In these cases, an

external 0~250mV voltage can be connected to the LD pin to adjust the LED current during operation.

To use the internal 250mV, the LD pin can be connected to VDD.

Note:

Although the LD pin can be pulled to GND, the output current will not go to zero. This is due to the presence of a mini-

mum on-time (which is equal to the sum of the blanking time and the delay to output time) which is about 450ns. This

will cause the MOSFET to be on for a minimum of 450ns and thus the LED current when LD = GND will not be zero.

This current is also dependent on the input voltage, inductance value, forward voltage of the LEDs and circuit parasitic.

To get zero LED current, the PWMD pin has to be used.