TS19450
Taiwan Semiconductor
Document Number: DS_P0000187 7 Version: B15
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
I
IN
≈ 1mA + Q
G
x f
S
In the above equation, f
S
is the switching frequency and Q
G
is the GATE charge of the external MOSFET (which
can be 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
external 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 between the external sense resistor (R
CS
) and the CS pin.
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
inductances 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
oscillator time period t
OSC
is given by:
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.
PWM Dimming
PWM Dimming can be achieved by driving the PWMD pin with a low frequency square wave signal. When the
PWM signal is zero, the GATE driver is turned off and when the PWMD signal if high, the GATE driver is enabled.
Since the PWMD signal does not turn off the other parts of the IC, the response of the TS19450 to the PWMD
signal is almost instantaneous. The rate of rise and fall of the LED current is thus determined solely by the rise and
fall times of the inductor current.
To disable PWM dimming and enable the TS19450 permanently, connect the PWMD pin to VDD.
