TS19450
Taiwan Semiconductor
Document Number: DS_P0000187 6 Version: B15
APPLICATION INFORMATION
The TS19450 is optimized to drive buck LED drivers using open-loop peak current mode control. This method of
control enables fairly accurate LED current control without the need for high side current sensing or the design of
any closed loop controllers. The IC uses very few external components and enables both linear and PWM dimming
of the LED current.
A resistor connected to the RT pin programs the frequency of operation (or the off-time). The oscillator produces
pulses at regular intervals. These pulses set the SR flip-flop in the TS19450 which causes the GATE driver to turn
on. The same pulses also start the blanking timer which inhibits the reset input of the SR flip flop and prevent false
turn-offs due to the turn-on spike. When the FET turns on, the current through the inductor starts ramping up. This
current flows through the external sense resistor R
CS
and produces a ramp voltage at the CS pin. The comparators
are constantly comparing the CS pin voltage to both the voltage at the LD pin and the internal 250mV. Once the
blanking timer is complete, the output of these comparators is allowed to reset the flip flop. When the output of
either one of the two comparators goes high, the flip flop is reset and the GATE output goes low. The GATE goes
low until the SR flip flop is set by the oscillator. Assuming a 30% ripple in the inductor, the current sense resistor
R
CS
can be set using:
Constant frequency peak current mode control has an inherent disadvantage – at duty cycles greater than 0.5, the
control scheme goes into sub harmonic oscillations. To prevent this, an artificial slope is typically added to the
current sense waveform. This slope compensation scheme will affect the accuracy of the LED current in the
present form. However, a constant off-time peak current control scheme does not have this problem and can easily
operate at duty cycles greater than 0.5 and also gives inherent input voltage rejection making the LED current
almost insensitive to input voltage variations. But, it leads to variable frequency operation and the frequency range
depends greatly on the input and output voltage variation. TS19450 makes it easy to switch between the two
modes of operation by changing one connection (see oscillator section).
Input Voltage Regulator
The TS19450 can be powered directly from its VIN pin and can work from 8.0V~450VDC at its VIN pin. When a
voltage is applied at the VIN pin, the TS19450 maintains a constant 7.5V at the VDD pin. This voltage is used to
power the IC and any external resistor dividers needed to control the IC. The VDD pin must be bypassed by a low
ESR capacitor to provide a low impedance path for the high frequency current of the output GATE driver.
The TS19450 can be also operated by supplying a voltage at the VDD pin greater than the internally regulated
voltage. This will turn off the internal linear regulator of the IC and the TS19450 will operate directly off the voltage
supplied at the VDD pin. Please note that this external voltage at the VDD pin should not exceed 12V.
Although the VIN pin of the TS19450 is rated up to 450V, the actual maximum voltage that can be applied is limited
by the power dissipation in the IC. For example, if an SOP-8 (junction to ambient thermal resistance Rθ
JA
= 128°C/
W) TS19450 draws about I
IN
= 2.0mA from the VIN pin, and has a maximum allowable temperature rise of the
junction temperature limited to about ∆T=10ºCm the maximum voltage at the VIN pin would be:
In these cases, to operate the TS19450 from higher input voltages, a Zener diode can be added in series with the
VIN pin to divert some of the power loss from the TS19450 to the Zener diode. In the above example, using a 100V
Zener diode will allow the circuit to easily work up to 450V.
