USL3533G-S08-R Datasheet, PDF (5/6 Pages) Unisonic Technologies – LINEAR INTEGRATED CIRCUIT

USL3533
Preliminary
LINEAR INTEGRATED CIRCUIT
 APPLICATION INFORMATION(Cont.)
Over Voltage Protection
The over voltage protection can be programmed by the R_OVP pin resistor. The R_OVP pin voltage is 0.5V.
When the LED is open circuit, the output voltage increases gradually, and the demagnetization time gets shorter.
The demagnetization time at OVP---- Tovp can be calculated by the open circuit protection voltage:
TOVP
≈^{L  IPK}
VOVP
Where，
Vovp is the open circuit protection voltage
And then the Rovp resistor value can be calculated by the equation:
ROVP ≈19  Toνo  10⁶ (kΩ)
Protection Function
The UTC USL3533 offers rich protection functions to improve the system reliability, including LED open/short
protection, CS resistor short protection, V_CC under voltage protection, thermal regulation. When the LED is open
circuit, the system will trigger the over voltage protection and stop switching.
When the LED short circuit is detected, the system works at low frequency (about 5kHz), and the CS pin turn off
threshold is reduced to 200mV. So the system power consumption is very low. At some catastrophic fault condition,
such as CS resistor shorted or inductor saturated, the internal fast fault detection circuit will be triggered, the system
stops switching immediately.
After the system enters into fault condition, the V_CC voltage will decrease until it reaches the UVLO threshold,
then the system will re-start again. If the fault condition is removed, the system will recover to normal operation.
Thermal Regulation
The UTC USL3533 integrates thermal regulation function. When the system is over temperature, the output
current is gradually reduced; the output power and thermal dissipation are also reduced. The system temperature is
regulated and the system reliability is improved. The thermal regulation temperature is set to 150°C internally.
PCB Layout
The following rules should be followed in UTC USL3533 PCB layout:
Bypass Capacitor
The bypass capacitor on V_CC pin should be as close as possible to the V_CC Pin and GND pin.
R_OVP Pin
The R_OVP resistor should be as close as possible to the R_OVP Pin.
Ground Path
The power ground path for current sense should be short, and the power ground path should be separated from
small signal ground path before connecting to the negative node of the bulk capacitor.
The Area of Power Loop
The area of main current loop should be as small as possible to reduce EMI radiation, such as the inductor, the
power MOSFET, the output diode and the bus capacitor loop.
NC pin
The NC pin should be connected to GND (pin1).
Drain Pin
To increase the copper area of DRAIN pin for better thermal dissipation. However too large copper area may
compromise EMI performance.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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QW-R103-100.a