USL3631

Preliminary

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

5 of 6

www.unisonic.com.tw

QW-R103-076.d



APPLICATION INFORMATION(Cont.)

Over Voltage Protection

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:

OVP

PK

OVP

V

I

L

T



≈

Where，

Vovp is the open circuit protection voltage

And then the Rovp resistor value can be calculated by the equation:

6

10

9

1

≈





p

To



(kΩ)

Protection Function

The UTC USL3631 offers rich protection functions to improve the system reliability, including LED open/short

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.

then the system will re-start again. If the fault condition is removed, the system will recover to normal operation.

Thermal Regulation

The UTC USL3631 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 USL3631 PCB layout:

Bypass Capacitor

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.