UPSL101

Preliminary

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

6 of 8

www.unisonic.com.tw

QW-R125-027.b



FUNCTION DESCRIPTION

The UTC UPSL101 is a primary side control offline LED lighting controller which can achieve high power factor

and accurate LED current for an isolate lighting application in a single converter. The UTC UPSL101 can control the

LED current accurately from the primary side information. It can significantly simplify the LED lighting system design

by eliminating the secondary side feedback components and the opto-coupler. And it integrates power factor

correction function and works in transition mode for reducing the MOSFET switching losses.

The principle of operation can be understood by referring to the block diagram.

Startup

be used to minimize the power loss in application because the start current of UTC UPSL101 is very low. When the

Operating Current

The operating current of UTC UPSL101 is as low as 1.5mA. Good efficiency and very low standby power can be

achieved.

Transition Mode Operation

During the external MOSFET on time, the rectified input voltage is applied across the primary side inductor and

the primary current increases linearly from zero to the peak value. When the external MOSFET turns off, the energy

stored in the inductor forces the secondary side diode to be turn on, and the current of the inductor begins to

decrease linearly from the peak value to zero, this decreasing is also reflected on the auxiliary winding. UTC

UPSL101

performs zero current detection (ZCD) through ZCD pin by monitoring the voltage activity on the auxiliary

windings in series with external resistors. When ZCD pin voltage falls below 0.3V, an internal ZCD comparator is

triggered and generates the turn on signal of the external MOSFET.

LED Constant Current Regulation

The proprietary real current control method allows the UTC UPSL101 controlling the secondary side LED current

from the primary side information. The LED mean current can be calculated approximately as:

CS

LED

R

f

re

V

*

2

N

≈

I

N-Turn ratio of primary side to secondary side

Current Sensing and Leading Edge Blanking

Cycle-by-cycle current limiting is offered in UTC UPSL101. The switch current is detected by a sense resistor into

the CS pin. When the power switch is turned on, a turn-on spike will occur on this resistor. A leading-edge blanking

is built in to avoid false-termination of the switching pulse so that the external RC filtering is no longer needed. PWM

duty cycle is determined by the current sense voltage and the voltage at pin COMP.

Threshold of OCP Selection

The threshold voltage level of OCP is selected by the voltage of OCP_TH pin. The UTC UPSL101 provides the

source current equal to approximately 28μA through OCP_TH pin. So the proper resistor can be connected with the

OCP_TH pin to GND in order to generate the voltage needed. If OCP_TH pin is connected with 33K resistor, the

threshold voltage of OCP is about 1.8V; If OCP_TH pin is connected with 110K resistor, the threshold voltage of

OCP is about 1.5V; If OCP_TH pin is connected with GND directly, the threshold voltage of OCP is about 2.1V.

Voltage of OCP_TH Pin

Connecting State of OCP_TH

Threshold Voltage of OCP

3V

110K

1.5V

0.9V

33K

1.8V

0V

GND

2.1V

Maximum and Minimum On-Time

The minimum on-time of the system is determined by the LEB time. The IC limits the on-time to a maximum time

of approximately 50μs.