USL3532K

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

4 of 6

www.unisonic.com.tw

QW-R125-053 .a



APPLICATION INFORMATION

The UTC USL3532K is a high performance non-isolated Buck converter specially designed for LED lighting. The

device integrates a 500V power MOSFET. With very few external components, the converter achieves excellent

constant current control. And it does not need auxiliary winding for powering the IC or voltage sensing, thus the

system size and cost is greatly reduced.

Start Up

reaches the start up threshold, the internal circuits start operating. The UTC USL3532K integrates a 17V zener

Constant Current Control

Cycle by Cycle current sense is adjusted in UTC USL3532K, the CS pin is connected to the current sense

comparator, and the voltage on CS pin is compared with the internal 400mV reference voltage. The MOSFET will be

switched off when the voltage on CS pin reaches the Peak Current threshold. The CS comparator includes a 350ns

leading edge blanking time. After the MOSFET switched off, the OFF time controller would open the MOSFET again

at time up. At that moment, CS pin voltage is 0mV usually.

The Delta inductor current is given by:

PK

CS

400

I=

(mA

R

）

The current in LED can be calculated by the equation:

PK

LED

CS

I

200

I=

=

(mA

2R

）

Inductor Selection

The UTC USL3532K works under inductor current critical conduction mode. When the power MOFET is switched

on, the current in the inductor rises up from zero, the on time of the MOSFET can be calculated by the equation:

LED

IN

PK

on

V

V

I

L

t

-





Where,

L is the inductance value

After the power MOSFET is switched off, the current in the inductor decreases. When the inductor current

reaches zero, the power MOSFET is turned on again by IC internal logic. The off time of the MOSFET is given by:

LED

PK

off

V

I

L

t





The inductance can be calculated by the equation:

IN

PK

LED

IN

LED

V

I

freq

V

V

V

L









)

-

(

The freq is the system switching frequency, which is proportional to the input voltage. So the minimum switching

frequency is set at lowest input voltage, and the maximum switching frequency is set at highest input voltage.

The minimum and maximum off time of UTC USL3532K is set at 4.5µs and 240µs, respectively. Referring to the

will operate in discontinuous conduction mode and the output current will be smaller than the designed value. If the

follow design value. So it is important to choose a proper inductance, we recommend user to regulate the inductance

value for 70KHz application.