USL1601

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

5 of 6

www.unisonic.com.tw

QW-R125-033.a



FUNCTION DESCRIPTION(Cont.)

Inductance Calculation

The UTC USL1601 is designed to work in inductor current critical mode, the energy will be stored in the inductor

when the MOSFET is turned on. The turn on time is given by:

LED

IN

PK

on

V

-

V

I

×

L

=

t

The L is the inductance.

When the power MOSFET is turned off, the inductor current will decrease from the peak current to zero. The turn

off time is given by:

LED

PK

off

V

I

×

L

=

t

The MOSFET will be turned on again when it detects the inductor current goes to zero.

The inductance can be calculated by the following equation:

IN

PK

LED

IN

LED

V

×

I

×

f

)

V

-

V

(

×

V

=

L

The UTC USL1601 internally set the minimum off time to 4µs and maximum off time to 130µs. When the

may goes beyond the maximum off time and the inductor current becomes continuous. And the output LED current

will be larger than the setting value. So it’s very important to select a right inductance.

The UTC USL1601 also internally set the maximum on time to 40µs. When the input voltage is very low or LED

even the inductor current still below the setting value. So the output LED current will be smaller than the setting

value.

Protection Functions

The UTC USL1601 has many protection functions, including LED short circuit protection, current sense resistor

short circuit protection and over temperature protection. All of the protection functions are designed to auto-recover.

The over temperature protection circuitry in the UTC USL1601 monitors the die junction temperature after start

up. When the temperature rises to 150°C, the power MOSFET will be shut down immediately and maintains at

switch off condition until the temperature on die falls 30°C below the thermal protection trigger point.

PCB Layout

The following guidelines should be followed in UTC USL1601 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 the negative node of the bus capacitor.

The Area of Power Loop

The area of main current loop should be as small as possible to reduce EMI radiation. And the controller should

be placed away from the heat generator, such as the power diode.