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Número de pieza USL1601
Descripción CONSTANT CURRENT CONTROLLER
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USL1601 datasheet

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USL1601 pdf
USL1601
Preliminary
CMOS IC
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:
ton
=
L ×IPK
VIN - VLED
The L is the inductance.
The IPK is the peak current in inductor.
The VIN is the input rectified voltage.
The VLED is the voltage on LED.
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:
toff
=
L ×IPK
VLED
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:
L
=
VLED × (VIN - VLED )
f ×IPK × VIN
The UTC USL1601 internally set the minimum off time to 4µs and maximum off time to 130µs. When the
inductance is very small, the toff may goes below the minimum off time and the inductor current becomes
discontinuous. So the output LED current will be smaller than the setting value. If the inductance is too large, the toff
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
output voltage is very high, the ton may goes beyond the maximum on time. The power MOSFET will be turned off
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
The bypass capacitor on VCC pin should be as close as possible to the VCC and GND pins.
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.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
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