UL22L-SH2-R datasheet(8/13 Pages) UTC

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UL22

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

8 of 13

www.unisonic.com.tw

QW-R125-088.a



FUNCTIONAL DESCRIPTION (Cont.)

LED Current Setting

LEDs current sensing resistors R10 and R17 connected to FB pin are selected for the LEDs current setting. The

LEDs current can be calculated by

()

ohm

252

ILED

Open LED Protection (OVP)

When either one of the LED strings is opened, the control loop intends to regulate the rated current flowing

through LEDs. The output voltage rises, reflected voltage on the auxiliary winding and the voltage at VDD will

therefore increase. UTC UL22 provides VDD pin Over Voltage Protection which stops all switching operation when

VDD voltage exceeds 25.5V and enters auto-recovery (hiccup) mode. This temporary shutdown of operation does not

allow circuit to resume until VDD drops to 9.5V followed by the rise to above 15.0V.

Under abnormal condition VDD level continues to rise, the. Internal to the chip, VDD is clamped at 28.5V with 20mA

current sinking capability.

The OVP protection threshold can therefore be designed by the following equations:

( )

()

Fig

;

Flyback

AUX

SEC

OVP

OUT

( )

()

Fig

;

Boost

Buck

AUX

OVP

OUT

( )

()

Fig

;

Buck

AUX

OVP

OUT

where NSEC is the number of turns of the secondary winding, and NAUX is the number of turns of the auxiliary

winding.

Short LED Protection

When a number of LEDs in a string are shorted, current flowing through the sense resistors R10 and R17 will be

increased. To prevent any damage, UTC UL22 provides Over Current Protection. When the voltage at FB pin

reaches 0.8V, the controller will turn off the driver signal DRV to the power MOSFET Q1. This operation does not

allow circuit to resume until VDD drops below 9.5V followed by the rise to above 15.0V.

If the output terminal for the LED load is shorted to ground, there is no sense signal at FB pin, and current flowing

through the power transformer rises sharply. Once the CS pin voltage exceeds 0.8V threshold, the controller turns

off the output driver. It will not be turned on again until the next switching cycle starts provided VDD voltage is still

above 9.5V. This cycle-by-cycle current-limit function prevents external components from being damaged under the

fault condition.

As the short-circuit condition continues, the auxiliary winding, coupled to the secondary winding, cannot support

and sustain the VDD bias voltage. The VDD voltage will drop below 9.5V and the controller will shut off the DRV signal.

Note that when controller is shut off, the rectified input AC voltage will supply the charging current for VDD via R1,

R2 and R3 start-up resistors. The current, however cannot sustain the operating current of controller. Therefore, the

VDD voltage will fall gradually, and DRV signal will be shut off. VDD will then be charged to 15.0V, the circuit operates

as a hiccup mode under the short-circuit condition.

Frequency Fold-back

A large current may flow in the primary-side of the transformer when output short circuit to ground. Even though

the VDD voltage will fall due to the collapse of the auxiliary winding voltage, but the 750ns leading-edge blanking time

blocks off the control function which may cause transformer to be saturated before VDD drops below 9.5V. In order to

protect the power MOSFET Q1 and other components from being damaged, UTC UL22 provides a frequency

fold-back function. When the voltage at CS pin exceeds 1.15V, the DRV signal will be interrupted and rested for 3

cycles followed by the next pulse output.

Thermal Protection

Thermal protection occurs when the IC experiences a high junction temperature. The MOSFET drive DRV is

turned off once the junction temperature exceeds approximately 140°C. It resumes normal operation when the

junction temperature drops to or below approximately 110°C.

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