UL22L-SH2-R datasheet(6/13 Pages) UTC

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UL22

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 13

www.unisonic.com.tw

QW-R125-088.a



FUNCTIONAL DESCRIPTION

Refer to both the Block Diagram in Figure 1 and a reference design circuit in Figure 4 for the following

discussions. All parameters mentioned below are typical values.

Start-up Circuit

Applying AC power to the LED driver shown in Figure 4, initiates the operation. To minimize power losses, large

start-up resistors are preferred to be connected between rectified line voltage and VDD pin. During start-up, capacitor

C5 is charged up to 15.0V (VDD-ON) by resistors R1, R2 and R3. The turn-on and turn-off thresholds of the controller

are 15.0V and 9.5V (VDD-OFF) respectively. Once the VDD exceeds 15.0V, all of the internal circuits are activated.

Referring to Figure 2, the hysteresis voltage for VDD pin is implemented to keep IC operating for a period of time with

external hold-up capacitor C5. When VDD drops below 9.5V, the controller shuts off the driver output yet keeping

color control function active. The controller will be in shut-off mode with control logic being reset when VDD drops

below 4.0V.

Power Off

VDD-ON

VDD-OFF

VDD-RESET

VDD

Figure 1. VDD Power Supply Hysteresis

Power On

Power Off

Only Color Control

Function On

Hysteresis

on to off

Hysteresis

off to color

control

VDD

Turn Off

Control

UTC UL22 operates with 66 KHz, fixed-frequency PWM voltage-mode control for converter operation. It can be

used for providing regulated current for high-brightness LED applications. To keep the converter operating under

discontinuous mode over various conditions, the primary-side inductance of the power inductor T1 must be designed

and operated in Critical Conduction Mode (CRM) at minimum line voltage and maximum load.

During normal operation conditions, current flowing through the LEDs is detected at the sense resistors R10 and

R17 connected to FB pin. The signal at FB pin is processed to compare with a reference voltage by internal OTA

(Operational Transconductance Amplifier) which produces a compensated signal (VCOMP) at COMP pin. The

compensated signal is then compared with an internal saw-tooth signal to generate a pulse-width-modulation signal

for the driver output. When the internal saw-tooth signal intersects with the compensated signal, the switch Q1 will

be turned off. Therefore, the COMP pin voltage level determines the turn-on time of power switch and regulates the

energy stored in power inductor. The higher level voltage at COMP results in larger duty cycle to the driver which

provides higher brightness on LEDs.

Capacitor C6 connected at COMP pin provides a stable close-loop response. The slower the response, the

higher power factor converter provides.

The driver output (DRV pin) provides a maximum duty cycle of 79%. For operating conditions with high

VIN-to-VOUT ratios, the minimum on-time of 250ns should be considered in design of the circuit.

Color Control Function - Free-Dimming

The controller implements a proprietary control technique to turn on a desired LED string (of the two) by using an

AC ON/OFF wall switch. It provides an example of color-temperature control for applications where each string

provide a unique color temperature.

CLK pin monitors a divided voltage signal from AC input through resistor divider (R1, R2, R4 and R5 in Figure 4).

The detected thresholds are set at 1.5V for CLK ON and 1.0V for CLK OFF, which can be designed appropriately

according to the AC input voltage range.

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