UCC28810_11 Datasheet, PDF (16/33 Pages) Texas Instruments – LED LIGHTING POWER CONTROLLER

UCC28810
UCC28811
SLUS865 – OCTOBER 2008.............................................................................................................................................................................................. www.ti.com
High Voltage Startup Circuit
In LED lighting applications it is important that the light source starts quickly after the input power is switched on.
In the circuit shown in Figure 18, the FET Q1 is turned on at a rate determined by R2 plus R3 and C4. The
source follows the gate as it rises. When the source reaches approximately 18.7 V, Q3 prevents the source from
increasing, and the UCC28810 starts. When the supply begins to deliver power to the output, V_BIAS is higher than
12.7 V, and Q2 lowers the gate voltage of Q1 to turn off the startup current path and improve efficiency. For
single range input supplies, or supplies that can start more slowly, R4, R8, and D9 can be populated to start the
supply. However, the power dissipation in normal operation is much higher and reduces the efficiency.
Figure 18. High-Voltage Startup Circuit
Primary-Side Soft Start
The circuit shown in Figure 19 provides an open-loop soft-start that allows the EAOUT pin to slowly rise on the
primary side until the secondary-side error amplifier and soft-start function can take over control of the power
stage.
When V_VDD is applied to the device, C20 is slowly charged to one-half of the voltage on the VDD pin and holds
the EAOUT pin to the voltage on C20 plus the V_BE of Q6. As the voltage on C20 slowly rises, the EAOUT pin
tracks it until the voltage on C20 is above the normal operating point of the EAOUT pin. At some point, the
secondary-side error amplifier takes control of the EAOUT pin as it also has a slowly ramping reference that
provides a closed-loop soft start.
Figure 19. Primary-Side Soft Start
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