UCC28810

UCC28811

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

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.

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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