UCC28810

UCC28811

Current Error Amplifier, Soft-Start, and Voltage Limit Amplifier

Component U4A, shown in Figure 24, fulfills the function of current error amplifier in this application.. A fixed

threshold is set by R39 and R44 and the current on the sense resistor, as represented by the signal from the

triac dimmer detection circuit, is compared to this voltage by U4A. R47 sums in the PWM dimming signal from

the triac dimming comparator. The voltage divider formed by R37 and R48 provides a maximum output of 2.5 V,

and the transistor Q9 reduces the voltage at the non-inverting input of the TLV272 to regulate the current in the

LED string.

When the supply is starting up, C30 and C31 provide a soft start set by the divider resistance and capacitance

value. After the primary soft start charges up, this secondary closed-loop soft start takes control and prevents

overshoot of the supply at startup. The soft-start time provided by the secondary soft start should be longer than

the time it takes for the power stage to fully charge the output capacitors, so that overshoot does not occur.

The second comparator of the TLV272 component, shown as U4B in Figure 24, provides a voltage limit.

Because the voltage at the non-inverting input of the TLV272 cannot go higher than 2.5 V, U4B can provide an

effective maximum voltage limit by increasing the LED current in the optocoupler when the voltage is moving

through a range determined by R35, R38, and R42. The gain is set by R45, and C27 ensures stability on the

amplifier. The voltage limit amplifier is not an integrator, it has a fixed gain. Two integrators in series would cause

stability issues due to phase shift. R41 is provided as a provision for disconnecting the current error amplifier and

regulating output voltage if desired. R49 and Q9 could be depopulated, and R41 and C27 could be changed to

make a voltage error amplifier that is an integrator.

Figure 24. Current Error Amplifier, Soft-Start, and Voltage Limit Amplifier

Copyright © 2008, Texas Instruments Incorporated

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Product Folder Link(s): UCC28810 UCC28811