AN3093 datasheet(25/29 Pages) STMICROELECTRONICS

Il numero della parte	AN3093

Spiegazioni elettronici	ALTAIR05T-800 5 W wide range CV-CC optoless adapter demonstration board

PDF	29 Pages

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Produttore elettronici	STMICROELECTRONICS [STMicroelectronics]

Homepage	http://www.st.com

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25 / 29 page

AN3093

Modes of operation

Doc ID16539 Rev 1

25/29

Figure 23.

Constant current operation: switching cycle waveforms

A.3

Current comparator speed

The propagation delay of the internal current comparator used in the CC control block

causes the MOSFET to be switched off with a peak current higher than the theoretical value:

this current overshoot is dependent on the input voltage and introduces an error in the

calculated CC setpoint, rendering Equation 5 no longer valid.

In order to make the cycle-by-cycle current limitation and the CC setpoint almost

independent of the input voltage, a feed-forward function has been implemented, according

the schematic in Figure 24.

During the MOSFET's ON-time, the current sourced from the ZCD/FB pin is mirrored inside

the “feed-forward logic” block to provide the feed-forward current, IFF.

This feed-forward current is proportional to the input voltage and provides an offset on the

current sense to compensate the propagation delay of the current comparator.

In order to achieve full compensation, the RZCD resistor should be selected based on the

following equation:

Equation 6

ZCD

AUX

PRI

-------------

SENSE

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