C2473 datasheet(10/18 Pages) ETC2

10 / 18 page

C2472, C2473 and C2474

Datasheet

RDFC Controllers for Offline Applications

Product data

Page 10 of 18

DS-1423-0709C

26-Sep-2007

IBASPBD is controlled by monitoring the voltage at the COL pin during T PBD; base current is increased

progressively as VCOL rises above threshold VCREF(see Figure 9). The desired on-state VCE of the switching

transistor is set by capacitors Cp and Ccol (see Figure 3), the COL pin capacitance (CINCOL) and VCREF.

C O L

C R E F

B A S P B D

Figure 9: IBASPBD characteristic

Power Control

Load conditions are sensed on a cycle-by-cycle basis via the CS pin. When low levels of output power

demand are detected, the controller progressively reduces the switching duty cycle to reduce power

consumption and to improve output voltage regulation. Power demand causes increase in duty up to the

maximum, or until Overload is detected.

The voltage at the CS pin is compared to two thresholds, one nominally a t GND voltage (VOCPL) to generate

an internal signal OCPL and the other at a negative threshold (VOCPH) to generate an internal signal OCPH.

The controller samples OCPL a short time (TOCPL) after turn-on of the primary switch. A negative voltage at

the CS pin indicates power demand so the controller increases the switching duty up to the maximum;

conversely a positive voltage causes a decrease in duty.

Excessive primary switch current, detected via OCPH, terminates the on -period of the primary switch to limit

power delivery (Overload mode). High levels of overload (when the converter output voltage held is low by

the load) causes OCPH to trigger soon after turn-on of the primary switch. This condition is detected by the

controller sampling OCPH at time TFBTHR after turn-on. If OCPH triggers within this time, the controller

changes to Foldback mode. To prevent mis-triggering, OCPH is blanked for a short period TCSBLANK after

turn-on.

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