C2473 datasheet(8/18 Pages) ETC2

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C2472, C2473 and C2474

Datasheet

RDFC Controllers for Offline Applications

Product data

Page 8 of 18

DS-1423-0709C

26-Sep-2007

Resonance Control

The natural resonance of the transformer and associated components is deduced from the current flowing

into and out of the COL pin via the collector coupling capacitor.

The voltage sensed on the COL pin is used to control saturation of the primary switch transistor (Q1 in

Figure 3) during the on-period (see “Optimised Base Drive”). During the off-period, timing of the resonance is

detected via the current in and out of the pin, which has a low impedance path to GND during this time. Rate

of change of voltage at the transformer primary causes current into or out of the COL pin, which is processed

to measure the resonance period TRES and to find the optimum turn-on time for the following conversion

cycle. The resonant period is also used to determine the maximum on-time of the primary switch transistor,

so that

TON = 0.75 x TRES

The maximum duty cycle (DNORMAL) is therefore nominally 43%. On-time of the switch is controlled to manage

power delivery and is reduced in both low-load and overload conditions. The minimum on-time in overload is

determined by the internal CS blanking, specified as TCSBLANK.

At turn-on of the primary switching transistor, its collector voltage can fall very rapidly, with correspondingly

large current out of or in to the COL pin via the coupling capacitor (Ccol). An on-chip clamp transistor,

controlled by an internal signal called ACTICLAMP, provides a low resistance path to GND. This transistor is

turned on shortly before turn-on of the primary switch and remains on until time TACT after turn-on of the

primary switch. It is then turned off during the remainder of the primary switch on-period. In some

applications, the current through the coupling capacitor may develop sufficient voltage across the clamp

transistor to cause conduction of the ESD protection diodes. This is permissible up to a limit ICOL which is

specified in ABSOLUTE MAXIMUM RATINGS. If, due to application design, the capacitor current could

exceed this level, external protection diodes and a resistor must be provided ( Dcol1, Dcol2 and Rcol in

Figure 3).

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