L6566A View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566A' PDF : 52 Pages View PDF
Application information
L6566A
5.2
Zero current detection and triggering block; oscillator block
The zero current detection (ZCD) and triggering blocks switch on the external MOSFET if a
negative-going edge falling below 50 mV is applied to the input (pin 11, ZCD). To do so the
triggering block must be previously armed by a positive-going edge exceeding 100 mV.
This feature is typically used to detect transformer demagnetization for QR operation, where
the signal for the ZCD input is obtained from the transformer’s auxiliary winding used also to
supply the L6566A. The triggering block is blanked for TBLANK = 2.5 µs after MOSFET turn-
off to prevent any negative-going edge that follows leakage inductance demagnetization
from triggering the ZCD circuit erroneously.
The voltage at the pin is both top and bottom limited by a double clamp, as illustrated in the
internal diagram of the ZCD block of Figure 8. The upper clamp is typically located at 5.7 V,
while the lower clamp is located at -0.4 V. The interface between the pin and the auxiliary
winding is a resistor divider. Its resistance ratio is properly chosen (see Section 5.11) and
the individual resistance values (RZ1, RZ2) are such that the current sourced and sunk by
the pin be within the rated capability of the internal clamps (± 3 mA).
At converter power-up, when no signal is coming from the ZCD pin, the oscillator starts up
the system. The oscillator is programmed externally by means of a resistor (RT) connected
from pin OSC (13) to ground. With good approximation the oscillation frequency fosc is:
Equation 2
fosc
≈
2 ⋅ 103
RT
(with fosc in kHz and RT in kW). As the device is turned on, the oscillator starts immediately;
at the end of the first oscillator cycle, the voltage on the ZCD pin being zero, the MOSFET is
turned on, therefore starting the first switching cycle right at the beginning of the second
oscillator cycle. At any switching cycle, the MOSFET is turned off as the voltage on the
current sense pin (CS, 7) hits an internal reference set by the line feedforward block, and the
transformer starts demagnetization. If this completes (so a negative-going edge appears on
the ZCD pin) after a time exceeding one oscillation period Tosc=1/fosc from the previous turn-
on, the MOSFET is turned on again - with some delay to ensure minimum voltage at turn-on
– and the oscillator ramp is reset. If, instead, the negative-going edge appears before Tosc
has elapsed, it is ignored and only the first negative-going edge after Tosc turns on the
MOSFET and synchronizes the oscillator. In this way one or more drain ringing cycles are
skipped (“valley-skipping mode”, Figure 9) and the switching frequency is prevented from
exceeding fosc.
Figure 9.
VDS
Drain ringing cycle skipping as the load is gradually reduced
VDS
VDS
TON
TFW
TV
Tosc
Pin = Pin'
(limit condition)
20/52
t
t
t
Tosc
Tosc
Pin = Pin'' < Pin'
Pin = Pin''' < Pin''
Doc ID 13794 Rev 4
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