NCP1339
www.onsemi.com
22
Fault
NTC
Vaux
Latch
Vfault(OVP)
Vfault(OTP)
S
R
Q
Q
BONOK
Ifault(OTP)
Rfault(clamp)
Vfault(clamp)
5 V
Figure 50. Fault Detection Schematic
As a matter of fact, the controller operates normally while
the Fault pin voltage is maintained within the upper and
lower fault thresholds. Upper and lower fault detector have
blanking delays to prevent noise from triggering them. Both
blanking timers (t
delay(Fault_OVP)
and t
delay(Fault_OTP)
) are
typically 27.5 ms.
When the part is latched−off, the drive is immediately
turned off. Also, V
CC
drops and stabilize to the 5.5−V
V
CC(bias)
level. The power supply needs to be un−plugged to
reset the part as a result of a BONOK (BO fault condition)
and/or the X2 circuitry activation.
PSM mode cannot be triggered in latched−off mode.
Zero Current Detection
The NCP1339 integrates a quasi−resonant (QR) flyback
controller. The power switch turn−off of a QR converter is
determined by the peak current set by the feedback loop. The
switch turn−on is determined by the transformer
demagnetization. The demagnetization is detected by
monitoring the transformer auxiliary winding voltage.
Turning on the power switch once the transformer is
demagnetized or reset reduces switching losses. Once the
transformer is demagnetized, the drain voltage starts ringing
at a frequency determined by the transformer magnetizing
inductance and the drain lump capacitance eventually
settling at the input voltage. A QR controller takes
advantage of the drain voltage ringing and turns on the
power switch at the drain voltage minimum or “valley” to
reduce switching losses and electromagnetic interference
(EMI).
As sketched by Figure 51, a valley is detected once the
ZCD pin voltage falls below the QR flyback
demagnetization threshold, V
ZCD(th)
, typically 55 mV. The
controller will switch once the valley is detected or
increment the valley counter depending on FB voltage.
Timeout
The ZCD block actually detects falling edges of the
auxiliary winding voltage applied to the ZCD pin. At
start−up or other transient phases, the ZCD comparator may
be unable to detect such an event. Also, in the case of
extremely damped oscillations, the system may not succeed
in detecting all the valleys required by VLO operation (see
next section). In this condition, the NCP1339 ensures
continued operation by incorporating a maximum timeout
period that resets when a demagnetization phase is detected.
The timeout signal substitutes ZCD signal for the valley
counter. Figure 51 shows the timeout period generator
circuit schematic. The steady state timeout period, t
(out2)
, is
set at 6 ms.
During startup, the output voltage is still low leading to
long demagnetization phases difficult to detect since the
auxiliary winding voltage is small as well. In this condition,
the 6−ms steady−state timeout is generally shorter than the
inductor demagnetization period and if used to restart a
switching cycle, it can cause continuous current mode
(CCM) operation for few cycles until the voltage on the ZCD
pin is high enough for proper valleys detection. A longer
timeout period, t
(out1)
, (typically 100 ms) is therefore set
during soft−start to prevent CCM operation.
In VLO operation, the timeout periods of time are counted
instead of valleys when the drain−source voltage
oscillations are too damped to be detected. For instance, if
the circuit must turn on at the fifth valley and if the ZCD
ringing only enables to detect:
• Valleys 1 to 4: the circuit generates a DRV pulse 6 ms
(steady−state timeout delay) after valley 4 detection.
• Valleys 1 to 3: the timeout delay must run twice so that
the circuit generates a DRV pulse 12 ms after valley 3
detection.
