NCP12700
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17
Fault (FLT) Pin
The FLT pin is intended to provide the system with a NTC
interface for thermal protection and a pull−up fault which
can be coupled to the auxiliary winding to provide output
over−voltage protection. The FLT pin can also be used as a
general purpose fault where it interfaces with a simple
pull−down BJT, open collector comparator, or optocoupler
for monitoring of secondary side faults. The internal
circuitry includes a precision pull−up current source, I
FLT
, of
85 mA and a window comparator to signal a fault whenever
the pin voltage goes below the OTP fault threshold,
V
FLT(OTP)
, of 0.5 V or above the OVP fault threshold,
V
FLT(OVP)
, of 3 V. Both of the fault comparators also include
a delay filter to prevent noise or glitches from setting the
fault. The over−temperature fault filter, t
OTP(DLY)
, is
nominally 20 ms and the over−voltage fault filter, t
OVP(DLY)
,
is typically 5 ms. An external filter capacitor is also
advisable.
Both faults have an option to permanently latch off the
controller or restart after a 1 s auto−recovery period. The
OVP fault is intended to monitor an auxiliary winding and
when triggered, the controller will disable switching which
will inhibit the aux winding from generating voltage and
allow the controller to restart after the auto−recovery timer
has expired. If the OVP fault comparator is continuously
held above 3 V, the NCP12700 will remain in the fault mode
and not restart.
The OTP fault detection is gated by the SS_END flag to
prevent the comparator from triggering while the external
filter capacitor charges up. Once the SS_END flag is set the
OTP fault can be acknowledged so there is a practical limit
on the size of the filter capacitor. Equation 6 and Equation 7
should assist the user with properly setting the external
capacitance of the fault pin.
t
SS_END
+
C
SS
V
SS_END
I
SS
(eq. 6)
C
FLT
t
I
FLT
t
SS_END
V
FLT(OTP)
(eq. 7)
When the OTP fault is triggered the NCP12700 will again
disable drive pulses and transition into a fault mode. The
OTP fault is auto−recoverable based on the auto−recovery
timer and a hysteresis set by the V
FLT(REC)
threshold of
0.9 V. The auto−recovery timer must expire and the voltage
at the fault pin must exceed 0.9 V. This methodology
guarantees a minimum amount of time for the system to
recover from thermal overstress but will not allow the
converter to restart unless the hysteresis is met. Given the
I
FLT
and V
FLT(OTP)
specifications the critical NTC
resistance for declaring a fault is ~ 5.9 kW. The critical
resistance for recovering from the OTP fault becomes ~
10.6 kW. This fault recovery threshold provides for about
~20°C of hysteresis for many NTC resistors.
Summary of Fault Handling
The NCP12700 has 6 fault detectors which will place the
device into the fault mode. In the fault mode switching is
inhibited and the controller bias is maintained by the HV
startup regulator. The controller also reduces current
consumption to I
CC(FLT)
, 500 mA maximum, so that the
regulator is not thermally overstressed. The NCP12700
remains in the fault mode until the fault signal has been
cleared and/or the auto−recovery timer has expired. The
fault signal can be cleared when the fault detector senses that
the fault has been removed or by a controller reset which
occurs if V
CC
drops below V
CC(OFF)
or the UVLO pin is
pulled below the V
RST(th)
level. Below is a brief summary
of the different fault detectors and their basic operation.
• Thermal Shutdown (TSD): Thermal shutdown is
declared when the internal junction temperature of the
device exceeds the T
SHDN
temperature of 165°C. The
thermal shutdown fault is auto−recoverable when the
device junction temperature reduces to T
SHDN
–
T
SHDN(hys)
where T
SHDN(hys)
is typically 25°C.
• Fault OTP: An OTP fault is declared when fault pin
voltage decreases below the V
FLT(OTP)
threshold of
0.5 V and the OTP filter, t
OTP(DLY)
, expires. The OTP
filter delay is typically 20 ms. The OTP fault is blanked
at startup until the SS_END flag has been set to allow
the external capacitance of the pin to charge up. For the
device to recover from the Fault OTP, the
auto−recovery timer must expire and the voltage at the
fault pin must recover to V
FLT(REC)
value of 0.9 V.
• Fault OVP: The OVP fault is declared when fault pin
the voltage exceeds the V
FLT(OVP)
threshold of 3 V and
the OVP filter, t
OVP(DLY)
, expiring. The OVP filter
delay is typically 5 ms. The OVP fault is cleared when
the auto−recovery timer expires. There is no hysteresis
on the OVP fault but if the pin voltage is permanently
held above 3 V, DRV will pulses will be permanently
inhibited.
• Overload (OVLD): The OVLD fault is set when the
overload timer, t
OVLD
, expires. The overload timer is
an integrating timer which counts up as long as the
Current Limit comparator is terminating DRV pulses.
The typical value for t
OVLD
is 30 ms. The controller
will recover from the OVLD fault when the
auto−recovery timer expires.
• SCP Fault: The SCP fault occurs when the N
SCP
counter has reaches 4 consecutive DRV pulses
terminated by the SCP comparator. The controller will
recover from the SCP fault when the auto−recovery
timer expires.
• V
CC
OVP: The V
CC
OVP is set when V
CC
voltage
exceeds the V
CC(OVP)
threshold of 28 V and the V
CC
OVP filter, t
VCC_OVP(DLY)
, expires. The V
CC
OVP
filter is typically 3 ms. V
CC
OVP will permanently latch
the device off so that it remains in the Fault mode
indefinitely until the controller is reset.
