FAN100_09 Datasheet, PDF (12/16 Pages) Fairchild Semiconductor – Primary-Side-Control PWM Controller

Protections
The FAN100 has several self-protective functions, such
as Over-Voltage Protection (OVP), Over-Temperature
Protection (OTP), and brownout protection. All the
protections are implemented as auto-restart mode.
When auto-restart protection is triggered, switching is
terminated and the MOSFET remains off. This causes
VDD to fall. When VDD reaches the VDD turn-off voltage of
5V, the current consumed by FAN100 reduces to the
startup current (maximum 10µA) and the current
supplied startup resistor charges the VDD capacitor.
When VDD reaches the turn-on voltage of 16V, FAN100
resumes normal operation. In this manner, the auto-
restart alternately enables and disables the switching of
the MOSFET until the fault condition is eliminated (see
Figure 28).
VDS
Power
On
Fault
Occurs
Fault Removed
VDD
16V
VDD Over-Voltage Protection (OVP)
VDD over-voltage protection prevents damage from over-
voltage conditions. If the VDD voltage exceeds 28V by
open-feedback condition, OVP is triggered. The OVP
has a debounce time (typical 250µs) to prevent false
triggering by switching noise. It also protects other
switching devices from over voltage.
Over-Temperature Protection (OTP)
The built-in temperature-sensing circuit shuts down
PWM output if the junction temperature exceeds 140°C.
Brownout Protection
FAN100 detects the line voltage using auxiliary winding
voltage since the auxiliary winding voltage reflects the
input voltage when the MOSFET is turned on. VS pin is
clamped at 1.15V while the MOSFET is turned on and
brownout protection is triggered if the current out of VS
pin is less than IVS-UVP (typical 180μA) during the
MOSFET conduction.
Pulse-by-pulse Current Limit
When the sensing voltage across the current sense
resistor exceeds the internal threshold of 1.3V, the
MOSFET is turned off for the remainder of the switching
cycle. In normal operation, the pulse-by-pulse current
limit is not triggered since the peak current is limited by
the control loop.
5V
Operating Current
3.5mA
10µA
Normal
Operation
Fault
Situation
Normal
Operation
Figure 28. Auto-Restart Operation
© 2009 Fairchild Semiconductor Corporation
FAN100 Rev. 1.0.2
12
www.fairchildsemi.com