FSEZ1216B Datasheet, PDF (12/15 Pages) Fairchild Semiconductor – Primary-Side-Regulation PWM Integrated Power MOSFET

Under-Voltage Lockout (UVLO)
The turn-on and turn-off thresholds of the FSEZ1216B
are fixed internally at 16V/5V. During startup, the hold-
up capacitor must be charged to 16V through the
startup resistor, so that the FSEZ1216B is enabled. The
hold-up capacitor continues to supply VDD until power
can be delivered from the auxiliary winding of the main
transformer. VDD must not drop below 5V during this
startup process. This UVLO hysteresis window ensures
that the hold-up capacitor is adequate to supply VDD
during startup.
V_DD Over-Voltage Protection (OVP)
VDD OVP is built in to prevent damage due to over-
voltage conditions. When the voltage VDD exceeds 28V
due to abnormal conditions, PWM pulses are disabled
until the VDD voltage drops below the UVLO and then
startup again. Over-voltage conditions are usually
caused by open feedback loops.
Over-Temperature Protection (OTP)
The FSEZ1216B has a built-in temperature-sensing
circuit to shut down PWM output once the junction
temperature exceeds 140°C. While PWM output is shut
down, the VDD voltage gradually drops to the UVLO
voltage. Some of the FSEZ1216B’s internal circuits are
shut down, and VDD gradually starts increasing again.
When VDD reaches 16V, all the internal circuits including
the temperature-sensing circuit starts operating
normally. If the junction temperature is still higher than
140°C, the PWM controller is shut down immediately.
Built-In Slope Compensation
The sensed voltage across the current-sense resistor is
used for current-mode control and pulse-by-pulse
current limiting. Built-in slope compensation improves
stability and prevents sub-harmonic oscillations due to
peak-current-mode control. The FSEZ1216B has a
synchronized, positively-sloped ramp built-in at each
switching cycle.
Noise Immunity
Noise from the current sense or the control signal can
cause significant pulse-width jitter, particularly in
continuous-conduction mode. While slope
compensation helps alleviate these problems, further
precautions should still be taken. Good placement and
layout practices should be followed. Avoid long PCB
traces and component leads, locating compensation
and filter components near the FSEZ1216B.
© 2008 Fairchild Semiconductor Corporation
FSEZ1216B Rev. 1.0.0
12
www.fairchildsemi.com