FAN6747 Datasheet, PDF (13/15 Pages) Fairchild Semiconductor – Highly Integrated Green-Mode PWM Controller

Built-In Slope Compensation
The sensed voltage across the current-sense resistor is
used for peak-current-mode control and pulse-by-pulse
current limiting. Built-in slope compensation improves
stability and prevents sub-harmonic oscillation.
FAN6747 inserts a synchronized, positive-going, ramp
at every switching cycle.
Constant Output Power Limit
When the SENSE voltage across sense resistor RS
reaches the threshold voltage, the output GATE drive is
turned off after a small delay, tPD. This delay introduces
an additional current proportional to tPD • VIN / LP. Since
the delay is nearly constant regardless of the input
voltage VIN, higher input voltage results in a larger
additional current and the output power limit is higher
than under low input line voltage. To compensate this
variation for a wide AC input range, a power-limiter is
controlled by HV pin to solve the unequal power-limit
problem. The power limiter is fed to the inverting input of
the OCP comparator. This results in a lower current limit
at high-line input than at low-line input.
Over-Temperature Protection (OTP)
A NTC thermistor, RNTC, in series with a resistor, RA, is
connected from the RT pin to GND pin. A constant
current IRT is output from this pin. The voltage of the RT
pin can be expressed as VRT = IRT • (RNTC + RA), where
IRT is 100µA, the headroom of VRT is limited at around
5V by internal circuitry. As high ambient temperatures
occur, RNTC is smaller, such that the VRT decreases.
When VRT is less than 1.05V(VOTP) but over 0.7V, the
PWM turns off after tD_OTP-LATCH. The other threshold,
VDD under 0.7V, is used for fast shut down of FAN6747
after a short time.
Noise Immunity
Noise on the current sense or control signal may cause
significant pulse-width jitter, particularly in continuous-
conduction mode. Slope compensation helps alleviate
this problem. Good placement and layout practices
should be followed. Avoiding long PCB traces and
component leads, locating compensation and filter
components near the FAN6747, and increasing the
power MOS gate resistance improve performance.
© 2009 Fairchild Semiconductor Corporation
FAN6747 • Rev. 1.0.1
13
www.fairchildsemi.com