FAN6751HLMY Просмотр технического описания (PDF) - Fairchild Semiconductor

Номер в каталоге

Компоненты Описание

производитель

FAN6751HLMY

Highly Integrated Green-Mode PWM Controller

Fairchild Semiconductor 

AN-6073

Figure 17. Overload Protection Waveforms

Pull-HIGH Protection Function in VIN Pin

The pull-high protection function is also included in the VIN

pin. When VIN is higher than 4.7V, FAN6751 latches up and

stops regulating. Figure 18 shows the external latch circuit

for secondary-side output OVP. If the output voltage (VO) is

higher than VZ (Zener diode voltage), VDD passes through

the RRESTRICT to VIN pin (there are three Zener diodes to

clamp this over-voltage at 6V) to achieve the latch mode.

APPLICATION NOTE

SENSE Pin Short-Circuit Protection

The FAN6751 provides a safety protection for power supply

production. When the sense resistor is shorted by soldering

during production, the pulse-by-pulse current limiting loses

efficiency for the purpose of providing over-power

protection of the unit. The unit may be damaged when the

loading is larger than the original maximum load. To protect

against a short circuit across the current-sense resistor, the

controller immediately shuts down if a continuously low

voltage (~0.15V/150µs) on the SENSE pin is detected.

Brownout Protection

Since the VIN pin is connected through a resistive divider to

the rectified AC input line voltage, it can also be used for

brownout protection. If the VIN voltage is less than 0.7V, the

PWM output is shut off. As the VIN voltage reaches 0.92V,

the PWM output is turned on again. The hysteresis window

for ON/OFF is around 0.22V. The recommended values for

RBo1, RBo2, and CBo1 are 10M (5M+5M), 100K, and 2.2µF.

Using these values in the test board, the power supply is

turned off at 66V (maximum load) and recovered at 70V.

Figure 19. Circuit for Brownout

Figure 18. External Circuit for Second OVP

Internal Over-Temperature Protection (OTP)

The FAN6751 has a built-in temperature sensing circuit to

shut down PWM output once the junction temperature

exceeds 135°C. While PWM output is shut down, the VDD

voltage gradually drops to the UVLO voltage (around 7.5V).

Then VDD is charged up to the startup threshold voltage of

16.5V through the startup resistor until PWM output is

restarted. This “hiccup” mode protection continues as long

as the temperature remains above 130°C. The temperature

hysteresis window for the OTP circuit is 25°C.

Leading-Edge Blanking (LEB)

A voltage signal proportional to the MOSFET current

develops on the current-sense resistor RS. Each time the

MOSFET is turned on, a spike induced by the diode reverse

recovery and by the output capacitances of the MOSFET

and diode, appears on the sensed signal. Inside the

FAN6751, a leading-edge blanking time of about 350ns

helps avoid premature termination of MOSFET by the spike.

Therefore, only a small-value RC filter (e.g. 100Ω + 470pF)

is required between the SENSE pin and RS. Still, a non-

inductive resistor for the RS is recommended.

© 2008 Fairchild Semiconductor Corporation

Rev. 1.0.0 • 9/26/08

7

www.fairchildsemi.com

Share Link: