6742HL Просмотр технического описания (PDF) - Fairchild Semiconductor
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6742HL Datasheet PDF : 13 Pages
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Functional Description (Continued)
Thermal Protection
An NTC thermistor, RNTC, in series with a resistor RA,
can be connected from the RT pin to ground. A
constant current IRT is output from the RT pin. The
voltage on the RT pin can be expressed as VRT = IRT •
(RNTC + RA), where IRT is 100µA. At high ambient
temperatures, RNTC is smaller, such that VRT decreases.
When VRT is less than 1.05V (VRTTH1), the PWM turns
off after 20ms (tD-OTP1). If VRT is less than 0.7V (VRTTH2),
PWM turns off immediately after 130µs (tD-OTP2).
Limited Power Control
The FB voltage increases every time the output of the
power supply is shorted or overloaded. If the FB voltage
remains higher than a built-in threshold for longer than
tD-OLP, PWM output is turned off. As PWM output is
turned off, VDD begins decreasing.
When VDD goes below the turn-off threshold (~9.5V) the
controller is totally shut down. VDD is charged up to the
turn-on threshold voltage of 15.5V through the startup
resistor until PWM output is restarted. This protection
feature continues as long as the overloading condition
persists. This prevents the power supply from
overheating due to overloading conditions. When VRT is
less than 1.05V (VRTTH1), the PWM is turned off after
20ms (tD-OTP1). If VRT is less than 0.7V (VRTTH2), PWM is
turned off immediately after 130µs (tD-OTP2).
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 SG6742HL/HR, and increasing
the power MOS gate resistance improve performance.
© 2008 Fairchild Semiconductor Corporation
SG6742HL/HR • Rev. 1.0.4
10
www.fairchildsemi.com
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