Datasheet

TL284x, TL384x
CURRENT-MODE PWM CONTROLLERS
SLVS038G − JANUARY 1989 − REVISED FEBRUARY 2008
10
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
shutdown technique
The PWM controller (see Figure 7) can be shut down by two methods: either raise the voltage at ISENSE above
1 V or pull the COMP terminal below a voltage two diode drops above ground. Either method causes the output
of the PWM comparator to be high (refer to block diagram). The PWM latch is reset dominant so that the output
remains low until the next clock cycle after the shutdown condition at the COMP or ISENSE terminal is removed.
In one example, an externally latched shutdown can be accomplished by adding an SCR that resets by cycling
V
CC
below the lower UVLO threshold. At this point, the reference turns off, allowing the SCR to reset.
330 Ω
1 kΩ
500 Ω
To Current-Sense
Resistor
REF
ISENSE
Shutdown
Shutdown
COMP
Figure 7. Shutdown Techniques
A fraction of the oscillator ramp can be summed resistively with the current-sense signal to provide slope
compensation for converters requiring duty cycles over 50% (see Figure 8). Note that capacitor C forms a filter
with R2 to suppress the leading-edge switch spikes.
R
T
0.1 µF
C
T
R1
R2
ISENSE
R
SENSE
C
REF
RT/CT
ISENSE
Figure 8. Slope Compensation