SLUA110 Datasheet, PDF (11/19 Pages) Texas Instruments – PRACTICAL CONSIDERATIONS IN CURRENT MODE POWER SUPPLIES

APPLICATION NOTE
U-111
simply pulling the error amplifier output below the lower
threshold of the PWM comparator of approximately 0.5
volts. This can be easily implemented via an NPN transistor
placed between the E/A output and ground, used to short
circuit the E/A output to zero volts. In most cases, this node
is internally current limited to prevent failures.
Another scheme is to pull the current limit or current sense
input above its upper threshold. A small transistor from this
input to the reference voltage will fulfill this requirement.
Vertical: 0.5 V/CM Both
Horizontal: 0.5 µS/CM
Figure 28. Operating Waveforms at 500 KHz
A. NONLATCHING
Figure 30. PWM Shutdown Circuits
Vertical: 0.5 V/CM Both
Horizontal: 0.5 µS/CM
Figure 29. Master/Slave Sync Waveforms at CT
IV. EXTERNALLY CONTROLLING THE PWM
Many of today’s sophisticated control schemes require
external control of the power supply for various reasons.
While most of these requirements can be incorporated
quite easily with a full functioned control chip, (typical of a
16 pin device), implementation may be more complex with
a low cost, 8 pin PWM. Circuits to provide these functions
with a minimum of external parts will be highlighted.
Shutdown
One of the most common requirements is to provide a
complete shutdown of the power supply for certain situa-
tions like remote on/off, or sequencing. Typically, a TTL
level input is used to disable the PWM outputs. Both vol-
tage and current mode control ICs can perform this task by
B. NONLATCHING
Figure 31.
Latching Shutdown
For those applications which require a latching shutdown
mechanism, an SCR can be used in conjunction with the
above circuits, or in lieu of them. The SCR can also be
placed from the PWM E/A output to ground, provided the
PWM E/A minimum short circuit current is greater than the
maximum holding current of the SCR, and the voltage
drop at I(hold) is less than the lower PWM threshold.
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