CM4 App Note 260424 issue 2.1.docx
Document No. 260424
Page 14 of 29
Reverse Current Protection (RCP)
The standard output modules use synchronous rectification in the output stages to achieve high efficiency and as a result the outputs can both
source and sink current. The sink current is internally limited to approximately -6% of the maximum rated current. However, in applications where
the output modules are connected to external power sources such as batteries or other power supplies certain precautions must be observed to
prevent damage to the unit.
The outputs should never be directly connected to to external power sources without some form of reverse current protection such as an external
diode or controlled mosfet. If protection is not used, large reverse currents which will ultimately result in damage to the unit will occur, especially
when the AC mains is disconnected.
Output Module Average and Peak Power
All modules have an average and peak power rating. The average power of each unit must at all times remain below it
’s specified limit. However,
each outp
ut can deliver up to 150% of it’s average power rating for a maximum of 5 seconds at 50% duty cycle, subject to the current limit not
being exceeded and subject to the overall average power drawn being less than the specified average power rating (including any input derating
due to temperature or line voltage). The available peak power is a function of the output voltage and maximum current for each module. Full peak
power is only possible when the output voltage is adjusted to V
MAX
and the maximum current is drawn from the module. Note that both average
and peak power ratings are subject to the same temperature derating as the input module (derate by 2.5% per °C above 50°C), but are not
subject to any line derating.
Start-up & Shut Down
All outputs are designed to have a regulated monotonic start-up with a rise time of
approximately 3 ms as shown in the diagram right. The power good signal will not
assert
until the voltage exceeds the power good threshold (≈90%).
Where multiple output modules are used, the default start up scheme is ratio-metric
with all outputs starting at the same time as shown in the diagram right. External
control circuits may be used to implement tracking or sequenced start up if
necessary.
The outputs are not designed to start into pre-biased loads and may discharge any
externally capacitance before beginning to ramp the output voltage up in the normal
way.
At shutdown, the outputs enter a high impedance state. Where no external load is
present it may take some time for the voltage to decay. When driving inductive loads,
care must be taken to limit the voltage at the output terminals to prevent damage to
the unit.
Synchronisation
All output modules in the same product are synchronised. The typical operating
frequency is 260 kHz and paralleled or series connected units will not produce ripple beat frequencies.
External capacitance
All outputs can support a large external capacitance as detailed in the table shown.
The capacitances specified ensure monotonic start-up with rated load applied.
Larger capacitances can be applied for lower load currents.
Module
C
EXT_MAX
S1
12mF
S2
4mF
S3
1mF
S4
500uF
