Specifications

Chapter 4: Theory of operation
63
voltage range. The global system reset provides a clean system restart whenever the
system becomes unstable due to power.
A Linear Technology 1643L Hot Swap power controller provides controlled
powering up and down of the CPU board. With its internal FETs for ±12V and
external FET gate control for 3.3V and 5V, the power controller ramps power up
and down for the CPU board upon insertion or extraction. It also provides
microsecond rate response to over current conditions that may exist on any of these
voltages.
A precision voltage reference in conjunction with a voltage comparator provides
monitoring of 3.3V, 5V, and the EMC2 module power good signal. All devices are
kept in reset for at least 100ms after all voltages are stable.
The CPU board will properly operate with power voltages that comply with the
tolerances in the PICMG 2.1 Hot Swap Revision 1.0 specification. The input DC
power levels pass through hot swap controller circuitry reducing the amplitude of
these signals slightly, based on how much current is being drawn. The 5V signal
passes through a single sense resistor and FET, while the 3.3V signal passes through
parallel sense resistors and FET switches. Each sense resistor is 7 milliohms while
the FET resistance is 13.5 milliohms.
Using this circuitry with 5A, the voltage drop for 5V due to the hot swap circuitry is
103mV. Similarly, with 10A, the voltage drop for 3.3V is 138mV. The +12V and –
12V signals only pass through the hot swap power controller creating a 600mV
drop for +12V (at 500mA) and a 250mV drop for –12V (at 100mA).
The following table represents a compilation of power estimates. It represents worst
case total power consumption.
1
The only power consumed by –12V on the CPU board is in the loss due to hot
swap controller switching.
Table 4-14. Power estimates
Voltage Maximum power, watts
3.3V 12W
5V 11S
12V 1W
–12V
0
1
Total power: 24W