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3.5.2
Sequencing Conditions Selections
4
EVM Design Configuration Information
4.1
Voltage Rail Set Points
V
RAILx
+
V
REF
ǒ
1
)
R
UP
R
DN
Ǔ
; V
REF
+
0.6663 typical for TPS76201
Set R
DN
+
66.5 k
W
(max); R
Up
+
ǒ
V
RAILx
V
REF
*
1
Ǔ
R
DN
(1)
Set R5
+
10 k
W
0.891
V
RAIL7
*
0.891
*
3.24 k
W
(2)
Set R2
+
10 k
W
0.8
V
RAIL8
*
0.8
*
2.49 k
W
(3)
4.2
Rail Bleed-Down Considerations
EVM Design Configuration Information
Sequencing conditions for a GPO
•
None
—GPO is not sequenced
•
Time
—number of milliseconds before this GPO is enabled
•
Parent (Reg)
—number of milliseconds to delay after the parent rail achieves regulation before
enabling this GPO
•
Parent (V)
—voltage at which the parent rail should be before this GPO is enabled
The EVM rail output voltage Set points are configurable by changing the output voltage set-point resistors.
RAIL1, RAIL2, RAIL3, and RAIL4 are monitored directly by the UCD9080 (no feedback divider resistors)
and these rail output voltages should never be set above the reference voltage being used. RAIL5, RAIL6,
RAIL7, and RAIL8 have the feedback divider resistors present and as such, these rail output voltages can
be higher than the currently used voltage reference.
•
RAIL1 through RAIL6: TPS76201 100-mA LDO
Note:
R
DN
has been set to 49.9
Ω
on the EVM to more quickly discharge the rails during shutdown
(see the rail bleed down considerations section).
•
RAIL7: PTH04070 3A Switching Regulator
•
RAIL8: PTH05050 6A Switching Regulator
When considering system shutdown (or unsequence time), loading on the power supply rails must be
taken into account. The rail bleed-down times are a function of the loading and the total (load and power
supply) output capacitance. As previously noted for RAIL1-RAIL6, the output voltage set-point resistors
have been chosen to provide a much quicker bleed-down time than could be achieved with a light load.
For RAIL7 and RAIL8, the feedback divider resistors have been sized to provide more loading for the
same reasons. If the user chooses to provide external rail loading, the output voltage set-point resistor
values can be adjusted. The EVM rail bleed-down times using the values shown in the BOM are on the
order of <10 ms, whereas for no-load conditions the rail bleed-down times can approach several seconds.
UCD9080 Power Supply Sequencer and Monitor Evaluation Module
10
SLVU184A – March 2007 – Revised February 2008
