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
4.3
Rail and GUI Divider Selection
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EVM Design Configuration Information
The EVM rail output voltage set points are configurable by changing the output voltage set-point resistors.
The UCD9081 directly monitors RAIL1, RAIL2, RAIL3, and RAIL4 (no feedback divider resistors), and
these rail output voltages must 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 3-A Switching Regulator
•
RAIL8: PTH05050 6-A 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 bill of materials are
on the order of <10 ms, whereas for no-load conditions, the rail bleed-down times can approach several
seconds.
Use these equations to choose the external rail and GUI divider resistor values.
Note:
Setting R
pullup
and R
pulldown
to 0 in the GUI yields a D
G
value of 1.
•
To accurately calculate the correct EVM divider ratio D
E
the rail, divider resistor, and reference voltage
tolerances must be taken into account so that the monitor input voltage does not exceed the device
reference voltage.
SLVU249 – September 2008
UCD9081 Power Supply Sequencer and Monitor EVM
13