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4.3
Rail and GUI Divider Selection
Required D
E
+
ǒ
1
)
TOL
R
100
Ǔ
V
RAILE
ǒ
1
)
TOL
RAILE
100
Ǔ
V
REF
ǒ
1
)
TOL
REF
100
Ǔ
4.4
ENx and GPOx Pin Default Voltage Levels
4.5
Reset
EVM Design Configuration Information
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.
•
Use the following equations to calculate the minimum divider ratio, D
E
:
TOL
R
= Divider resistor tolerance
TOL
RAILE
= V
RAILE
tolerance
TOL
REF
= V
REF
tolerance
V
RAILE
= Rail voltage being monitored via divider
V
REF
= Reference voltage
R
PU
= R
PD
×
(D
E
– 1); Choose R
PD
≤
10 k
Ω
•
EVM Actual Voltages:
–
EVM divider, D
E
= R
HI
+ R
LO
) / R
LO
= 2
–
RAIL1 through RAIL4: No dividers; V
MON
= V
RAILE
; V
MON
= MONx pin voltage, V
RAILE
= EVM RAILx
voltage
–
RAIL 5 through RAIL8: Divider, D
E
= 2; V
MON
= V
RAIL
/ 2
•
GUI Calculated Voltages:
–
GUI divider, D
G
= R
PULLDOWN
+ R
PULLUP
) / R
PULLDOWN
–
Displayed rail voltages, V
RAILG
= V
MON
×
D
G
= V
RAILE
×
D
G
/ D
E
; V
RAILG
= GUI displayed voltage
When the device is reset (see the Reset section), the ENx and GPOx outputs go to a high-impedance
state (3-state). Pullup or pulldown resistors on these signals are recommended to cause the signals to go
to the desired voltage level during reset. For example, if it is desired to keep a RAILx enabled during reset
and the enable is active-low, then a pulldown resistor should be located on the corresponding ENx output.
Note that on the EVM, the ENx and GPOx outputs have selectable pullup or pulldown jumpers installed.
Also note that for the ENx outputs, the ENx status LEDs provide a stronger pullup current than the jumper
pulldowns so that the power supplies become disabled during reset.
Device reset occurs when:
•
Power is applied to the device (power on reset) or power is cycled.
•
The external RST pin is driven low.
•
The GUI Update Parameters and Sequence button is pressed, causing new configuration data to be
written to the device.
•
The GUI Sequence button is pressed, causing the device to sequence.
•
A Sequence occurs as a result of a configured rails alarm event.
Any of these reset conditions causes the device I
2
C address to be re-evaluated. Keep in mind that the
EN8/GPO1 pin, as well as GPO2, GPO3, and GPO4 pins, provide the I
2
C address inputs that are sampled
during device reset. If the pin pullup/pulldown configuration jumpers are changed while the GUI is
operating and a device reset occurs, then the GUI communication may be affected (perform Discover to
re-acquire the EVM I
2
C address.
SLVU184A – March 2007 – Revised February 2008
UCD9080 Power Supply Sequencer and Monitor Evaluation Module
11
