LTC2928CUHF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2928CUHF' PDF : 34 Pages View PDF
LTC2928
Applications Information
g. Undervoltage thresholds
The positive supply monitor undervoltage threshold at all
of the monitor inputs is 0.5V. Connect a resistive divider
from the sensed voltage to ground. Connect the tap point
to the respective high impedance monitor input. Specify
the required undervoltage threshold (UVTH) and calculate
the divider ratio using
RnB = UVTH(V) – 1
RnA 0.5V
This application requires –6.5% undervoltage thresholds
for all supplies. For the 1.8V supply monitored on V3, the
undervoltage threshold is 1.683V (1.8V x 0.935). The
necessary divider ratio is
R3B
R3A
=
1.683
0.5V
–
1≈
2.37
CPTMR
=
400ms
4.0MΩ
=
0.1µF
i. Reset delay time
The reset delay time is the additional time that RST is held
low after all monitor inputs are above their undervoltage
threshold. It is also the additional time that OV is held low
(after an overvoltage event) after all monitor inputs are
below their overvoltage threshold. When the reset timer
expires, RST and/or OV is allowed to pull high. The reset
delay time (tRTMR) is set with a capacitor from the RTMR
pin to ground. Calculate the reset capacitance from
CRTMR(F) =
tRTMR (s)
4.0MΩ
For this application, the reset delay time is 190ms,
yielding
A good choice for R3B is 23.7k and R3A is 10k. All sequence
thresholds are a percentage of the configured undervoltage
threshold. The remaining ratios are:
R1B
R1A
=
2.338
0.5V
–
1≈
3.68
R2B = 1.403 – 1≈ 1.81
R2A 0.5V
R4B
R4A
=
3.086
0.5V
–
1≈
5.17
h. Power-good timing
The “power-good” time defines the maximum time allowed
for all monitored voltages to reach their undervoltage
threshold when sequencing-up, or the sequence-down
threshold (when sequencing-down) with respect to the time
of the first enabled (disabled) supply. If the “power-good”
timer expires, a sequence fault occurs. The “power-good”
time (tPTMR) is set with a capacitor from the PTMR pin
to ground. Calculate the “power-good” capacitance from
CPTMR(F) =
tPTMR (s)
4.0MΩ
CRTMR (F)
=
190ms
4.0MΩ
=
0.047µF
j. Overvoltage thresholds
Configure the OVA pin to set the global overvoltage
thresholds. The application requires overvoltage thresholds
at approximately 25% above the nominal supply voltage.
For the 1.8V supply, the overvoltage threshold (OVTH)
equals 2.25V (1.8V x 1.25). Compute the required value
for VOVA from:
VOVA
(V)
=
RnA
RnA +RnB
•
OVTH
(V)
For this application,
VOVA
(V)
=
10k
10k
+ 23.7k
•
2.25
=
0.667V
From the curves in Figures 16 and 17, the approximate
value for VOVA is achieved by leaving the OVA pin open.
Since the other monitor inputs were configured for the
same relative undervoltage level, the relative overvoltage
levels for the other supplies are the same (+ 25%).
For this application, the “power-good” time is 400ms,
yielding
The application requires a fast shutdown upon overvoltage.
To generate a fault upon an overvoltage condition, tie OV to
FLT. The fault condition shuts down all controlled supplies.
2928fa
26
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