參數(shù)資料
型號(hào)： LTI2911IDDB-3#TRM
廠商： LINEAR TECHNOLOGY CORP
元件分類： 電源管理
英文描述： POWER SUPPLY SUPPORT CKT, PDSO8
封裝： 3 X 2 MM, PLASTIC, MO-229, DFN-8
文件頁(yè)數(shù)： 4/20頁(yè)
文件大小： 245K
代理商： LTI2911IDDB-3#TRM
LTC2911
12
2911f
Again, to minimize errors arising from the PFI input bias
current, a value of less than 100k is recommended for
R3.
R4 can be chosen either using the PFI falling threshold or
the PFI rising threshold.
For the falling edge threshold, use the equation:
R4
=R3
VTRIP_ PFI_FALL
0.5V
– 1
Alternatively, for the rising edge threshold, use the
equation:
R4
=R3
VTRIP_ PFI_RISE
0.515V
– 1
where VTRIP_PFI_FALL is the supply threshold when the PFI
pinfallsbelowthe0.5Vfallingthreshold,andVTRIP_PFI_RISE
is the supply threshold when the PFI pin rises above the
0.515V rising threshold.
Note that VTRIP_PFI_RISE is typically 3% above the
VTRIP_PFI_FALL due to the fact that the PFI 515mV rising
threshold is 3% above its 500mV falling threshold.
In applications that require an accurate trip point, the R4
and R3 resistors should have small tolerances.
Hysteresis for Power-Fail Comparator
The power-fail comparator uses a positive 3% accurate
hysteresis to combat spurious triggering while maintain-
ing accurate thresholds for both the rising and falling
edges. The nominal threshold is 500mV at the falling edge
and 515mV at the rising edge. The hysteresis prevents
oscillation when the monitored voltage passes through
the thresholds. If the PFI pin is connected to an external
resistive divider, it may be bypassed with a capacitor for
additional noise filtering.
Increasing the Power-Fail Hysteresis
The power-fail comparator hysteresis can be increased by
adding two resistors, R5 and R6, as shown in Figure 3b.
When PFO is low, R5 sinks current from the center tap
of the R3 and R4 resistive divider. The upper threshold is
therefore given by:
VH = 0.515V 1+
R4
R3
+
R4
R5
When PFO is high, the series combination of R5 and R6
sources current into the center tap of the R3 and R4 resis-
tive divider. This leads to a lower threshold of:
VL = 0.5V 1+
R4
R3
–
3.3V – 0.5V
(
)R4
R5
+R6
The addition of R5 and R6 increases the hysteresis to:
VHYST = VH – VL
= 0.015 1+
R4
R3
+ 0.515
R4
R5
+
3.3V – 0.5V
(
)R4
R5
+R6
APPLICATIONS INFORMATION
Figure 3a. Setting the Power-Fail (PFI) Trip Point
–
+
+–
0.5V
2911 F03a
PFI
V1
114k
PFO
LTC2911
R4
R3
VTRIP
Figure 3b. Increasing Power-Fail Hysteresis
–
+
+–
0.5V
2911 F03b
PFI
V1
PFO
LTC2911
R4
R6
R5
R3
VTRIP
114k
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