LTC2934 Ver la hoja de datos (PDF) - Linear Technology
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LTC2934 Datasheet PDF : 12 Pages
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LTC2934
APPLICATIONS INFORMATION
R3 is easily obtained from:
R3 = RSUM – R1 – R2 = 8.1M – 1.87M – 130k = 6.1M
The closest 1% value is 6.04M. Plugging the standard
values back into the equations yields the design values
for the falling power-fail and reset voltages:
VPF = 1.720V, VR = 1.608V
Figure 2 demonstrates how the inputs can be biased
to monitor two voltages (V1, V2). In this example, four
resistors are required. Calculate each divider ratio for the
desired falling threshold (VFT) using:
RnB = VFT − 1= VFT − 1
RnA VTH
0.4V
In Figure 2, PFO is tied back to the MR input, making the
state of the RST output dependent upon both V1 and V2. If
V1 and V2 are both above the configured falling threshold
plus hysteresis, RST is allowed to pull high. If independent
operation of the status outputs is desired, simply omit the
PFO to MR connection.
R1B
V1
ADJ
RST
R2B
V2
LTC2934
PFI
PFO
R2A
R1A
MR
Figure 2. Dual Voltage Monitoring
Selecting Output Logic Style
The LTC2934 status outputs are available in two options:
open-drain (LTC2934-1) or active pull-up (LTC2934-2).
The open-drain option (LTC2934-1) allows the outputs
to be pulled up to a user defined voltage with a resistor.
The open-drain pull-up voltage may be greater than VCC
(5.5V maximum), which is not always possible with
inferior battery supervisors, due to internal diode clamps.
When the status outputs are low, power is dissipated in
the pull-up resistors. Recommended resistor values lie in
the range between 10k and 470k. Figure 3 demonstrates
typical LTC2934-1 RST output behavior.
The active pull-up option (LTC2934-2) eliminates the
need for external pull-up resistors on the status outputs.
Integrated pull-up devices pull the outputs up to VCC.
Actively pulled up outputs may not be driven above VCC.
Some applications require the RST and/or PFO outputs to
be valid with VCC down to ground. Active pull-up handles
this requirement with the addition of an external resistor
from the output to ground. The resistor provides a path
for leakage currents, preventing the output from floating to
undetermined voltages when connected to high impedance
(such as CMOS logic inputs). The resistor value should
be small enough to provide effective pull-down without
excessively loading the pull-up circuitry. A 100k resistor
from output to ground is satisfactory for most applications.
When the status outputs are high, power is dissipated in
the pull-down resistors. Figure 4 demonstrates typical
LTC2934-2 RST output behavior.
8
3.5
LTC2934-1
3.0
EXTERNALLY CONFIGURED
FOR 3V FALLING THRESHOLD
2.5
2.0
1.5
1.0
0.5
0
0 0.5 1 1.5 2 2.5 3 3.5
VCC (V)
2934 F03
Figure 3. RST vs VCC with 10k Pull-Up
3.5
LTC2934-2
3.0
EXTERNALLY CONFIGURED
FOR 3V FALLING THRESHOLD
2.5
2.0
1.5
1.0
0.5
0
0 0.5 1 1.5 2 2.5 3 3.5
VCC (V)
2934 F04
Figure 4. RST vs VCC
2934f
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