MIC2755BMM(2000) Просмотр технического описания (PDF) - Micrel
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MIC2755BMM Datasheet PDF : 12 Pages
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MIC2755
VBAT(low)
=
3.1V
=
1.24V
1MΩ
R10
R10 = 400k
1MΩ = R10+R11
VBAT(dead)
=
2.9V
=
1.24V
1MΩ
R9 + 400k
R9 = 27k
R8 = 1MΩ – R9 – R10 = 573k
The accuracy of the resistors can be chosen based upon the
accuracy required by the system.
Input Transients
The MIC2755 is inherently immune to very short negative-
going “glitches.” Very brief transients may cross the VBAT(lo)
or VBAT(dead) thresholds without tripping the output(s).
As shown in Figures 3 and 4, the narrower the transient, the
deeper the threshold overdrive that will be ignored by the
MIC2755. The graph represents the typical allowable tran-
sient duration for a given amount of threshold overdrive that
will not cause the corresponding output to change state.
Alternate Configurations
The MIC2755 can be used in a variety of ways. It is especially
flexible due to the fact that the NMI comparator is completely
independent. There are other useful configuration besides a
three-state battery monitor. The NMI comparator can be used
to provide power-fail indication (PFI/PFI), monitor an auxiliary
battery (LBI/LBO), or detect the presence of an ac adapter.
Voltage Supervisor and Backup Battery Monitor
Figure 5 illustrates the MIC2755 being used as a voltage
supervisor and a battery monitor in a 3.3V system with a
Lithium coin-cell backup. The primary voltage monitor is
configured as a voltage supervisor with a nominal trip point of
3.034V and 33mV of hysteresis as set by R1, R2, and R3. The
NMI comparator is used to detect a low-battery condition so
the system is aware that the backup battery is discharged. In
this example, the /NMI output will be asserted if battery
voltage falls below 2.2V. Manual reset capability can be
added as discussed in the Manual Reset and Configuration
With Manual Reset sections.
This same configuration can be used to detect the presence
of an auxiliary power source such as an ac adapter instead of
monitoring a battery. R4 and R5 would be selected such that
the /NMI output is deasserted when the proper input voltage
is applied.
Voltage Supervisor with Power Fail Warning
Figure 6 illustrates the MIC2755 being used as a voltage
supervisor and a power-fail detector in a 3.3V system. The
primary voltage monitor is configured as a voltage supervisor
Micrel
Input Transient
200
Response (VPOF)
180
160
140
120
100
80
60
40
20
0
0 5 10 15 20 25 30
RESET COMP. OVERDRIVE, VREF–VPTH (mV)
Figure 3. Input Transient Response
Input Transient
120
Response (VNMI)
100
80
60
40
20
0
0 5 10 15 20 25 30 35 40 45
RESET COMP. OVERDRIVE, VREF–VNTH (mV)
Figure 4. Input Transient Response
with a nominal trip point of 3.034V and 33mV of hysteresis as
set by R1, R2, and R3. The NMI comparator is used to detect
an impending power failure such as a low-battery condition or
ac power outage. The /NMI output will be asserted if the input
voltage to the LDO regulator falls below 3.55V. (The MIC5245
has a specified maximum dropout of 250mV at 150mA output
current. If the input voltage falls below 3.55V, the output may
droop.)
By monitoring the input of the LDO regulator, the system
receives the earliest warning of an impending power loss.
Manual reset capability can be added as discussed in the
Manual Reset and Configuration With Manual Reset sec-
tions.
Supervised Boost Converter and Microcontroller or
Microprocessor
In Figures 7 and 8, the MIC2755 is used to monitor the battery
and the MIC3172 is used to maintain the output voltage at
3.3V by boosting the input voltage. When the Li-ion battery
voltage drops to 3.1V, the MIC2755 alerts the microcontroller
or the microprocessor. When the battery voltage drops to
2.9V, the MIC2755 turns off the MIC3172.
February 2000
7
MIC2755
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