MIC2755 View Datasheet(PDF) - Micrel
Part Name
Description
MFG CO.
MIC2755 Datasheet PDF : 10 Pages
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MIC2755
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 transient
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 su-
pervisor 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 volt-
age 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
with a nominal trip point of 3.034V and 33mV of hysteresis as
Micrel, Inc.
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
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 re-
ceives 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 sections.
Supervised Boost Converter and Microcontroller or Mi-
croprocessor
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.
January 2006
7
MIC2755
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