MIC2778
Micrel
MIC2778
Voltage Monitor with Adjustable Hysteresis
General Description
The MIC2778 is a voltage monitor—uniquely designed to
detect two separate voltage thresholds—combined with a
delay generator and logic. It is designed for monitoring the
battery supply of portable digital systems, particularly PDAs,
pagers, and cellular telephones.
High- and low-voltage thresholds can be adjusted inde-
pendently, allowing for wide hysteresis. Voltage detection
thresholds are accurate to 2%.
If the battery voltage falls below the low-voltage threshold,
the output (RST) is asserted and latched, preventing system
operation until the battery is replaced or recharged. Internal
logic prevents the output from chattering due to battery
recovery or load removal. The output is asserted for 140ms
(minimum) when a fresh battery is inserted. For applications
not requiring built-in delay, see MIC841.
The IC’s power supply input is separate from the detector
inputs, allowing the MIC2778 to be powered from a down-
stream supply, such a boost converter. Supply current is
extremely low (1µA, typical), making it ideal for portable
applications.
A high-precision 1% grade is available. The MIC2778 is sup-
plied in Micrel’s IttyBitty™ 5-lead SOT-23-5 package.
Features
• Optimized for PDAs, cellular telephones, pagers,
and other battery-powered devices
• Independently adjustable high- and
low-voltage thresholds
• Internal logic prevents battery-voltage-fluctuation chatter
• High ±2% voltage threshold accuracy; 1% available
• Built in 140ms (minimum) delay deglitches output
• Extremely low 1µA typical supply current
• For applications not requiring built-in delay, see MIC841
• Immune to brief power supply transients
• 5-lead SOT-23 package
Applications
•
•
•
•
•
•
PDAs
Pagers
Cordless phones
Consumer electronics
Embedded controllers
Personal electronics
Ordering Information
Part Number
Standard
MIC2778-2BM5
MIC2778-1BM5
Marking
UFB
UFA
Pb-Free
MIC2778-2YM5
MIC2778-1YM5
Marking
UFB*
UFA*
2%
1%
-40°C to +85°C
-40°C to +85°C
SOT-23-5
SOT-23-5
Accurary
Temperature Range
Package
* Underscore indicates a Pb-Free part.
Typical Application
MIC5207-3.0BM5
Li-Ion
Cell
604k
1%
56k
1%
340k
1%
MIC2778
VDD
LTH
HTH
R ST
GND
IN
100k
OUT
BYP
GND
V
OUT
3.0V
470pF
4.7µF
EN
V
BAT(hi)
= 3.6V
V
BAT(lo)
= 3.1V
Cellular Telephone Battery Monitor
IttyBitty™ is a trademark of Micrel, Inc.
Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
January 2005
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MIC2778
Micrel
Pin Configuration
LTH GND HTH
3
2
1
4
5
R ST
VDD
SOT-23-5 (M5)
Pin Description
Pin Number
1
Pin Name
HTH
Pin Function
High-Voltage Theshold (Input): Analog input to a comparator. When the level
on this pin initially rises above V
REF
, the delay generator cycles and the RST
remains low for a minimum of 140ms.
Ground
Low-Voltage Threshold (Input): Analog input to a comparator. This is the
voltage monitor input assigned to detect a low voltage condition. When
the level on this pin falls below V
REF
, RST is asserted and the condition is
latched until V
HTH
> V
REF
.
2
3
GND
LTH
4
RST
5
VDD
Reset (Output): Active-low, open-drain output. This output is asserted and
latched when V
LTH
<V
REF
, indicating a low voltage condition. This state
remains latched until V
HTH
> V
REF
.
Power Supply (Input): Independent supply input for internal circuitry.
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Micrel
Absolute Maximum Ratings
(Note 1)
Supply Voltage (V
DD
).......................................–0.3V to +7V
Input Voltages (V
HI
, V
LO
) .................................–0.3V to +7V
RST Output Current (I
RST
) ......................................... 20mA
Storage Temperature (T
S
) ........................ –65°C to +150°C
ESD Rating,
Note 3
....................................................... 2kV
Operating Ratings
(Note 2)
Supply Voltage (V
DD
) .................................. +1.5V to +5.5V
Input Voltages (V
RST
, V
LTH
, V
HTH
)................–0.3V to +6.0V
Ambient Temperature Range (T
A
) .............. –40°C to +85°C
Package Thermal Resistance................................ 256°C/W
Electrical Characteristics
Symbol
I
DD
1.5V ≤ V
DD
≤ 5.5V; T
A
= +25°C,
bold
values indicate –40°C ≤ T
A
≤ +85°C; unless noted
Parameter
Condition
Supply Current
Input Leakage Current
Reference Voltage
MIC2778-2
MIC2778-1
t
D
t
RESET
V
RST
Propagation Delay
Reset Pulse Width
Voltage-Low Reset
V
LTH
= V
REF(max)
+100mV to
V
REF(min)
– 100mV
RST asserted, I
SINK
= 1.6mA, V
DD
≥ 1.6V
1.215
1.228
5
140
420
0.3
0.4
RST not asserted
Min
Typ
1
5
10
1.240
1.265
1.252
Max
2
Units
µA
pA
nA
V
V
V
µs
ms
V
V
I
LTH,
I
HTH
V
REF
RST asserted, I
SINK
= 100µA, V
DD
≥ 1.2V,
Exceeding the absolute maximum rating may damage the device.
The device is not guaranteed to function outside its operating rating.
Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF.
V
DD
operating range is 1.5V to 5.5V. Output is guaranteed to be held low down to V
DD
= 1.2V.
Note 1.
Note 2.
Note 3.
Note 4.
Timing Diagram
V
BATT
V
HTH
V
LTH
0V
t
RST
A
t
RST
A
V
RST
V
RSTH
V
RSTL
Propagation delays not shown for clarity.
Note A.
The MIC2778 ignores very brief transients.
See “Applications Information” for details.
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Micrel
Functional Diagram
V
BATT
Low-Voltage
Detect
V
LO
LTH
R Q
High-Voltage
Detect
RST
V
HI
HTH
S
Q
Delay
Line
1.24V
Bandgap
Reference
MIC2778
GND
Functional Description
The MIC2778 monitors the voltage of a battery and detects
when it is discharged below a programmed level. Upon be-
ing replaced, or being recharged above a second higher
programmed trip point, the output remains low for a minimum
of 140ms and then sends a reset signal to a microprocessor
or other downstream component.
Voltage Low Output
The voltage-low output (RST) is an active-low, open-drain
output which sinks current when the MIC2778 detects a low
input voltage.
Trip Points
Battery voltage is monitored by a comparator via a voltage
divider network. The divided voltage is compared to an in-
ternal reference voltage. When the voltage at the LTH input
pin drops below the internal reference voltage, the output
pulls low. At this point, the voltage at HTH is assumed to be
below the reference voltage.
Delay
At power-on or when the battery is replaced or recharged,
and the voltage at HTH exceeds the reference voltage, the
output goes high after a minimum delay of 140ms.
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Micrel
of the two remaining resistor values.
1M
Ω
V
BAT(lo)
=
3.1V
=
1.24
R2
+
344k
R2
=
56k
Ω
R1
=
1M
Ω −
R2
−
R3
R1
=
600k
Ω
The accuracy of the resistors can be chosen based upon the
accuracy required by the system.
Input Transients
The MIC2778 is inherently immune to very short negative-
going “glitches.” Very brief transients may exceed the V
BAT(lo)
threshold without tripping the output.
As shown in Figure 2, the narrower the transient, the deeper
the threshold overdrive that will be ignored by the MIC2778.
The graph represents the typical allowable transient dura-
tion for a given amount of threshold overdrive that will not
generate a reset.
140
Applications Information
Output
Since the MIC2778 output is an open-drain MOSFET, most
applications will require a pull-up resistor. The value of the
resistor should not be too large or leakage effects may domi-
nate. 470kΩ is the maximum recommended value.
Programming the Thresholds
The low-voltage threshold is calculated using:
R1
+
R2
+
R3
V
BAT(lo)
=
V
REF
R2
+
R3
The high-voltage threshold is calculated using:
R1
+
R2
+
R3
V
BAT(hi)
=
V
REF
R3
where, for both equations:
V
REF
=
1.240V
In order to provide the additional criteria needed to solve
for the resistor values, the resistors can be selected such
that they have a given total value, that is, R1 + R2 + R3 =
R
TOTAL
. A value such as 1MΩ for R
TOTAL
is a reasonable
value because it draws minimum battery current but has no
significant effect on accuracy.
When working with large resistors, a small amount of leak-
age current can cause voltage offsets that degrade system
accuracy. The maximum recommended total resistance from
V
BAT
to ground is 3MΩ.
V
BATT
Input T rans ient
R es pons e
MAX. TRANSIENT DURATION (µs)
120
100
80
60
40
20
0
1
10
100
1000
R1
R2
R3
604k
1%
56k
1%
340k
1%
MIC2778
VDD
LTH
HTH
R ST
GND
470k
RESET COMP. OVERDRIVE, V
REF
–V
LTH
(mV)
Figure 2. Input Transient Response
Interfacing to Processors
With Bidirectional Reset Pins
Some microprocessors have reset signal pins that are bidi-
rectional, rather than input only. The Motorola 68HC11 family
is one example. Because the MIC2778’s output is open drain,
it can be connected directly to the processor’s reset pin using
only the pull-up resistor normally required. See Figure 3.
V
BATT
Microprocessor
Figure 1. Example Circuit
Once the desired trip points are determined, set the V
BAT(hi)
threshold first.
For example, use a total of 1MΩ = R1 + R2 + R3. For a typical
single-cell lithium ion battery, 3.6V is a good “high threshold”
because at 3.6V the battery is moderately charged. Solving
for R3:
1M
Ω
V
BAT(hi)
=
1.24
R3
R3
=
344k
Ω
Once R3 is determined, the equation for V
BAT(lo)
can be
used to determine R2. A single lithium-ion cell should not be
discharged below 2.5V. Many applications limit the drain to
3.1V. Using 3.1V for the V
BAT(lo)
threshold allows calculation
R1
R2
R3
604k
1%
56k
1%
340k
1%
MIC2778
VDD
LTH
HTH
R ST
GND
Supply In
100k
Reset In
GND
Figure 3. Interfacing to Bidirectional Reset Pin
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