June 2000
1
MIC2779
MIC2779
Micrel
MIC2779
Voltage Monitor with Adjustable Hysteresis
Preliminary Information
General Description
The MIC2779 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 indepen-
dently, allowing for wide hysteresis. Voltage detection thresh-
olds are accurate to 2%.
If the battery voltage falls below the low-voltage threshold, the
output /RST or 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 requiring open-drain output, see MIC2778/
MIC833.
The IC's power supply input is separate from the detector
inputs, allowing the MIC2779 to be powered from a down-
stream supply, such a boost converter. Supply current is
extremely low (1
A, typical), making it ideal for portable
applications.
A high-precision 1% grade is available. The MIC2779 is
supplied in Micrel's IttyBittyTM 5-lead SOT-23-5 package.
Typical Application
HTH
/RST
VDD
LTH
GND
MIC2779L
604k
1%
56k
1%
340k
1%
EN
OUT
IN
BYP
GND
MIC5207-3.0BM5
Li-Ion
Cell
470pF
V
OUT
3.0V
4.7F
V
BAT(hi)
= 3.6V
V
BAT(lo)
= 3.1V
Cellular Telephone Battery Monitor
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 1
A typical supply current
For applications requiring open-drain output, see
MIC2778/MIC833
Immune to brief power supply transients
5-lead SOT-23 package
Applications
PDAs
Pagers
Cordless phones
Consumer electronics
Embedded controllers
Personal electronics
Micrel, Inc. 1849 Fortune Drive San Jose, CA 95131 USA tel + 1 (408) 944-0800 fax + 1 (408) 944-0970 http://www.micrel.com
IttyBittyTM is a trademark of Micrel, Inc.
MIC2779
Micrel
MIC2779
2
June 2000
Pin Configuration
HTH
VDD
RST
LTH
1
3
4
5
2
GND
SOT-23-5 (M5)
"H" Version
Ordering Information
Part Number
Marking
Output Polarity
Accuracy
Temperature Range
Package
MIC2779H-2BM5
UPA2
Active-High RST
2%
40
C to +85
C
SOT-23-5
MIC2779H-1BM5
UPA1
Active-High RST
1%
40
C to +85
C
SOT-23-5
MIC2779L-2BM5
UPB2
Active-Low /RST
2%
40
C to +85
C
SOT-23-5
MIC2779L-1BM5
UPB1
Active-Low /RST
1%
40
C to +85
C
SOT-23-5
HTH
VDD
/RST
LTH
1
3
4
5
2
GND
SOT-23-5 (M5)
"L" Version
Pin Description
Pin Number
Pin Number
Pin Name
Pin Function
MIC2779H
MIC2779L
1
1
HTH
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 or RST remains high for a minimum of 140ms.
2
2
GND
Ground
3
3
LTH
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 or RST is asserted and the condition
is latched until V
HTH
> V
REF
.
4
RST
Reset (Output): Push-pull 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
. The polarity of this signal (active-high or low) is deter
mined by the part number suffix. See ordering information.
4
/RST
Reset (Output): Push-pull 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
. The polarity of this signal (active-high or low) is deter
mined by the part number suffix. See ordering information.
5
5
VDD
Power Supply (Input): Independent supply input for internal circuitry.
June 2000
3
MIC2779
MIC2779
Micrel
Absolute Maximum Ratings
(Note 1)
Supply Voltage (V
DD
) ..................................... 0.3V to +7V
Input Voltages (V
LTH
, V
HTH
, V
/RST
, V
RST
) ..... 0.3V to +7V
/RST, RST Output Current (I
/RST
), (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
LTH
, V
HTH
, V
/RST
, V
RST
) .. 0.3V to +6.0V
Ambient Temperature Range (T
A
) ............. 40
C to +85
C
Package Thermal Resistance ............................... 256
C/W
Electrical Characteristics
1.5V
V
DD
5.5V; T
A
= +25
C, bold values indicate 40
C
T
A
+85
C; unless noted
Symbol
Parameter
Condition
Min
Typ
Max
Units
I
DD
Supply Current
/RST, RST not asserted
1
2
A
I
LTH,
I
HTH
Input Leakage Current
5
pA
10
nA
V
REF
Reference Voltage
1.240
V
MIC2779H/L-2
1.215
1.265
V
MIC2779H/L-1
1.228
1.252
V
t
D
Propagation Delay
V
LTH
= V
REF(max)
+100mV to
5
s
V
REF(min)
100mV
t
RESET
Reset Pulse Width
140
420
ms
V
OL
Reset Output Low Voltage
/RST asserted or RST not asserted,
I
SINK
= 1.6mA, V
DD
1.6V
0.3
V
/RST asserted or RST not asserted,
I
SINK
= 100
A, V
DD
1.2V,
0.4
V
V
OH
Reset Output High Voltage
/RST not asserted or RST asserted,
I
SOURCE
= 500
A, V
DD
1.6V
0.8V
DD
V
/RST not asserted or RST asserted,
I
SOURCE
= 50
A, V
DD
1.2V,
0.8V
DD
V
Note 1.
Exceeding the absolute maximum rating may damage the device.
Note 2.
The device is not guaranteed to function outside its operating rating.
Note 3.
Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF.
Note 4.
V
DD
operating range is 1.5V to 5.5V. Output is guaranteed to be held low down to V
DD
= 1.2V.
Timing Diagram
V
HTH
V
BATT
V
LTH
0V
V
OH
V
OL
t
RST
t
RST
A
A
V
/RST
V
OH
V
OL
V
RST
Propagation delays not shown for clarity.
Note A.
The MIC2779 ignores very brief transients.
See "Applications Information" for details.
MIC2779
Micrel
MIC2779
4
June 2000
Functional Description
The MIC2779 monitors the voltage of a battery and detects
when it is discharged below a programmed level. Upon being
replaced, or being recharged above a second higher pro-
grammed trip point, the output remains low (MIC2779L) or
high (MIC2779H) for a minimum of 140ms and then sends a
reset signal to a microprocessor or other downstream com-
ponent. See "Timing Diagram."
/RST, RST Low Output
The output is a push-pull logic signal which is asserted when
the MIC2779 detects a low input voltage. The MIC2779L's
/RST output is active-low; the MIC2779H's RST output is
active-high.
Functional Diagram
GND
V
BATT
RST*
1.24V
Bandgap
Reference
Low-Voltage
Detect
High-Voltage
Detect
R
S
Q
Q
Delay
Line
LTH
HTH
V
LO
V
HI
MIC2779
V
DD
* Pinout and polarity vary by device type.
See ordering information table.
/RST*
One Shot
Trip Points
Battery voltage is monitored by a comparator via a voltage
divider network. The divided voltage is compared to an
internal reference voltage. When the voltage at the LTH input
pin drops below the internal reference voltage, the output is
asserted. 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 is deasserted after a minimum delay of 140ms.
June 2000
5
MIC2779
MIC2779
Micrel
Applications Information
Programming the Thresholds
The low-voltage threshold is calculated using:
V
V
R1 R2 R3
R2 R3
BAT(lo)
REF
=
+
+
+
The high-voltage threshold is calculated using:
V
V
R1 R2 R3
R3
BAT(hi)
REF
=
+
+
where, for both equations:
V
1.240V
REF
=
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 1M
for R
TOTAL
is a reasonable value
because it draws minimum battery current but has no mea-
surable effect on accuracy.
When working with large resistors, a small amount of leakage
current can cause voltage offsets that degrade system accu-
racy. The maximum recommended total resistance from
V
BAT
to ground is 3M
.
HTH
/RST
VDD
LTH
GND
MIC2779
604k
1%
56k
1%
340k
1%
V
BATT
R1
R2
R3
RST
Figure 1. Example Circuit
Once the desired trip points are determined, set the V
BAT(hi)
threshold first.
For example, use a total of 1M
= 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:
V
1.24
1M
R3
BAT(hi)
=
R3
344k
=
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
of the two remaining resistor values.
V
3.1V
1.24
1M
R2
344k
BAT(lo)
=
=
+
R2
56k
=
R1 1M
R2 R3
=
-
-
R1 600k
=
The accuracy of the resistors can be chosen based upon the
accuracy required by the system.
Input Transients
The MIC2779 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 MIC2779.
The graph represents the typical allowable transient duration
for a given amount of threshold overdrive that will not gener-
ate a reset.
0
20
40
60
80
100
120
140
1
10
100
1000
MAX. TRANSIENT DURATION (
s)
RESET COMP. OVERDRIVE, V
REF
V
LTH
(mV)
Input Transient
Response
Figure 2. Input Transient Response
MIC2779
Micrel
MIC2779
6
June 2000
Package Information
0.20 (0.008)
0.09 (0.004)
0.60 (0.024)
0.10 (0.004)
3.02 (0.119)
2.80 (0.110)
10
0
3.00 (0.118)
2.60 (0.102)
1.75 (0.069)
1.50 (0.059)
0.95 (0.037) REF
1.30 (0.051)
0.90 (0.035)
0.15 (0.006)
0.00 (0.000)
DIMENSIONS:
MM (INCH)
0.50 (0.020)
0.35 (0.014)
1.90 (0.075) REF
SOT-23-5 (M5)
June 2000
7
MIC2779
MIC2779
Micrel
MIC2779
Micrel
MIC2779
8
June 2000
MICREL INC.
1849 FORTUNE DRIVE
SAN JOSE, CA 95131
USA
TEL
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or
other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.
2000 Micrel Incorporated
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