MIC2779
Micrel, Inc.
MIC2779
Voltage Monitor with Adjustable Hysteresis
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 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 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 (1µA, typical), making it ideal for portable
applications.
A high-precision 1% grade is available. The MIC2779 is sup-
plied in Micrel’s IttyBitty™ 5-pin 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 requiring open-drain output, see
MIC2778/MIC833
• Immune to brief power supply transients
• 5-pin SOT-23 package
Applications
•
•
•
•
•
•
PDAs
Pagers
Cordless phones
Consumer electronics
Embedded controllers
Personal electronics
Typical Application
MIC5207-3.0BM5
Li-Ion
Cell
604k
1%
56k
1%
340k
1%
MIC2779L
VDD / R S T
LTH
HTH GND
IN
EN
OUT
BYP
GND
V
OU T
3.0V
4.7µF
470pF
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 2006
1
MIC2779
MIC2779
Micrel, Inc.
Ordering Information
Part Number
Standard
MIC2779H-2BM5
MIC2779H-1BM5
MIC2779L-2BM5
MIC2779L-1BM5
Marking
UPA2
UPA1
UPB2
UPB1
Pb-Free
MIC2779H-2YM5
MIC2779H-1YM5
MIC2779L-2YM5
MIC2779L-1YM5
Marking
UPA2
UPA1
UPB2
UPB1
Output Polarity
Active-High RST
Active-High RST
Active-Low /RST
Active-Low /RST
Accuracy
2%
1%
2%
1%
Temperature Range
–40ºC to +85ºC
–40ºC to +85ºC
–40ºC to +85ºC
–40ºC to +85ºC
Package
SOT-23-5
SOT-23-5
SOT-23-5
SOT-23-5
Pin Configuration
LTH
GND
HTH
3
2
LTH
GND
HTH
3
2
1
1
4
5
4
5
RST
VDD
/RST
VDD
SOT-23-5 (M5)
“H” Version
SOT-23-5 (M5)
“L” Version
Pin Description
Pin Number
MIC2779H
1
Pin Number
MIC2779L
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 or RST remains high 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 or RST is asserted and the condition
is latched until V
HTH
> V
REF
.
2
3
2
3
GND
LTH
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.
Power Supply (Input): Independent supply input for internal circuitry.
5
5
VDD
MIC2779
2
January 2006
MIC2779
Micrel, Inc.
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
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
MIC2779H/L-2
MIC2779H/L-1
t
D
t
RESET
V
OL
Propagation Delay
Reset Pulse Width
Reset Output Low Voltage
V
LTH
= V
REF(max)
+100mV to
V
REF(min)
– 100mV
/RST asserted or RST not asserted,
I
SINK
= 1.6mA, V
DD
≥ 1.6V
/RST asserted or RST not asserted,
I
SINK
= 100µA, V
DD
≥ 1.2V,
/RST not asserted or RST asserted,
I
SOURCE
= 500µA, V
DD
≥ 1.6V
/RST not asserted or RST asserted,
I
SOURCE
= 50µA, V
DD
≥ 1.2V,
Note 1.
Note 2.
Note 3.
Note 4.
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.
Min
Typ
1
5
Max
2
10
Units
µA
pA
nA
V
V
V
µs
/RST, RST not asserted
I
LTH,
I
HTH
V
REF
1.240
1.215
1.228
5
140
420
0.3
0.4
0.8V
DD
0.8V
DD
1.265
1.252
ms
V
V
V
V
V
OH
Reset Output High Voltage
Timing Diagram
V
BATT
V
HTH
V
LTH
0V
V
/RST
V
RST
A
A
t
RST
t
RST
V
OH
V
OL
V
OH
V
OL
Propagation delays not shown for clarity.
Note A.
The MIC2779 ignores very brief transients.
See “Applications Information” for details.
January 2006
3
MIC2779
MIC2779
Micrel, Inc.
Functional Diagram
V
BATT
V
DD
Low-Voltage
Detect
V
LO
LTH
High-Voltage
Detect
One Shot
Delay
Line
R Q
S Q
/RST*
RST*
V
HI
HTH
1.24V
Bandgap
Reference
MIC2779
GND
* Pinout and polarity vary by device type.
See ordering information table.
Functional Description
The MIC2779 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 (MIC2779L)
or high (MIC2779H) for a minimum of 140ms and then sends
a reset signal to a microprocessor or other downstream
component. 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.
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 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.
MIC2779
4
January 2006
MIC2779
Micrel, Inc.
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.
1MΩ
V
BAT(lo)
= 3.1V = 1.24
R2 + 344k
R2 = 56k Ω
R1 = 1MΩ − R2 − R3
R1 = 600k Ω
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 dura-
tion for a given amount of threshold overdrive that will not
generate a reset.
Input Transient
Response
Applications Information
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
measurable 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
R 1
604k
1%
R 2
56k
1%
R 3
340k
1%
MIC2779
VDD / R S T
LTH
R ST
HTH
GND
MAX. TRANSIENT DURATION (µs)
140
120
100
80
60
40
20
0
1
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:
1MΩ
V
BAT(hi)
= 1.24
R3
R3 = 344kΩ
10
100
1000
RESET COMP. OVERDRIVE, V
REF
–V
LTH
(mV)
Figure 2. Input Transient Response
January 2006
5
MIC2779
嵌入式系统
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