October 1999

MIC2778

Micrel

MIC2778

Voltage Monitor with Adjustable Hysteresis

Preliminary Information

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 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) 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 MIC833.

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 (1

A, typical), making it ideal for portable

applications.

A high-precision 1% grade is available. The MIC2778 is
supplied in Micrel's IttyBittyTM 5-lead SOT-23-5 package.

Typical Application

HTH

RST

VDD

LTH

GND

MIC2778

604k
1%

56k
1%

340k
1%

OUT

BYP

GND

MIC5207-3.0BM5

100k

Li-Ion

Cell

470pF

OUT

3.0V

4.7µF

BAT(hi)

= 3.6V

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 not requiring built-in delay, see 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.

Ordering Information

Part Number

Marking

Accuracy

Temperature Range

Package

MIC2778-2BM5

UFB

C to +85

SOT-23-5

MIC2778-1BM5

UFA

C to +85

SOT-23-5

MIC2778

Micrel

MIC2778

October 1999

Pin Description

Pin Number

Pin Name

Pin Function

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 for a minimum of 140ms.

GND

Ground

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 is asserted and the condition is

latched until V

HTH

> V

REF

RST

Reset (Output): Active-low, open-drain output. This output is asserted and
latched when V

LTH

REF

, indicating a low voltage condition. This state

remains latched until V

HTH

> V

REF

VDD

Power Supply (Input): Independent supply input for internal circuitry.

Pin Configuration

HTH

VDD

RST

LTH

GND

SOT-23-5 (M5)

October 1999

MIC2778

Micrel

Absolute Maximum Ratings

(Note 1)

Supply Voltage (V

) ..................................... 0.3V to +7V

Input Voltages (V

, V

) ............................... 0.3V to +7V

RST Output Current (I

RST

) ......................................... 20mA

Storage Temperature (T

) ....................... 65

C to +150

ESD Rating, Note 3 ...................................................... 2kV

Operating Ratings

(Note 2)

Supply Voltage (V

) .................................. +1.5V to +5.5V

Input Voltages (V

RST

, V

LTH

, V

HTH

) ............. 0.3V to +6.0V

Ambient Temperature Range (T

) ............. 40

C to +85

Package Thermal Resistance ............................... 256

C/W

Electrical Characteristics

1.5V

5.5V; T

= +25

C, bold values indicate 40

+85

C; unless noted

Symbol

Parameter

Condition

Min

Typ

Max

Units

Supply Current

RST not asserted

LTH,

HTH

Input Leakage Current

REF

Reference Voltage

1.240

MIC2778-2

1.215

1.265

MIC2778-1

1.228

1.252

Propagation Delay

LTH

= V

REF(max)

+100mV to

REF(min)

100mV

RESET

Reset Pulse Width

140

420

RST

Voltage-Low Reset

RST asserted, I

SINK

= 1.6mA, V

1.6V

0.3

RST asserted, I

SINK

= 100

A, V

1.2V,

0.4

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.

operating range is 1.5V to 5.5V. Output is guaranteed to be held low down to V

= 1.2V.

Timing Diagram

HTH

BATT

LTH

RSTH

RSTL

RST

RST

Propagation delays not shown for clarity.

Note A.

The MIC2778 ignores very brief transients.
See "Applications Information" for details.

MIC2778

Micrel

MIC2778

October 1999

Functional Description

The MIC2778 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 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.

Functional Diagram

GND

BATT

RST

1.24V
Bandgap
Reference

Low-Voltage

Detect

High-Voltage

Detect

Delay

Line

LTH

HTH

MIC2778

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
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.

October 1999

MIC2778

Micrel

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
dominate. 470k

is the maximum recommended value.

Programming the Thresholds

The low-voltage threshold is calculated using:

R1 R2 R3

R2 R3

BAT(lo)

REF

The high-voltage threshold is calculated using:

R1 R2 R3

BAT(hi)

REF

where, for both equations:

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 signifi-
cant 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

MIC2778

604k
1%

56k
1%

340k
1%

470k

BATT

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:

1.24

BAT(hi)

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.

3.1V

1.24

344k

BAT(lo)

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 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 duration
for a given amount of threshold overdrive that will not gener-
ate a reset.

100

120

140

100

1000

MAX. TRANSIENT DURATION (

RESET COMP. OVERDRIVE, V

REF

LTH

(mV)

Input Transient

Response

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 4.

HTH

RST

VDD

LTH

GND

MIC2778

604k
1%

56k
1%

340k
1%

BATT

Microprocessor

Supply In

Reset In

GND

100k

Figure 4. Interfacing to Bidirectional Reset Pin

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