MIC2778-1YM5TR - Micrel, Inc.

January 2005 1 M9999-011405

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.

Typical Application

HTH

R STVDD

LTH

GND

MIC2778

604k

56k

340k

E N

OUT

B Y P

GND

MIC5207-3.0BM5

100k

Li-Ion

Cell

470pF

VOUT

3.0V

4.7µF

VBAT(hi) = 3.6V

VBAT(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 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

Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com

IttyBitty™ is a trademark of Micrel, Inc.

Ordering Information

Part Number Accurary Temperature Range Package

Standard Marking Pb-Free Marking

MIC2778-2BM5 UFB MIC2778-2YM5 UFB* 2% -40°C to +85°C SOT-23-5

MIC2778-1BM5 UFA MIC2778-1YM5 UFA* 1% -40°C to +85°C SOT-23-5

* Underscore indicates a Pb-Free part.

MIC2778 Micrel

M9999-011405 2 January 2005

Pin Description

Pin Number Pin Name Pin Function

1 HTH High-Voltage Theshold (Input): Analog input to a comparator. When the level

on this pin initially rises above VREF, the delay generator cycles and the RST

remains low for a minimum of 140ms.

2 GND Ground

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 VREF, RST is asserted and the condition is

latched until VHTH > VREF.

4 RST Reset (Output): Active-low, open-drain output. This output is asserted and

latched when VLTH <VREF, indicating a low voltage condition. This state

remains latched until VHTH > VREF.

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

Pin Conﬁguration

HTH

VDDR ST

LTH

4 5

GND

SOT-23-5 (M5)

January 2005 3 M9999-011405

MIC2778 Micrel

Absolute Maximum Ratings (Note 1)

Supply Voltage (VDD) .......................................–0.3V to +7V

Input Voltages (VHI, VLO) .................................–0.3V to +7V

RST Output Current (IRST) ......................................... 20mA

Storage Temperature (TS) ........................ –65°C to +150°C

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

Operating Ratings (Note 2)

Supply Voltage (VDD) .................................. +1.5V to +5.5V

Input Voltages (VRST, VLTH, VHTH) ................–0.3V to +6.0V

Ambient Temperature Range (TA) .............. –40°C to +85°C

Package Thermal Resistance ................................ 256°C/W

Electrical Characteristics

1.5V ≤ VDD ≤ 5.5V; TA = +25°C, bold values indicate –40°C ≤ TA ≤ +85°C; unless noted

Symbol Parameter Condition Min Typ Max Units

IDD Supply Current RST not asserted 1 2 µA

ILTH, IHTH Input Leakage Current 5 pA

10 nA

VREF Reference Voltage 1.240 V

MIC2778-2 1.215 1.265 V

MIC2778-1 1.228 1.252 V

tD Propagation Delay VLTH = VREF(max) +100mV to 5 µs

VREF(min) – 100mV

tRESET Reset Pulse Width 140 420 ms

VRST Voltage-Low Reset RST asserted, ISINK = 1.6mA, VDD ≥ 1.6V 0.3 V

RST asserted, ISINK = 100µA, VDD ≥ 1.2V, 0.4 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. VDD operating range is 1.5V to 5.5V. Output is guaranteed to be held low down to VDD = 1.2V.

Timing Diagram

VHTH

VBATT

VLTH

VRSTH

VRSTL

tRST

VRST

Propagation delays not shown for clarity.

Note A. The MIC2778 ignores very brief transients.

See “Applications Information” for details.

MIC2778 Micrel

M9999-011405 4 January 2005

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.

Functional Diagram

GND

VBAT T

R S T

1.24V

Bandgap

Reference

Low-Voltage

Detect

High-Voltage

Detect

Delay

Line

LTH

HTH

VLO

VHI

MIC2778

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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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 domi-

nate. 470kΩ is the maximum recommended value.

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

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 =

RTOTAL. A value such as 1MΩ for RTOTAL 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

VBAT to ground is 3MΩ.

HTH

R STVDD

LTH

GND

MIC2778

604k

56k

340k

470k

VBATT

Figure 1. Example Circuit

Once the desired trip points are determined, set the VBAT(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:

V 1.24 1M

BAT(hi) =Ω











R3 344k= Ω

Once R3 is determined, the equation for VBAT(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 VBAT(lo) threshold allows calculation

of the two remaining resistor values.

V 3.1V 1.24 1M

R2 344k

BAT(lo) = = Ω











R2 56k= Ω

R1 1M R2 R3= − −Ω

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 VBAT(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.

100

120

140

1 10 100 1000

MAX. TRANSIENT DURATION (µs)

RESET COMP. OVERDRIVE, VREF –V LTH (mV)

Input T rans ient

R es pons e

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.

HTH

R STVDD

LTH

GND

MIC2778

604k

56k

340k

VBATT

Microprocessor

Supply In

Reset In

GND

100k

Figure 3. Interfacing to Bidirectional Reset Pin

MIC2778 Micrel

M9999-011405 6 January 2005

Package Information

SOT-23-5 (M5)

MICREL INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA

TEL + 1 (408) 944-0800 FAX + 1 (408) 474-1000 WEB http://www.micrel.com

This information furnished by Micrel in this data sheet is believed to be accurate and reliable. However no responsibility is assumed by Micrel for its use.

Micrel reserves the right to change circuitry and speciﬁcations at any time without notiﬁcation to the customer.

Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can

reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into

the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a signiﬁcant injury to the user. A Purchaser's

use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser's own risk and Purchaser agrees to fully indemnify

Micrel for any damages resulting from such use or sale.