Features

Safe management of fast charge

for NiCd and NiMH battery

packs.

High-frequency switching con-

troller for efficient and simple

charger design

Pre-charge qualification for

detecting shorted, damaged, or

overheated cells

Fast-charge termination by

peak voltahge (PVD), maximum

temperature, and maximum

charge time

Selectable top-off mode for

achieving maximum capacity in

NiMH batteries

Programmable trickle-charge

mode for reviving deeply dis-

charged batteries and for post-

charge maintenance

Built-in battery removal and in-

sertion detection

Sleep mode for low power con-

sumption

General Description

The bq24400 is a programmable,

monolithic IC for fast-charge manage-

ment of nickel cadmium (NiCd) and

nickel metal-hydride (NiMH) batter-

ies.
The bq24400 provides a number of

charge termination criteria:

Peak voltage, PVD (for NiCd and

NiMH)

Maximum temperature

Maximum charge time

For safety, the bq24400 inhibits fast

charge until the battery voltage and

temperature are within user-defined

limits. If the battery voltage is below

the low-voltage threshold, the

bq24400 uses trickle-charge to

condition the battery. For NiMH

batteries, the bq24400 provides an

optional top-off charge to maximize

the battery capacity.
The integrated high-frequency com-

parator allows the bq24400 to be the

basis for a complete, high-efficiency

power-conversion circuit.

bq24400

SNS

Current-sense input

System ground

LED

Charge-status

output

BAT

Battery-voltage

input

PN-2000.eps

8-Pin DIP or Narrow SOIC

or TSSOP

SNS

LED

BAT

VSS

MOD

VCC

Pin Connections

Pin Names

Programmable NiCd/NiMH

Fast-Charge Management Device

SLUS498 -SEPTEMBER 2001

Temperature-sense

input

Timer-program input

Supply-voltage input

MOD

Modulation-control

output

Pin Descriptions

SNS

Current-sense input

Enables the bq24400 to sense the battery cur-

rent via the voltage developed on this pin by

an external sense-resistor connected in series

with the battery pack.

System Ground

LED

Charge-status output

Open-drain output that indicates the charg-

ing status by turning on, turning off, or flash-

ing an external LED.

BAT

Battery-voltage input

Battery-voltage sense input. A simple resistive

divider, across the battery terminals, generates

this input.

Temperature-sense input

Input for an external battery-temperature

monitoring circuit. An external resistive di-

vider network with a negative tempera-

ture-coefficient thermistor sets the lower and

upper temperature thresholds.

Timer-program input

RC input used to program the maximum

charge-time, hold-off period, and trickle rate

during the charge cycle, and to disable or

enable top-off charge.

Supply-voltage input

MOD

Modulation-control output

Push-pull output that controls the charging

current to the battery. MOD switches high to

enable charging current to flow and low to

inhibit charging- current flow.

Functional Description

The bq24400 is a versatile, NiCd, NiMH battery- charge

control device. See Figure 1 for a functional block dia-

gram and Figure 2 for the state diagram.

bq24400

BD2000T.eps

Voltage

Reference

Internal

OSC

Clock

Phase

Generator

ADC

OSC

SNS

MOD

BAT

LED

Timer

ALU

Voltage

Comparator

Voltage

Comparator

Charge
Control

Figure 1. Functional Block Diagram

bq24400

4.0 V <V

<6.0 V

Battery Voltage

(PVD checked at rate

of MTO/64)

Sleep

Mode

Charge

Suspended

Charge

Initialization

Battery

Conditioning

Battery Temperature

(checked 1,750 times

per second)

BAT

SLP

BAT

MCV

BAT

SLP

Current

Regulation

BAT

LBAT

LTF

HTF

LBAT

BAT

MCV

and

HTF

LTF

LBAT

BAT

MCV

and

HTF

LTF

PVD (after hold-off period), or

TCO

Time =MTO

Top-Off

Selected?

Maintenance

Charge

Top-Off

Yes

Done

Time =MTO

TCO

BAT

MCV

BAT

MCV

Reset or Battery Replacement

Reset

Figure 2. State Diagram

UDG-01088

Initiation and Charge Qualification

The bq24400 initiates a charge cycle when it detects

Application of power to V

Battery replacement

Exit from sleep mode

Immediately following initiation, the IC enters a

charge-qualification mode. The bq24400 charge qualifica-

tion is based on battery voltage and temperature. If

voltage on pin BAT is less than the internal threshold,

LBAT

, the bq24400 enters the charge-pending state.

This condition indicates the possiblility of a defective or

shorted battery pack. In an attempt to revive a fully

depleted pack, the bq24400 enables the MOD pin to

trickle-charge at a rate of once every 1.0s. As explained in

the section "Top-Off and Pulse-Trickle Charge," the

trickle pulse-width is user-selectable and is set by the

value of the resistance connected to pin RC.
During this period, the LED pin blinks at a 1Hz rate, in-

dicating the pending status of the charger.
Similarly, the bq24400 suspends fast charge if the battery

temperature is outside the V

LTF

to V

HTF

range. (See Table

4.) For safety reasons, however, it disables the pulse trickle,

in the case of a battery over-temperature condition (i.e., V

< V

HTF

). Fast charge begins when the battery temperature

and voltage are valid.

NiCd and NiMH Batteries

Following qualification, the bq24400 fast-charges NiCd or

NiMH batteries using a current-limited algorithm. Dur-

ing the fast-charge period, it monitors charge time, tem-

perature, and voltage for adherence to the termination

criteria. This monitoring is further explained in later sec-

tions. Following fast charge, the battery is topped off, if

top-off is selected. The charging cycle ends with a trickle

maintenance-charge that continues as long as the voltage

on pin BAT remains below V

MCV

Charge Termination

Maximum Charge Time

The bq24400 sets the maximum charge-time through pin

RC. With the proper selection of external resistor and ca-

pacitor, various time-out values may be achieved. Figure

3 shows a typical connection.
The following equation shows the relationship between

the R

MTO

and C

MTO

values and the maximum charge

time (MTO) for the bq24400:

MTO = R

MTO

35,988

MTO is measured in minutes, R

MTO

in ohms, and C

MTO

in farads. (Note: R

MTO

and C

MTO

values also determine

other features of the device. See Tables 2 and 3 for de-

tails.)

Maximum Temperature

A negative-coefficient thermistor, referenced to V

and

placed in thermal contact with the battery, may be used

as a temperature-sensing device. Figure 4 shows a typical

temperature-sensing circuit.
During fast charge, the bq24400 compares the battery

temperature to an internal high-temperature cutoff

threshold, V

TCO

. As shown in Table 4, high-temperature

termination occurs when voltage at pin TS is less than

this threshold.

Peak Voltage

The bq24400 uses a peak-voltage detection (PVD) scheme

to terminate fast charge for NiCd and NiMH batteries.

The bq24400 continuously samples the voltage on the

BAT pin, representing the battery voltage, and triggers

the peak detection feature if this value falls below the

maximum sampled value by as much as 3.8 mV (PVD).

As shown in figure 5, a resistor voltage-divider between

the battery pack's positive terminal and V

scales the

battery voltage measured at pin BAT.
The resistor values R

And R

are calculated by the fol-

lowing equation:

bq24400

Battery Chemistry

Charge Algorithm

NiCd or NiMH

1. Charge qualification

2. Trickle charge, if required

3. Fast charge (constant current)

4. Charge termination (

t, time)

5. Top-off (optional)

6. Trickle charge

Table 1. Charge Algorithm

N 1

where N is the number of cells in series.
The end-to-end input impedance of this resistive divider

network should be at least 200kW and no more than

1MW.

Initial Hold-Off Period

The values of the external resistor and capacitor con-

nected to pin RC set the initial hold-off period. During

this period, the bq24400 avoids early termination by dis-

abling the

t feature. This period is fixed at the pro-

grammed value of the maximum charge time divided by

32.

hold-off period =

maximum time - out

Top-Off and Pulse-Trickle Charge

bq24400

Parameter

Value

Maximum cell voltage (V

MCV

)

Minimum pre-charge qualification voltage (V

LBAT

)

950mV

High-temperature cutoff voltage (V

TCO

)

0.225

High-temperature fault voltage (V

HTF

)

0.25

Low-temperature fault voltage (V

LTF

)

0.5

bq24400 fast-charge maximum time out (MTO)

MTO

35,988

Fast-charge charging current (I

MAX

)

0.05/R

SNS

Hold-off period

MTO/32

Top-off charging current (optional)

MAX

/16

Top-off period (optional)

MTO

Trickle-charge frequency

1Hz

Table 2. Summary of NiCd or NiMH Charging Characteristics

Temperature

Condition

Action

> V

LTF

Cold battery--checked at all times

Suspends fast charge or top-off and timer

Allows trickle charge--LED flashes at 1Hz rate

during pre-charge qualification and fast charge

HTF

< V

LTF

Optimal operating range

Allows charging

< V

HTF

Hot battery--checked during charge qualifi-

cation and top-off and trickle-charge

Suspends fast-charge initiation, does not allow

trickle charge--LED flashes at 1Hz rate during

pre-charge qualification

< V

TCO

Battery exceeding maximum allowable tem-

perature--checked at all times

Terminates fast charge or top-off

Table 3. Temperature-Monitoring Conditions

Table 4. Charge Status Display

Charge Action State

LEDStatus

Battery absent

High impedance

Pre-charge qualification

1Hz flash

Trickle charge (before fast charge)

1Hz flash

Fast charging

Low

Top-off or trickle

High impedance

Charge complete

High impedance

Sleep mode

High impedance

Charge suspended (V

> V

LTF

)

1Hz flash

Top-off may be desirable on batteries that have a ten-

dency to terminate charge before reaching full capacity.

To enable this option, the capacitance value of C

MTO

con-

nected to pin RC (Figure 3) should be greater than

0.13

F, and the value of the resistor connected to this pin

should be less than 15k

. To disable top-off, the capaci-

tance value should be less than 0.07

F. The tolerance of

the capacitor needs to be taken into account in compo-

nent selection.
Once enabled, the top-off is performed over a period

equal to the maximum charge time at a rate of

that of

fast charge.
Following top-off, the bq24400 trickle-charges the battery

by enabling the MOD to charge at a rate of once every 1.0

second. The trickle pulse-width is user-selectable and is

set by the value of the resistor R

MTO

, which is on pin RC.

Figure 6 shows the relationship between the trickle

pulse-width and the value of R

MTO

. The typical tolerance

of the pulsewidth below 150k

10%.

During top-off and trickle-charge, the bq24400 monitors

battery voltage and temperature. These functions are

suspended if the battery voltage rises above the

maximum cell voltage (V

MCV

) or if the temperature

exceeds the high-temperature fault threshold (V

HTF

Charge Current Control

The bq24400 controls the charge current through the

MOD output pin. The current-control circuit supports a

switching-current regulator with frequencies up to

500kHz. The bq24400 monitors charge current at the

SNS input by the voltage drop across a sense-resistor,

SNS

, in series with the battery pack. See Figure 8 for a

typical current-sensing circuit. R

SNS

is sized to provide

the desired fast-charge current (I

MAX

0.05
R

SNS

If the voltage at the SNS pin is greater than V

SNSLO

less than V

SNSHI

, the bq24400 switches the MOD output

high to pass charge current to the battery. When the SNS

voltage is less than V

SNSLO

or greater than V

SNSHI

, the

bq24400 switches the MOD output low to shut off

charging current to the battery. Figure 7 shows a typical

multi-chemistry charge circuit.

bq24400

100

150

200

250

Pulse

width--ms

MTO

--k

100

120

140

160

2000PNvB3.eps

Shows Tolerance

Figure 6. Relationship Between Trickle Pulse-Width and Value of R

MTO

Temperature Monitoring

The bq24400 measures the temperature by the voltage at

the TS pin. This voltage is typically generated by a nega-

tive-temperature-coefficient thermistor. The bq24400

compares this voltage against its internal threshold

voltages to determine if charging is safe. These

thresholds are the following:

High-temperature cutoff voltage: V

TCO

= 0.225

This voltage corresponds to the maximum temperature

(TCO) at which fast charging is allowed. The bq24400

terminates fast charge if the voltage on pin TS falls

below V

TCO

High-temperature fault voltage: V

HTF

= 0.25

This

voltage corresponds to the temperature (HTF) at which

fast charging is allowed to begin.

Low-temperature fault voltage: V

LTF

= 0.5

This voltage corresponds to the minimum temperature

(LTF) at which fast charging or top-off is allowed. If the

voltage on pin TS rises above V

LTF,

the bq24400

suspends fast charge or top-off but does not terminate

charge. When the voltage falls back below V

LTF,

fast

charge or top-off resumes from the point where

suspended. Trickle-charge is allowed during this

condition.

Table 3 summarizes these various conditions.

Charge Status Display

The charge status is indicated by open-drain output LED.

Table 4 summarizes the display output of the bq24400.

Sleep Mode

The bq24400 features a sleep mode for low power con-

sumption. This mode is enabled when the voltage at pin

BAT is above the low-power-mode threshold, V

SLP

. Dur-

ing sleep mode, the bq24400 shuts down all internal cir-

cuits, drives the LED output to high-impedance state,

and drives pin MOD to low. Restoring BAT below the

MCV

threshold initiates the IC and starts a fast-charge

cycle.

bq24400

2000TCS.eps

BAT-

Power Supply ground

bq2000 ground

SNS

bq2000T

Figure 8. Current-Sensing Circuit

bq24400

Absolute Maximum Ratings

Symbol

Parameter

Minimum

Maximum

Unit

Notes

relative to V

-0.3

+7.0

DC voltage applied on any pin, ex-

cluding V

relative to V

-0.3

+7.0

OPR

Operating ambient temperature

-20

+70

°C

STG

Storage temperature

-40

+125

°C

SOLDER

Soldering temperature

+260

°C

10s max.

Note:

Permanent device damage may occur if Absolute Maximum Ratings are exceeded. Functional operation

should be limited to the Recommended DC Operating Conditions detailed in this data sheet. Exposure to

conditions beyond the operational limits for extended periods of time may affect device reliability.

DC Thresholds

(TA = TOPR; VCC = 5V

20% unless otherwise specified)

Symbol

Parameter

Rating

Tolerance

Unit

Notes

TCO

Temperature cutoff

0.225 * V

Voltage at pin TS

HTF

High-temperature fault

0.25 * V

Voltage at pin TS

LTF

Low-temperature fault

0.5 * V

Voltage at pin TS

MCV

Maximum cell voltage

2.00

2.5%

BAT

> V

MCV

inhibits

fast charge

LBAT

Minimum cell voltage

950

Voltage at pin BAT

PVD

BAT input change for PVD detection

3.8

20%

SNSHI

High threshold at SNS, resulting in

MOD-low

Voltage at pin SNS

SNSLO

Low threshold at SNS, resulting in

MOD-high

-50

Voltage at pin SNS

SLP

Sleep-mode input threshold

- 1

0.5

Applied to pin BAT

RCH

Recharge threshold

MCV

- 0.1

0.02

At pin BAT

bq24400

bq24400

Impedance

Symbol

Parameter

Minimum

Typical

Maximum

Unit

BAT

Battery input impedance

TS input impedance

SNS

SNS input impedance

Recommended DC Operating Conditions

(TA = TOPR)

Symbol

Condition

Minimum

Typical

Maximum

Unit

Notes

Supply voltage

4.0

5.0

6.0

Supply current

0.5

Exclusive of external loads

CCS

Sleep current

BAT

= V

SLP

Thermistor input

0.5

< 0.5V prohibited

Output high

- 0.2

MOD, I

= 20mA

Output low

0.2

MOD, LED, I

= 20mA

High-impedance leakage

current

LED

snk

Sink current

MOD, LED

MTO

Charge timer resistor

250

MTO

Charge timer capacitor

0.001

1.0

Note:

All voltages relative to V

except as noted.

Timing

(TA = TOPR; VCC = 5V

20% unless otherwise specified)

Symbol

Parameter

Minimum

Typical

Maximum

Unit

MTO

MTO time-base variation

-5

TRKL

Pulse-trickle frequency

0.9

1.0

1.1

bq24400

Ordering Information

bq24400

Package Option:

P = 8-pin narrow plastic DIP

D = 8-pin narrow SOIC

PW = 8-pin TSSOP

Device:

bq24400 NiCd/NiMH Fast-Charge IC with

t Detection

0.015 (0,38)

Gage Plane

0.325 (8,26)
0.300 (7,62)

0.010 (0,25) NOM

MAX

0.430 (10,92)

4040082/D 05/98

0.200 (5,08) MAX

0.125 (3,18) MIN

0.355 (9,02)

0.020 (0,51) MIN

0.070 (1,78) MAX

0.240 (6,10)

0.260 (6,60)

0.400 (10,60)

0.015 (0,38)

0.021 (0,53)

Seating Plane

0 .01 0 (0 ,2 5)

0.100 (2,54)

8-Pin DIP (P)

NOTES:

A. All linear dimensions are in inches (millimeters).
B. This drawing is subject to change without notice.
C. Falls within JEDEC MS-001

bq24400

14 PINS SHOWN

4 0 4 0 0 4 7 / D 1 0 /9 6

0.228 (5,80)

0.244 (6,20)

0.069 (1,75) MAX

0.010 (0,25)
0.004 (0,10)

0.014 (0,35)

0.020 (0,51)

0.157 (4,00)

0.150 (3,81)

0.044 (1,12)

0.016 (0,40)

Seating Plane

0.010 (0,25)

PINS **

0.008 (0,20) NOM

A MIN

A MAX

DIM

Gage Plane

0.189

(4,80)

(5,00)

0.197

(8,55)

(8,75)

0.337

0.344

(9,80)

0.394

(10,00)

0.386

0.004 (0,10)

0 .0 1 0 (0 ,2 5 )

0.050 (1,27)

05- 85

8-Pin SOIC Narrow (D)

NOTES:

A. All linear dimensions are in inches (millimeters).
B. This drawing is subject to change without notice.
C. Body dimensions do not include mold flash or protrusion, not to exceed 0.006 (0,15).
D. Falls within JEDEC MS-012

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