HT7470.PDF

HT7470

Fast Battery Charger

Features

Fast charge NiCd battery

Direct drive LED display and alarm buzzer

V detector to determine

peak charging voltage

Automatic switching from fast to trickle

charge

Auto cutoff for defective battery

Warm up option for battery

Fast charge from C/2 to 4C rate

Maximum voltage protection

C/10 trickle charge rate

Auto-start function

Adjustable thermo protection

12-bit A/D converter

Built-in safety timer

General Description

The HT7470 is a 4-bit single-chip microcom-

puter specially designed for NiCd battery fast

charge/discharge control. The advanced unique

design and multiple protection features of the

HT7470 eliminate excessive external compo-

nents in application circuits, and ensure bat-

tery safety and high performance.

Pin Assignment

Block Diagram

Pin Description

Pin No. Pin Name I/O

Description

EOC

End of A/D conversion. The A/D conversion process takes place

during the high period on this pin. When this pin falls low, this

indicates the end of the A/D conversion.

I Over temperature input, active low.

LED1

O LED 1 indicates the charge, discharge and battery status.

VDD

— Positive voltage supply. (5V)

OSCO

OSCI

OSCI and OSCO are connected to a resistor to generate the system

clock. The frequency of the system clock is 1MHz.

VSS

— Negative power supply.

RST

I Input to reset internal LSI, active low.

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HT7470

Pin No. Pin Name I/O

Description

S0/ BZ

S1/BZ

I/O

S0, S1 are charge rate inputs.

BZ, BZ are a buzzer driving pair.

DCHI

I Discharge input, active low. Discharge before charging, active low.

WARM

I Warm-up key, active low. Used only to warm up the battery.

LED2

O LED2 indicates the charge, discharge and battery status.

DCHO

O Discharge control output, active high.

CHGO

O Charge control output, active high.

O A/D converter start conversion control output, active low.

Absolute Maximum Ratings

Supply Voltage .............................. –0.3V to 5.5V

Input Voltage................. V SS –0.3V to V DD +0.3V

°C to 125°C

Operating Temperature .............. –20

°C to 70°C

Electrical Characteristics

Test condition

Symbol

Parameter

Min. Typ. Max. Unit

V DD

Condition

V DD

Operating voltage

—

3.5

5.5

I DD

Operating current

No load

f SYS =1MHz

—

1.5

V IL

Input low voltage

—

V SS

— 0.2V DD V

V IH

Input high voltage

—

0.8V DD — DD

I OL

LED1, LED2 output sink

current

5V V OL =0.5V

2.5

—

I OH

LED1 output source current

5V V OH =4.5V

–2.5

—

I OL1

CHGO, DCHO, BZ, BZ, SC

output sink current

5V V OL =0.5V

2.5

—

I OH1

CHGO, DCHO, BZ, BZ, SC,

LED2 output source current

5V V OH =4.5V

–1.5

—

R PH

Pull-high resistance

S0, S1 , WARM

DC HI, RST, EOC,

—50

—K

f SYS

System clock

5V R OSC =18K

—

MHz

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Storage Temperature ................ –50

HT7470

Fundamental Function

Charge Function

The charge function of the HT7470 consists of

two periods, the charging period and the status

detection period. During the charging period,

current is supplied to the battery for charging,

while during the status detection period, the

battery voltage and temperature are measured

to determine if the battery is fully charged,

open, shorted or damaged.

Warm-up Function

The HT7470 provides a battery warm-up func-

tion to warm up the battery and improve bat-

tery efficiency. The warm-up key is enabled only

in the warm-up waiting period. A low 20ms

pulse in the warm-up waiting period will force

the HT7470 to warm-up the battery. During

this time the discharge input will be inhibited.

The warm-up function is identical to the charge

function.

The warm-up interval is dependent on the

charge rate inputs and is described below:

Trickle Function (C/10)

When the battery has been detected to have

reached full charge by the HT7470, the charge

rate will switch to trickle charge automatically.

The trickle charge function is dependent upon

the charge inputs and is shown below.

The trickle charge function is different but the

trickle charge rate is identical to C/10. This

function is accomplished by adjusting the

charge duty cycle which is also implemented in

the HT7470. The charge rate KC, is calculated

by the following equation:

Charge rate

inputs

4C 2C 1C C/2

S1,S0

Charge Count

150 300 600 1200

ie (i)

.5748

During battery warm-up, full charge detection

and abnormal charge detection are always en-

abled.

Charge current

Battery capacity

= Battery capacity

Full Charge Detection

There are four full charge detection methods

implemented in the HT7470 as described be-

low:

Discharge Function

The HT7470 provides a discharge function to

prevent battery memory effect from occuring. If

battery memory effect has occured this function

will eliminate the effect gradually. The dis-

charge input can be selected to discharge the

battery before the charge procedure has

started. When the battery is empty the HT7470

will start the charge procedure automatically.

V detection

Over Temperature: The TC input pin is logi-

cal low

Trickle Function (C/10)

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HT7470

Safety Timer Overflow: Internal event

counter has overflowed

ternal pull-high resistor. In order to guarantee

all circuits are reset it is combined with an

external 0.1

Charge rate

inputs

mF capacitor to provide an internal

reset pulse. If the charge loops is open the

HT7470 will enter the stand-by mode. In this

mode the HT7470 continues to detect whether

the battery is ready or not.

1C C/2

S1,S0

Charge Count

1200 2400 4800 9600

Time (min)

112 224

Charge and Discharge Inputs

Charge rate inputs: 4C, 2C, 1C, C/2

S1 and S0 are the charge rate inputs with inter-

nal pull-high resistors. The HT7470 adjusts the

internal parameters such as safety timer,

trickle charge duty cycle, and warm-up interval

by referring to these charge rate inputs to make

the system safe. These charge rate inputs are

determined by the external charge current. The

relationship between the inputs “S1,S0” and

charge rate are listed in the following table.

Battery Maximum Voltage Protection

If any of the conditions described above oc-

cur the charge rate will switch to trickle

charge automatically.

Abnormal Charge Detection (Open, Short, Bat-

tery Damage)

The HT7470 can detect abnormal battery states

which include charge loop open or short, battery

high impedance or short. There is a precharge

procedure to pull-up the battery voltage for the

detection of abnormal states. The HT7470 pre-

charges the battery and looks for abnormal

states before the charge procedure begins. The

detection of abnormal states is always enabled

when the charge procedure is running. Any ab-

normal state will force the HT7470 to enter the

abnormal mode and display the corresponding

indicator or turn on the buzzer. In abnormal

mode the HT7470 stops charging until the ab-

normal battery is removed or the charge loop is

open.

Charge rate inputs 4C 2C 1C C/2

S1,S0

11 10 01 00

Discharge Input: Discharge before charge pro-

cedu re.

The DCHI pin is the discharge input with inter-

nal pull-high resistor. The user can force this

pin low to select the discharge function of the

HT7470.

War m-Up K ey

The WARM pin is the warm-up input with in-

ternal pull-high resistor and 20ms debounce

time.

The warm-u p funct ion is activated by a low

pulse on the WARM pin. The pulse width must

be larger than 20ms during the warm-up wait-

ing period.

Note that the discharge input will be inhibited

if the warm-up key is pushed.

The charge and discharge inputs are enabled

until the warm-up key is pushed or until the

end of the warm-up waiting period.

A/D Converter

The battery voltage is read into the HT7470

through a 12 bit wide integral type A/D con-

verter. The resolution of the A/D converter is 68

s/count (1MHz system clock).

Auto - Start and Reset Key (RST)

The HT7470 can detect whether the battery is

ready or not. When the battery has been de-

tected to be ready the HT74 70 ch arge procedure

will run automatically. The RST pin is used for

system initialization. This reset pin has an in-

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HT7470

Indicator

The HT7470 can indicate all the battery’s states

with the indicators LED1, LED2 and the buzzer

(BZ, BZ). Each state is represented in the fol-

lowing table.

Mode

38.54ms and the minimum battery rated volt-

age is 2.4V in this case.

= Battery rated voltage – 2.4

2.4

* 2K

LED1* LED2* Buzzer

Charging

off

Warming

off

Discharging

off

Full charge

Flash

(slow)

off

Warm-up

waiting

period

Flash

(fast)

off

Abnormal

Flash

Flash tone

(1 minute)

Standby

off

* Note: LED1 and LED2 are active low.

Input RC-Network

The HT7470 is a universal fast charge control-

ler for NiCd batteries. It can be used to charge

batteries of any rated voltage by adjusting the

CR resistance of the RC-network and the inte-

gral time constant of the A/D converter. The

RC-network is configured as shown and the CR

resistance is listed in the table. Note that the

integral time constant of the A/D converter is

Input RC-Network

If the minimum battery rated voltage is 1.2V

the CR1 resistance must be 1K and the integral

time constant of the A/D converter must be

77.08ms. The CR resistance can be calculated

by the following equation

= Battery rated voltage – 1.2

1.2

* 1K

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