LT1510IGN datasheet(12/16 Pages) LINER

12 / 16 page

LT1510/LT1510-5

APPLICATIONS INFORMATION

Figure 7. Current Comparator for Initiating Float Time-Out

0.18

Ω

GND

NEGATIVE EDGE

TO TIMER

INTERNAL

SENSE

RESISTOR

1510 F06

3.3V OR 5V

ADAPTER

OUTPUT

LT1510

1N4148

0.1

µF

BAT

SENSE

R1*

1.6k

470k

430k

560k

LT1011

1N4148

* TRIP CURRENT =

R1(VBAT)

(R2 + R3)(0.18

Ω)

–

voltage decrease is detected as an indication of near full

charge. The charging current is then reduced to a much

lower value and maintained as a constant trickle charge.

An intermediate “top off” current may be used for a fixed

time period to reduce 100% charge time.

NiMH batteries are similar in chemistry to NiCd but have

two differences related to charging. First, the inflection

characteristic in battery voltage as full charge is ap-

proached is not nearly as pronounced. This makes it more

difficult to use dV/dt as an indicator of full charge, and

change of temperature is more often used with a tempera-

ture sensor in the battery pack. Secondly, constant trickle

charge may not be recommended. Instead, a moderate

level of current is used on a pulse basis (

≈ 1% to 5% duty

cycle) with the time-averaged value substituting for a

constant low trickle.

Thermal Calculations

If the LT1510 is used for charging currents above 0.4A, a

thermal calculation should be done to ensure that junction

temperature will not exceed 125

°C. Power dissipation in

the IC is caused by bias and driver current, switch resis-

tance, switch transition losses and the current sense

resistor. The following equations show that maximum

practical charging current for the 8-pin SO package

(125

° C/W thermal resistance) is about 0.8A for an 8.4V

battery and 1.1A for a 4.2V battery. This assumes a 60

°C

maximum ambient temperature. The 16-pin SO, with a

thermal resistance of 50

°C/W, can provide a full 1.5A

charging current in many situations. The 16-pin PDIP falls

between these extremes. Graphs are shown in the Typical

Performance Characteristics section.

mA V

BIAS

BAT

DRIVER

BAT

SENSE

()( )+

()

+ ()

()( )

[]

()( ) +









()

= () (

)( )

()( )( )( )

0012

..18

Ω

()( )

IBAT

RSW = Switch ON resistance ≈ 0.35Ω

tOL = Effective switch overlap time ≈ 10ns

f = 200kHz (500kHz for LT1510-5)

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