LT1769IGN datasheet(10/16 Pages) LINER

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LT1769

APPLICATIONS INFORMATION

R5 =

R6(V

– V

)

VUV = Rising lockout threshold on the UV pin

VIN = Charger input voltage that will sustain full load power

Example: With R6 = 5k, VUV = 6.7V and setting VIN at 12V;

R5 = 5k (12V – 6.7V)/6.7V = 4k

The resistor divider should be connected directly to the

adapter output as shown, not to the VCC pin, to prevent

battery drain with no adapter voltage. If the UV pin is not

used, connect it to the adapter output (not VCC) and

connect a resistor no greater than 5k to ground. Floating

this pin will cause reverse battery current to increase from

µA to 200µA.

If connecting the unused UV pin to the adapter output is

not possible, it can be grounded. Although it would seem

that grounding the pin creates a permanent lockout state,

the UV circuitry is arranged for phase reversal with low

voltages on the UV pin to allow the grounding technique to

work.

ally, batteries will automatically be charged at the maximum

possible rate of which the adapter is capable.

This is accomplished by sensing total adapter output

current and adjusting the charge current downward if a

preset adapter current limit is exceeded. True analog

control is used, with closed-loop feedback ensuring that

adapter load current remains below the limit. Amplifier

CL1 in Figure 2 senses the voltage across RS4, connected

between the CLP and CLN pins. When this voltage exceeds

100mV, the amplifier will override the programmed charge

current to limit adapter current to 100mV/RS4. A lowpass

filter formed by 500

Ω and 1µF is required to eliminate

switching noise. If the input current limit is not used, both

CLP and CLN pins should be connected to VCC.

Charge Current Programming

The basic formula for charge current is (see Block

Diagram):

IBAT = IPROG

2.465V

RPROG

RS2

RS1

()( )

RS2

RS1

()

where RPROGisthetotalresistancefromPROGpintoground.

For the sense amplifier CA1 biasing purpose, RS3 should

have the same value as RS2 and SPIN should be connected

directly to the sense resistor (RS1) as shown in the Block

Diagram.

For example, 2A charge current is needed. For low power

dissipation on RS1 and enough signal to drive the amplifier

CA1, let RS1 = 100mV/2A = 0.05Ω. This limits RS1 power

to 0.2W. Let RPROG = 5k, then:

RS2 = RS3 =

= 200

Ω

(IBAT)(RPROG)(RS1)

2.465V

(2A)(5k)(0.05)

2.465V

Charge current can also be programmed by pulse width

modulating IPROG with a switch Q1 to RPROG at a frequency

higher than a few kHz (Figure 3). Charge current will be

proportional to the duty cycle of the switch with full current

at 100% duty cycle.

100mV

500

Ω

CLP

CLN

VCC

1769 F02

LT1769

µF

RS4*

VIN

AC ADAPTER

OUTPUT

*RS4 =

100mV

ADAPTER CURRENT LIMIT

–

CL1

Figure 2. Adapter Input Current Limiting

Adapter Current Limiting

An important feature of the LT1769 is the ability to

automatically adjust charge current to a level which avoids

overloading the wall adapter. This allows the product to

operate at the same time the batteries are being charged

without complex load management algorithms. Addition-

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