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LT1511

Constant-Current/Constant-Voltage 3A Battery Charger with Input Current Limiting

Linear Technology 

LT1511

APPLICATIONS INFORMATION

capacitor. A resistor divider is used to set the desired VCC

lockout voltage as shown in Figure 2. A typical value for R6

is 5k and R5 is found from:

R5 = R6(VIN – VUV )

VUV

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

the pin will cause reverse battery current to increase from

3µA to 200µA.

If connecting the unused UV pin to the adapter output is

not possible for some reason, it can be grounded. Al-

though 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.

100mV

+

CLP

CL1

LT1511

1µF

CLN

500Ω

VCC

+

UV

RS4*

AC ADAPTER

OUTPUT

VIN

R5

*RS4

=

100mV

ADAPTER CURRENT

LIMIT

R6

1511 • F02

Figure 2. Adapter Current Limiting

Adapter Limiting

An important feature of the LT1511 is the ability to

automatically adjust charging current to a level which

avoids overloading the wall adapter. This allows the

product to operate at the same time that batteries are

being charged without complex load management algo-

rithms. Additionally, batteries will automatically be charged at

the maximum possible rate of which the adapter is capable.

This feature is created by sensing total adapter output

current and adjusting charging 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 within limits. 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 programmed charging

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

filter formed by 500Ω and 1µF is required to eliminate

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

and CLN pins should be connected to VCC.

Charging Current Programming

The basic formula for charging current is (see Block

Diagram):

( ) ( ) ( ) IBAT = IPROG

RS2

RS1

=

2.465V

RPROG

RS2

RS1

where RPROG is the total resistance from PROG pin to ground.

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, 3A charging current is needed. To have low

power dissipation on RS1 and enough signal to drive the

amplifier CA1, let RS1 = 100mV/3A = 0.033Ω. This limits

RS1 power to 0.3W. Let RPROG = 5k, then:

RS2

=

RS3

=

(IBAT)(RPROG)(RS1)

2.465V

=

(3A)(5k)(0.033)

2.465V

=

200Ω

Charging 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). Charging current will be

proportional to the duty cycle of the switch with full current

at 100% duty cycle.

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