10

LT1511

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

is 5k and R5 is found from:

R5 =

R6(V

– V

)

V

UV

UV

IN

R5 = 5k (12V – 6.7V)/6.7V = 4k

The resistor divider should be connected directly to the

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

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

the pin will cause reverse battery current to increase from

3

µA 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.

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

between the CLP and CLN pins. When this voltage exceeds

100mV, the amplifier will override programmed charging

filter formed by 500

Ω and 1µF is required to eliminate

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

Charging Current Programming

The basic formula for charging current is (see Block

Diagram):

=

2.465V

()( )

()

Diagram.

For example, 3A charging current is needed. To have low

=

= 200

Ω

2.465V

(3A)(5k)(0.033)

2.465V

Charging current can also be programmed by pulse width

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.

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

100mV

–

+

500

Ω

CLP

CLN

UV

1511 • F02

R5

LT1511

R6

1

µF

+

CL1

AC ADAPTER

OUTPUT

100mV

ADAPTER CURRENT LIMIT

+