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LT1505CG-1

Constant-Current/Voltage High Efficiency Battery Charger

Linear Technology 

LT1505

APPLICATIONS INFORMATION

EMI considerations usually make it desirable to minimize

ripple current in the battery leads. Beads or inductors may

be added to increase battery impedance at the 200kHz

switching frequency. Switching ripple current splits be-

tween the battery and the output capacitor depending on

the ESR of the output capacitor and the battery imped-

ance. If the ESR of COUT is 0.2Ω and the battery impedance

is raised to 4Ω with a bead or inductor, only 5% of the

ripple current will flow in the battery.

Soft Start and Undervoltage Lockout

The LT1505 is soft started by the 0.33µF capacitor on the

VC pin. On start-up, the VC pin voltage will rise quickly to

0.5V, then ramp up at a rate set by the internal 45µA pull-

up current and the external capacitor. Battery charge

current starts ramping up when VC voltage reaches 0.7V

and full current is achieved with VC at 1.1V. With a 0.33µF

capacitor, time to reach full charge current is about 10ms

and it is assumed that input voltage to the charger will

reach full value in less than 10ms. The capacitor can be

increased up to 1µF if longer input start-up times are

needed.

In any switching regulator, conventional timer-based soft

starting can be defeated if the input voltage rises much

slower than the time out period. This happens because the

switching regulators in the battery charger and the com-

puter power supply are typically supplying a fixed amount

of power to the load. If input voltage comes up slowly

compared to the soft start time, the regulators will try to

deliver full power to the load when the input voltage is still

well below its final value. If the adapter is current limited,

it cannot deliver full power at reduced output voltages and

the possibility exists for a quasi “latch” state where the

adapter output stays in a current limited state at reduced

output voltage. For instance, if maximum charger plus

computer load power is 30W, a 15V adapter might be

current limited at 2.5A. If adapter voltage is less than

(30W/2.5A = 12V) when full power is drawn, the adapter

voltage will be pulled down by the constant 30W load until

it reaches a lower stable state where the switching regu-

lators can no longer supply full load. This situation can be

prevented by setting undervoltage lockout higher than the

minimum adapter voltage where full power can be achieved.

92m+V

CL1

LT1505

CLP

1µF

CLN

500Ω

VCC

RS4*

+

CIN

UV

AC ADAPTER

OUTPUT

VIN

R5

*RS4

=

ADAPTER

92mV

CURRENT

LIMIT

R6

1505 F02

Figure 2. Adapter Current Limiting

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 16V;

R5 = 5k (16V – 6.7V)/6.7V = 6.9k

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

10µA to 200µA.

Adapter Current Limiting

(Not Applicable for the LT1505-1)

An important feature of the LT1505 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 batteries are being charged

without complex load management algorithms. Addition-

ally, batteries will automatically be charged at the maximum

possible rate of which the adapter is capable.

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