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1510

Constant-Voltage/Constant-Current Battery Charger

Linear Technology 

LT1510/LT1510-5

APPLICATIONS INFORMATION

capacity ceramic capacitor (5µF to 10µF) from Tokin or

United Chemi-Con/MARCON, et al., and the old standby,

aluminum electrolytic, which will require more microfarads

to achieve adequate ripple rating. OS-CON can also be used.

The output capacitor COUT is also assumed to absorb

output switching current ripple. The general formula for

capacitor current is:

( ( ))( ) IRMS =

0.29



VBAT 1−

L1 f

VBAT

VCC





For example, with VCC = 16V, VBAT = 8.4V, L1 = 30µH and

f = 200kHz, IRMS = 0.2A.

EMI considerations usually make it desirable to minimize

ripple current in the battery leads, and 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 impedance.

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

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

ripple will flow in the battery.

Soft Start

The LT1510 is soft started by the 0.1µF capacitor on VC

pin. On start-up, VC pin voltage will rise quickly to 0.5V,

then ramp at a rate set by the internal 45µA pull-up current

and the external capacitor. Battery charging current starts

ramping up when VC voltage reaches 0.7V and full current

is achieved with VC at 1.1V. With a 0.1µF capacitor, time to

reach full charge current is about 3ms and it is assumed

that input voltage to the charger will reach full value in less

than 3ms. Capacitance can be increased up to 0.47µF 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 20W, a 24V adapter might be

current limited at 1A. If adapter voltage is less than (20W/1A

= 20V) when full power is drawn, the adapter voltage will be

sucked down by the constant 20W load until it reaches a

lower stable state where the switching regulators can no

longer supply full load. This situation can be prevented by

utilizing undevoltage lockout, set higher than the minimum

adapter voltage where full power can be achieved.

A fixed undervoltage lockout of 7V is built into the VCC pin.

Internal lockout is performed by clamping the VC pin low.

The VC pin is released from its clamped state when the VCC

pin rises above 7V. The charger will start delivering current

about 2ms after VC is released, as set by the 0.1µF at VC

pin. Higher lockout voltage can be implemented with a

Zener diode (see Figure 3 circuit).

VIN

VZ

D1

1N4001

2k

VCC

VC LT1510

GND

1510 F03

Figure 3. Undervoltage Lockout

The lockout voltage will be VIN = VZ + 1V.

For example, for a 24V adapter to start charging at 22VIN,

choose VZ = 21V. When VIN is less than 22V, D1 keeps VC

low and charger off.

Charging Current Programming

The basic formula for charging current is (see Block

Diagram):

( )( ) ( ) IBAT =

IPROG

2000

=





2.465V

RPROG





2000

9

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