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LTC1960

1960fa

Watchdog Timer

Charging will begin when either CHARGE_BAT1 or

CHARGE_BAT2 bits are set in the charger register

(address: 111). Charging will stop if the charger register is

not updated prior to the expiration of the watchdog timer.

Simply repeating the same data transmission to the charger

that charging will continue uninterrupted.

Extending System to More than 2 Batteries

The LTC1960 can be extended to manage systems with

more than 3 sources of power. Contact Linear Technology

Applications Engineering for more information.

Charging Depleted Batteries

Some batteries contain internal protection switches that

disconnect a load if the battery voltage falls below what is

considered a reasonable minimum. In this case, the charger

may not start because the voltage at the battery terminal

is less than 5V. The low current mode of the IDAC must be

used in this case to condition the battery. In low current

mode, there is no minimum voltage requirement (but dual

charging is not allowed). Usually, the battery will detect

that it is being charged and then close its protection

switch, which will allow the IDAC to switch to normal

mode. Smart batteries require that charging current not

exceed 100mA until valid charging voltage and charging

current parameters are transmitted via the SMBus. The

low current IDAC mode is ideal for this purpose.

Starting Charge with Dissimilar Batteries

in Dual Charge Mode

When charging batteries of different charger termination

voltages, the charger should be started using the following

procedure:

Step 1. Select only the lowest termination voltage battery

for charging, and set the charger to its charging param-

eters.

Step 2. When the battery current is flowing into that

battery, change to dual charging mode (without stopping

the charger) and set the appropriate charging parameters

for this dual charger condition.

APPLICATIONS INFORMATION

If this procedure is not followed, and BAT2 is significantly

higher voltage than BAT1, the charger could refuse to

charge either battery.

Charge Termination Issues

Batteries with constant-current charging and voltage-

based charger termination might experience problems

with reductions of charger current caused by adapter

limiting. It is recommended that input limiting feature be

defeated in such cases. Consult the battery manufacturer

for information on how your battery terminates charging.

Setting Output Current Limit

The full scale output current setting of the IDAC will

produce VMAX = 102.3mV between CSP and CSN. To set

the full scale current of the DAC simply divide VMAX by

RSNS.

This is expressed by the following equation:

RSNS = 0.1023/IMAX

Table 1. Recommended RSNS Resistor Values

IMAX (A)

RSNS (Ω) 1%

RSNS (W)

1.023

0.100

0.25

2.046

0.050

0.25

4.092

0.025

0.5

8.184

0.012

Use resistors with low ESL.

Inductor Selection

Higher operating frequencies allow the use of smaller

inductor and capacitor values. A higher frequency gener-

ally results in lower efficiency because of MOSFET gate

charge losses. In addition, the effect of inductor value on

ripple current and low current operation must also be

considered. The inductor ripple current

∆IL decreases

with higher frequency and increases with higher VIN.

∆I

OUT

= ()( )

−

⎛

⎝⎜

⎞

⎠⎟

Accepting larger values of

∆IL allows the use of low

inductances, but results in higher output voltage ripple

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