LTC1960 datasheet(21/24 Pages) LINER

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LTC1960

1960fa

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 COUTis 0.2Ω and the battery impedance

is raised to 4

Ω with a bead or inductor, only 5% of the

current ripple will flow in the battery.

Power Path and Charge MUX MOSFET Selection

Three pairs of P-channel MOSFETs must be used with the

wall adapter and the two battery discharge paths. Two

pairs of N-channel MOSFETs must be used with the

battery charge path. The nominal gate drive levels are set

by the clamp drive voltage of their respective control

circuitry. This voltage is typically 6.25V. Consequently,

logic-level threshold MOSFETs must be used. Pay close

attention to the BVDSS specification for the MOSFETs as

well; many of the logic level MOSFETs are limited to 30V

or less.

Selection criteria for the power MOSFETs include the “ON”

resistance RDS(ON), input voltage and maximum output

current. For the N-channel charge path, the maximum

current is the maximum programmed current to be used.

For the P-channel discharge path maximum current typi-

cally occurs at end of life of the battery when using only

one battery. The upper limit of RDS(ON) value is a function

of the

actual power dissipation capability of a given

MOSFET package that must take into account the PCB

layout. As a starting point, without knowing what the PCB

dissipation capability would be, derate the package power

rating by a factor of two.

DS ON MAX

MOSFET

MAX

()

If you are using a dual MOSFET package with both MOSFETs

in series, you must cut the package power rating in half

again and recalculate.

DS ON MAX

MOSFETDUAL

MAX

()

APPLICATIONS INFORMATION

If you use identical MOSFETs for both battery paths,

voltage drops will track over a wide current range. The

LTC1960 linear 25mV CV drop regulation will not occur

until the current has dropped below:

LINEARMAX

DS ON MAX

()

However, if you try to use the above equation to determine

RDS(ON) to force linear mode at full current, the MOSFET

RDS(ON) value becomes unreasonably low for MOSFETs

available at this time. The need for the LTC1960 voltage

drop regulation only comes into play for parallel battery

configurations that terminate charge or discharge using

voltage. At first this seems to be a problem, but there are

several factors helping out:

1. When batteries are in parallel current sharing, the

current flow through any one battery is less than if it is

running stand-alone.

2. Most batteries that charge in constant voltage mode,

such as Li-ion, charge terminate at a current value of

C/10 or less which is well within the linear operation

range of the MOSFETs.

3. Voltage tracking for the discharge process does not

need such precise voltage tracking values.

The LTC1960 has two transient conditions that force the

discharge path P-channel MOSFETs to have two additional

parameters to consider. The parameters are gate charge

QGATE and single pulse power capability.

When the LTC1960 senses a LOW_POWER event, all the

P-channel MOSFETs are turned on simultaneously to

allow voltage recovery due to a loss of a given power

source. However, there is a delay in the time it takes to turn

on all the MOSFETs. Slow MOSFETs will require more bulk

capacitance to hold up all the system’s power supply

function during the transition and fast MOSFET will require

less bulk capacitance. The transition speed of a MOSFET

to an on or off state is a direct function of the MOSFET gate

charge.

t = QGATE/IDRIVE

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