LTC3335 datasheet(24/32 Pages) AD

24 / 32 page

LTC4120/LTC4120-4.2

Rev. G

For more information www.analog.com

APPLICATIONS INFORMATION

the operating frequency of the basic transmitter can vary

with component selection.

LTC4120 customers can also choose more advanced

transmitter options such as the LTC4125. With addi-

tional features such as: foreign metal detection; optimum

power search and AutoResonant™ operating frequency.

For more information on advanced transmitter options

refer to the Wireless Power Users Guide.

Maximum Battery Power Considerations

Using one of the approved transmitter options with this

wireless power design provides a maximum of 2W at the

input to the LTC4120. It is optimized for supplying 400mA

of charge current to a 4.2V Li-Ion battery. If a higher bat-

tery voltage is selected, then a lower charge current must

be used as the maximum power available is limited. The

maximum battery charge current, ICHG(MAX), that may be

programmed for a given float voltage, VFLOAT, can be cal-

culated based on the charger efficiency, ηEFF, as:

ICHG(MAX) ≤

ηEFF • 2W

VFLOAT

The charger efficiency, ηEFF, depends on the operating

conditions and may be estimated using the Buck Efficiency

curve in the Typical Performance Characteristics. Do not

select a charge current greater than this limit when select-

ing RPROG.

Input Voltage and Minimum On-Time

The LTC4120 can operate from input voltages up to 40V.

The LTC4120 maintains constant frequency operation

under most operating conditions. Under certain situa-

tions with high input voltage and high switching frequency

selected and a low battery voltage, the LTC4120 may not

be able to maintain constant frequency operation. These

factors, combined with the minimum on-time of the

LTC4120, impose a minimum limit on the duty cycle to

maintain fixed-frequency operation. The on-time of the

top switch is related to the duty cycle (VBAT/VIN) and the

switching frequency, fOSC in Hz:

tON =

VBAT

fOSC • VIN

When operating from a high input voltage with a low bat-

tery voltage, the PWM control algorithm may attempt

to enforce a duty cycle which requires an on-time lower

than the LTC4120 minimum, tMIN(ON). This minimum

duty cycle is approximately 18% for 1.5MHz operation

or 9% for 750kHz operation. Typical minimum on-time

is illustrated in graph G11 in the Typical Performance

Characteristics section. If the on-time is driven below

tMIN(ON), the charge current and battery voltage remain

in regulation, but the switching duty cycle may not remain

fixed, and/or the switching frequency may decrease to an

integer fraction of its programmed value.

The maximum input voltage allowed to maintain constant

frequency operation is:

VIN(MAX) =

VLOWBAT

fOSC • tMIN(ON)

where VLOWBAT, is the lowest battery voltage where the

switcher is enabled.

Exceeding the minimum on-time constraint does not

affect charge current or battery float voltage, so it may not

be of critical importance in most cases and high switch-

ing frequencies may be used in the design without any

fear of severe consequences. As the sections on Inductor

Selection and Capacitor Selection show, high switching

frequencies allow the use of smaller board components,

thus reducing the footprint of the applications circuit.

Fixed-frequency operation may also be influenced by drop-

out and Burst Mode operation as discussed previously.

Switching Inductor Selection: LSW

The primary criterion for switching inductor value selec-

tion in an LTC4120 charger is the ripple current created in

that inductor. Once the inductance value is determined, the

saturation current rating for that inductor must be equal

to or exceed the maximum peak current in the inductor,

IL(PEAK). The peak value of the inductor current is the sum

of the programmed charge current, ICHG, plus one-half of

the ripple current, ∆IL. The peak inductor current must

also remain below the current limit of the LTC4120, IPEAK:

IL(PEAK) = ICHG +

∆IL

< IPEAK

LTC3335 Datasheet(PDF) 24 Page - Analog Devices

LTC3335 Datasheet(HTML) 24 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL