LTC3305 datasheet(21/28 Pages) LINER

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LTC3305

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For more information www.linear.com/LTC3305

APPLICATIONS INFORMATION

The tOFF parameter is programmed by a capacitor from

the CTOFF pin to GND. tOFF is given by:

tOFF = 0.48hrs•

CTOFF

10nF

C0Gtypecapacitorsarerecommendedduetotheirsuperior

temperature characteristics.

In the continuous operation mode (MODE = 1), the CTON

and CTOFF pins are unused and should be connected to

GND.

Selecting Charge Pump Components

Referring to Figure 2, recommended values for R1, R2

and CFLY are 249Ω, 1.33k & 10µF respectively for all ap-

plications. For applications in which V4 is no lower than

32V a 10µF capacitor is recommended for CBOOST. For

applications which may have lower voltages at V4, the

recommended value for CBOOST is 22µF. Schottky diodes

with a breakdown voltage larger than the maximum V4

voltage are recommended for diodes D1 and D2.

Selecting Decoupling Capacitors

Decoupling capacitors of at least 10µF must be placed

across each battery, from the BOOST pin to V4 and from

the AUXP pin to the AUXN pin. These capacitors must be

placedascloseaspossibletotheLTC3305.Thecapacitors

must be capable of withstanding the maximum voltage

across each battery. Capacitors with an X5R or X7R type

dielectric should be used.

Thermal Considerations and Limiting On-Chip Power

Dissipation

Excessive on-chip power dissipation will cause the

LTC3305 to enter thermal shutdown. It is important to

understand the source of the power dissipation and how

power dissipation can be reduced. The two contributions

of on-chip power dissipation on the LTC3305 that may be

controlled by the user are the loading on the low voltage

regulator and the power dissipated through the current

sources that provide the NGATE pin currents.

The low voltage linear regulator provides a 2.5V output.

Any current provided by the regulator will cause power

dissipation in the internal switch connected from V4 to

VREG,whichcausesdietemperaturetoincrease.InFigure8,

an external switching regulator generates a 3.3V rail that

backdrivestheVREGpinandprovidespowertotheexternal

microprocessor and other low voltage circuits. There is

no on-chip loading on the VREG pin and thus no on chip

power dissipation in the low voltage regulator.

In Figure 8, external resistors R1, R2, R3, R4, and R5 are in

serieswiththeon-chipcurrentsourcesthatprovideNGATE

pin current. These resistors reduce the voltage across the

on-chip current sources and thus reduce on-chip power

dissipation. As an example, the current source at NGATE1

delivers current from the V4 pin. When this current source

is turned on, the voltage across it is V4-VNGATE1. For a

typical application with V4 = 52.8V, programmed INGATE =

506µA and VNGATE1 = 6.12V, the on-chip power dissipated

in the current source is 23mW. In Figure 8, resistor R1

operates with approximately 30.5V across it. The on-chip

power dissipated in the current source is reduced to ap-

proximately 8mW. In similar fashion resistors R2, R3,

R4, and R5 reduce the on-chip power dissipation on the

respectivecurrentsources.Whenchoosingtheseresistors

it is recommended to have the internal current sources

biased with at least 6V across them under all operating

conditions. Power dissipation through the on-chip current

sources may be further reduced by programming a lower

gate current through the NGATE pins.

Balancing Battery Stacks with more than Four Batteries

To balance battery stacks that have more than four batter-

ies, multiple LTC3305 devices may be stacked together. In

this scenario, it is recommended that each LTC3305 be

run in continuous mode and at least one battery in each

sub-stack of four is common to two LTC3305s. Each

LTC3305 needs an auxiliary cell for the balancing opera-

tion. Figure 9 shows an eight battery stack being balanced

using three LTC3305 devices connected together. Figure

10 shows a stack of six batteries being balanced using

two LTC3305 devices. To balance a battery stack with

n batteries, the minimum number of LTC3305 devices

required is [(n-1)/3] rounded up to the nearest integer.

In this calculation, each LTC3305 is assumed to be used

in a four-battery configuration and at least one battery

interleaves two LTC3305 devices.

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