LTC4162EUFD-FSTM#PBF datasheet(31/52 Pages) AD

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LTC4162-F

Rev A

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APPLICATIONS INFORMATION

Power Path Isolation in Ship Mode

In ship mode, the LTC4162 shuts down nearly all internal

circuits and reduces its quiescent current to only a few

micro-Amperes. The body diodes of the power path tran-

sistors still provide a conduction path from the battery to

the system load however. If circuits down stream of the

LTC4162 power path must be completely cut off in ship

mode, an external PMOS transistor and one small signal

NMOS transistor can provide this isolation. The circuit of

Figure 6 exploits the fact that the VCC2P5 pin drops to

ground in ship mode. MP1 should be chosen to have a

fairly low threshold voltage if full conductance is needed

for single cell applications because in the single cell case

MN1 drops out and RA and RB form a voltage divider

preventing full gate drive to MP1.

4162F F06

MN1

RYM002N05

MP1

Si5411EDU

390k

VCC2P5

VOUT

VLOAD

TO DOWN STREAM

CIRCUITRY

Figure 6. Isolating Downstream Circuits in Ship Mode.

Choosing the BOOST Capacitor

The BOOST capacitor should be a low ESR surface mount

ceramic type rated to at least 6.3V and should have a

value of 22nF.

Choosing the Inductor

To ensure proper ripple current and control loop stability

the inductor value as a function of switching frequency

and maximum input voltage should be computed from

the following expression:

L(µH) =

0.3 • VIN(MAX)

fOSC(MHz)

Once the value for L is known, the type of inductor core

must be selected. Ferrite cores are recommended for their

very low core loss at frequencies above 100kHz, such as

is the operating frequency of the LTC4162. Ferrite core

material saturates hard, however, which means that in-

ductance collapses abruptly when the peak design current

is exceeded. This causes an abrupt increase in inductor

ripple current and consequent output voltage ripple. The

saturation current for the inductor should be about 30%

higher than the maximum regulated current, ICHG(MAX).

Setting the Switching Frequency (RT Resistor)

The operating frequency and inductor selection are inter-

related. Higher operating frequencies allow the use of

smallerinductorsandcapacitorsbutgenerallyalsoresults

in lower efficiency because of switching and charge trans-

fer losses. The feedback loops of LTC4162 are internally

compensated and cannot be adjusted. The LTC4162 is

designedtooperateproperlywithfrequenciesrangingfrom

1MHz to 2.5MHz. Operation at lower or higher frequen-

cies jeopardizes control loop stability. A resistor on the

RT pin sets the LTC4162's step-down switching charger

operating frequency. To keep the inductor size down and

ensure peak efficiency and stability, the LTC4162 has been

optimized to run at 1.5MHz with an RT value of 63.4kΩ.

Small changes in oscillator frequency can be achieved

by altering RT from this value. The oscillator frequency is

inversely proportional to RT as given by the expression:

fOSC MHz

(

) =

RT kΩ

( )

Choosing the VOUT, BATSENS+, INTVCC and VCC2P5

Bypass Capacitors

ThestyleandvalueofthecapacitorsusedwiththeLTC4162

determineimportantparameters,suchasregulatorcontrol

loopstabilityandinputvoltageripple.BecausetheLTC4162

uses a step-down switching power supply from VOUT to

BATSENS+, its input current waveform contains very high

frequencycomponents.Itisimperativethatlowequivalent

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