LTC4162EUFD-FST#PBF datasheet(32/52 Pages) AD

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LTC4162-F

Rev A

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APPLICATIONS INFORMATION

series resistance (ESR) multilayer ceramic capacitors be

used to bypass VOUT. Tantalum and aluminum capacitors

will not work because of their high ESR and ESL. The

value of the total capacitance on VOUT directly controls the

amount of input ripple for a given load current. Increas-

ing the size of this capacitor will reduce the input ripple.

The LTC4162 has been designed with VOUT and PGND as

two corner pin groups so there is ample room to fit an

appropriatebypasscapacitor.Theneedforlowimpedance

capacitance directly adjacent to the VOUT and PGND pins

cannot be overemphasized. PCB distance of only a few

millimeters will introduce nano-Henrys of inductance and

compromise the high frequency "hot-loop" (See Printed

Circuit Board Layout Considerations).

It is also recommended that a ceramic capacitor be used

to bypass BATSENS+. At least 10µF with low ESR is re-

quired. Multilayer ceramic chip capacitors typically have

exceptional ESR performance. MLCCs combined with a

tight board layout and an unbroken ground plane will yield

very good performance and low EMI emissions.

The INTVCC and VCC2P5 pins are the outputs of onboard

lowdropoutregulatorsandalsorequireceramiccapacitors.

The INTVCC and VCC2P5 capacitors should be as close

to the LTC4162 as possible and returned immediately to

an analog ground plane. The INTVCC pin requires at least

4.7µF of capacitance rated to at least 6.3V and the VCC2P5

pin requires at least 1µF rated to 4V.

The actual capacitance of any ceramic capacitor should

be measured with a small AC signal and DC bias, as is

expected in-circuit. Many vendors specify the capacitance

versus voltage with a 1VRMS AC test signal with no bias

and, as a result, grossly overstate the capacitance that

the capacitor will present in the application. Using similar

operating conditions as the application, the user must

measure,orrequestfromthevendor,theactualcapacitance

to determine if the selected capacitor meets the minimum

capacitance that the application requires.

INFET and BATFET MOSFET Selection

An external N-channel MOSFET is required for both the

input and battery paths. Important parameters for the se-

lection of these MOSFETs are the maximum drain-source

voltage, VDSS, gate threshold voltage and on-resistance

(RDS(ON)). When the input is grounded, the battery stack

voltage is applied across the input MOSFET. When VBAT

is at 0V, the input voltage is applied across the battery

MOSFET. Therefore, the VDSS of the input MOSFET must

withstand the maximum voltage on VBAT while the VDSS

of the output MOSFET must withstand the highest voltage

on VIN. The gate drive for both is 5V. This requires the use

of logic-level threshold N-channel MOSFETs. As a general

rule, select MOSFETs with a low enough RDS(ON) to obtain

the desired VDS and power dissipation while operating at

full load current.

Operation Without a Battery

The LTC4162 has built in battery detection. Its switching

regulator will generally not start if the battery is missing.

However,ifabatteryispresentatthebeginningofacharge

cycle and is removed, the LTC4162 will operate without

a battery. Typically the BATSENS+ pin will rise quickly

to the programmed constant-voltage level and remain

there. However, it is important that the impedance on the

BATSENS+ node be kept relatively low at the switching

frequency. Therefore a ceramic capacitor of 10µF or more

near the LTC4162 is necessary. Note that without a load on

the BATSENS+ pin, the switching regulator will eventually

terminate due to tcvtimer reaching max_cv_time.

Operation With Long Battery Leads

The LTC4162 is generally resilient to operation with long

battery leads, however a ceramic capacitor of 10µF or

more of appropriate voltage tolerance near the LTC4162

is necessary. Note that any parasitic battery resistance,

such as long cabling, will push the LTC4162 into constant

voltage charging sooner, dramatically extending charging

LTC4162EUFD-FST#PBF Datasheet(PDF) 32 Page - Analog Devices

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