LTC4121 datasheet(21/30 Pages) LINER

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LTC4121/LTC4121-4.2

4121fc

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APPLICATIONS INFORMATION

BAT Capacitor and Output Ripple: CBAT

The LTC4121 charger output requires bypass capacitance

connected from BAT to GND (CBAT). A 22µF ceramic ca-

pacitor is required for all applications. In systems where

the battery can be disconnected from the charger output,

additional bypass capacitance may be desired. In this

type of application, excessive ripple and/or low amplitude

oscillations can occur without additional output bulk

capacitance. For optimum stability, the additional bulk

capacitance should also have a small amount of ESR. For

these applications, place a 100µF low ESR non-ceramic

capacitor(chiptantalumororganicsemiconductorcapaci-

tors such as Sanyo OS-CONs or POSCAPs) from BAT to

GND, in parallel with the 22µF ceramic bypass capacitor,

or use large ceramic capacitors with an additional small

series ESR resistor of less than 1Ω. This additional bypass

capacitance may also be required in systems where the

battery is connected to the charger with long wires. The

voltage rating of all capacitors applied to CBAT must meet

or exceed the battery float voltage.

Boost Supply Capacitor

The BOOST pin provides a bootstrapped supply rail that

provides power to the top gate drivers. The operating volt-

age of the BOOST pin is internally generated from INTVCC

whenever the SW pin pulls low. This provides a floating

voltage of INTVCC above SW that is held by a capacitor

tied from BOOST to SW. A low ESR ceramic capacitor

of 10nF to 33nF is sufficient, with a voltage rating of 6V.

INTVCC Supply and Capacitor

Power for the top and bottom gate drivers and most other

internal circuitry is derived from the INTVCC pin. A low

ESR ceramic capacitor of 2.2µF is required on the INTVCC

pin. The INTVCC supply has a relatively low current limit

(about 20mA) that is dialed back when INTVCC is low to

reduce power dissipation. Do not use the INTVCC voltage

to supply power for any external circuitry except for the

NTCBIAS network. When the RUN pin is above VEN, the

INTVCC supply is enabled, and when INTVCC rises above

UVINTVCC, the charger is enabled.

Figure 8. Reverse Blocking with a P-Channel MOSFET in Series with the BAT Pin

BAT

22µF

10µF

4.7µF

2.2µF

RPROG

RFB1

49.9k

4.99k*

470k

*ADD 4.99k WHEN MAX BAT VOLTAGE APPROACHES 85%

OF VGS LIMIT FOR Si2343.

RFB2

Li-Ion

FBG

GND

CHRG

RUN

INTVCC

VIN

LTC4121

SI2343DS

PROG

BAT

22µF

10µF

4.7µF

2.2µF

RPROG

49.9k

470k

Li-Ion

BATSNS

GND

CHRG

RUN

INTVCC

VIN

LTC4121-4.2

SI2343DS

PROG

4121 F08

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