5

dc1977afb

DEMO MANUAL

DC1977A-A/DC1977A-B

theory of operation

Battery capacitors C1 and C2

The maximum battery voltage for the LTC4121EUD is

18V, and for the LTC4121EUD-4.2, it is 4.2V. Analog

Devices recommends 47µF of capacitance on the BAT

pin, if the battery is not present. For the LTC4121EUD

the voltage rating of the capacitor will need to be 25V, so

two 22µF, 25V, MLCC capacitors are used. In the case of

the LTC4121EUD-4.2, a 6.3V capacitor will suffice, and

a single 47µF, 6.3V, MLCC capacitor is used for C1, with

C2 not placed.

Reverse-Blocking Circuit

Components Q1, R16 and C6 comprise a reverse-blocking

circuit. The circuit performs two functions. First, the cir-

cuit prevents the battery from back-charging the power

source when the power source is dormant (e.g., a solar

cell in the absence of illumination). Note that this func-

tionality can also be provided through D1. Second, when a

charged battery is connected to the circuit in the absence

of input voltage, current will flow from the battery into the

BAT pin and out the IN pin, charging C4. With battery volt-

ages in excess of ~10V, this current surge can destroy the

device. Note that this is not a problem when only one or

two series Li-Ion cells are employed. Thus, this reverse-

blocking circuit may not be necessary depending on the

application. If reverse-blocking is not required, C6 and

R16 also become unnecessary. These two components

provide a path to the BAT pin from which the LTC4121

derives bias for internal circuits which would be provided

by the battery directly in the absence of Q1. See, for exam-

ple, the application on the first page of this manual.