LTC4098-1 datasheet(23/32 Pages) LINER

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LTC4098/LTC4098-1

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Multilayer ceramic chip capacitors typically have excep-

tional ESR performance. MLCCs combined with a tight

board layout and an unbroken ground plane will yield very

good performance and low EMI emissions.

There are several types of ceramic capacitors avail-

able each having considerably different characteristics.

For example, X7R ceramic capacitors have the best voltage

and temperature stability. X5R ceramic capacitors have

apparently higher packing density but poorer performance

over their rated voltage and temperature ranges. Y5V

ceramic capacitors have the highest packing density,

but must be used with caution, because of their extreme

nonlinear characteristic of capacitance versus voltage. The

actual in-circuit capacitance of a 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 and, as a result,

over state the capacitance that the capacitor will present

in the application. Using similar operating conditions as

the application, the user must measure or request from

the vendor the actual capacitance to determine if the

selected capacitor meets the minimum capacitance that

the application requires.

Overprogramming the Battery Charger

The USB high power speciﬁcation allows for up to 2.5W

to be drawn from the USB port. The switching regulator

transforms the voltage at VBUS to just above the voltage

at BAT with high efﬁciency, while limiting power to less

than the amount programmed at CLPROG. The charger

should be programmed (with the PROG pin) to deliver the

maximum safe charging current without regard to the USB

speciﬁcations. If there is insufﬁcient current available to

charge the battery at the programmed rate, it will reduce

charge current until the system load on VOUT is satisﬁed

and the VBUS current limit is satisﬁed. Programming the

charger for more current than is available will not cause

the average input current limit to be violated. It will merely

allow the battery charger to make use of all available

power to charge the battery as quickly as possible, and

with minimal power dissipation within the charger.

Overvoltage Protection

It is possible to protect both VBUS and WALL from

overvoltage damage with several additional components,

as shown in Figure 7. Schottky diodes D1 and D2 pass the

larger of V1 and V2 to R1 and OVSENS. If either V1 or V2

MN1

MN2

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WALL

OVSENS

OVGATE

LTC4098/

LTC4098-1

VBUS

Figure 7. Dual Input Overvoltage Protection

APPLICATIONS INFORMATION

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