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LTC4098/LTC4098-1

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Introduction

The LTC4098/LTC4098-1 are high efﬁciency power manage-

ment and Li-Ion charger solutions designed to make optimal

use of the power available from a variety of sources, while

minimizing power dissipation and easing thermal budgeting

constraints. The innovative PowerPath architecture ensures

that the application is powered immediately after external

voltage is applied, even with a completely dead battery by

prioritizing power to the application over the battery.

The LTC4098/LTC4098-1 include a Bat-Track monolithic

step-down switching regulator for USB, wall adapters and

other 5V sources. Designed speciﬁcally for USB applica-

tions, the switching regulator incorporates a precision

average input current limit for USB compatibility. Because

power is conserved, the LTC4098/LTC4098-1 allow the load

current on VOUT to exceed the current drawn by the USB

port, making maximum use of the allowable USB power

for battery charging.

The switching regulator and battery charger communicate

to ensure that the average input current never exceeds the

USB speciﬁcations.

For automotive, Firewire, and other high voltage applica-

tions, the LTC4098/LTC4098-1 provide Bat-Track control

of an external LTC step-down switching regulator to

maximize battery charger efﬁciency and minimize heat

production.

When power is available from both the USB and high

voltage inputs, the high voltage input is prioritized and

the USB input is automatically disabled.

The LTC4098/LTC4098-1 feature an overvoltage protec-

tion circuit which is designed to work with an external

N-channel FET to prevent damage to its inputs caused by

accidental application of high voltage.

The LTC4098/LTC4098-1 contain both an internal 180m

ideal diode and an ideal diode controller designed for use

with an external P-channel FET. The ideal diodes from BAT

to VOUT guarantee that ample power is always available

to VOUT even if there is insufﬁcient or absent power at

VBUS or WALL.

Finally, to prevent battery drain when a device is connected

to a suspended USB port, an LDO from VBUS to VOUT

provides either low power or high power USB suspend

current to the application.

Bat-Track Input Current Limited Step Down Switching

Regulator

The power delivered from VBUS to VOUT is controlled

by a 2.25MHz constant-frequency step-down switching

regulator. To meet the USB maximum load speciﬁcation,

the switching regulator contains a measurement and

control system that ensures that the average input cur-

rent remains below the level programmed at CLPROG.

VOUT drives the combination of the external load and the

battery charger.

If the combined load does not cause the switching power

supply to reach the programmed input current limit, VOUT

will track approximately 0.3V above the battery voltage.

By keeping the voltage across the battery charger at this

low level, power lost to the battery charger is minimized.

Figure 1 shows the power path components.

If the combined external load plus battery charge current is

large enough to cause the switching power supply to reach

the programmed input current limit, the battery charger

will reduce its charge current by precisely the amount

necessary to enable the external load to be satisﬁed. Even

if the battery charge current is programmed to exceed the

allowable USB current, the USB speciﬁcation for average

input current will not be violated; the battery charger will

reduce its current as needed. Furthermore, if the load cur-

rent at VOUT exceeds the programmed power from VBUS,

load current will be drawn from the battery via the ideal

diodes even when the battery charger is enabled.

The current at CLPROG is a precise fraction of the VBUS

current. When a programming resistor and an averaging

capacitor are connected from CLPROG to GND, the voltage

on CLPROG represents the average input current of the

switching regulator. As the input current approaches the

programmed limit, CLPROG reaches 1.188V and power

delivered by the switching regulator is held constant.

Several ratios of current are available which can be set

to correspond to USB low and high power modes with a

single programming resistor.

The input current limit is programmed by various com-

binations of the D0, D1 and D2 pins as shown in Table 1.

OPERATION

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