LTC4098-1 datasheet(15/32 Pages) LINER

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LTC4098/LTC4098-1

40981fb

For very low battery voltages, the battery charger acts like

a load and, due to limited input power, its current will tend

to pull VOUT below the 3.6V instant-on voltage. To prevent

VOUT from falling below this level, an undervoltage circuit

automatically detects that VOUT is falling and reduces the

battery charge current as needed. This reduction ensures

that load current and voltage are always prioritized while

allowing as much battery charge current as possible. Refer

to Overprogramming the Battery Charger in the Applica-

tions Information section.

The voltage regulation loop compensation is controlled by

the capacitance on VOUT. An MLCC capacitor of 10μF is

required for loop stability. Additional capacitance beyond

this value will improve transient response.

An internal undervoltage lockout circuit monitors VBUS and

keeps the switching regulator off until VBUS rises above

the rising UVLO threshold (4.3V). If VBUS falls below the

falling UVLO threshold (4V), system power at VOUT will

be drawn from the battery via the ideal diodes. The volt-

age at VBUS must also be higher than the voltage at BAT

by approximately 170mV for the switching regulator to

operate.

Bat-Track High Voltage External Switching Regulator

Control

The WALL, ACPR and VC pins can be used in conjunction

with an external high voltage step-down switching regulator

such as the LT3653 or LT3480 to minimize heat production

when operating from higher voltage sources, as shown in

Figures 3 and 4. Bat-Track control circuitry regulates the

external switching regulator’s output voltage to the larger

of BAT + 300mV or 3.6V. This maximizes battery charger

efﬁciency while still allowing instant-on operation when

the battery is deeply discharged.

When using the LT3480, the feedback network should be set

to generate an output voltage between 4.5V and 5.5V. When

high voltage is applied to the external regulator, WALL will

rise toward this programmed output voltage. When WALL

exceeds approximately 4.3V, ACPR is brought low and the

Bat-Track control of the LTC4098/LTC4098-1 overdrives

the local VC control of the external high voltage step-down

switching regulator. Therefore, once the Bat-Track control

is enabled, the output voltage is set independent of the

switching regulator feedback network.

Bat-Track control provides a signiﬁcant efﬁciency advantage

over the simple use of a 5V switching regulator output to

drive the battery charger. With a 5V output driving VOUT,

battery charger efﬁciency is approximately:

ηη

TOTAL

BUCK

BAT

•

where

ηBUCK is the efﬁciency of the high voltage switching

regulator and 5V is the output voltage of the switching

regulator. With a typical switching regulator efﬁciency of

87% and a typical battery voltage of 3.8V, the total battery

charger efﬁciency is approximately 66%. Assuming a 1A

charge current, nearly 2W of power is dissipated just to

charge the battery!

OPERATION

ACPR

40981 F03

LTC4098/

LTC4098-1

VOUT

ISENSE

WALL

LT3653

HVOK

SYSTEM

LOAD

ACPR

40981 F04

LTC4098/

LTC4098-1

VOUT

WALL

LT3480

SYSTEM

LOAD

Figure 3. LT3653 Typical Interface

Figure 4. LT3480 Typical Interface

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