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LTC4110

4110fa

and ranges. It should be noted that even if the LTC4110

TYPE pin is not set to a smart battery mode, any SMBus

commands sent by a host or a smart battery are still

acted upon. For SuperCap support, see the Applications

Information section.

BATTERY BACKUP MODE

Figure 1 shows the LTC4110 in backup mode and the

corresponding PowerPath enabled. The LTC4110 use the

DCDIV pin to typically monitor the DCIN voltage through

an external resistor divider. The DCDIV pin sets the backup

mode threshold voltage and senses the need to enter

backup mode. DCDIV can alternately be driven with other

signals such as logic. When the DCDIV pin voltage drops

below the AC present threshold voltage (see VAC) backup

mode is entered. Backup mode is also entered whenever

the internal undervoltage lockout, UVLO, senses that DCIN

(VUVD) or DCOUT has fallen to excessively low voltages.

In backup mode the battery P-MOSFET ideal diode is

enabled to backup the load from the battery. The supply

input P-MOSFET ideal diode isolates the main supply

input from the load and the ﬂyback switcher N-MOSFETs

are inhibited from turning on. Also, after the threshold is

passed, hysteresis (VACH) is switched in. When the supply

is returning and the AC present threshold voltage plus the

hysteresis voltage is reached on the DCDIV pin, both of the

battery P-MOSFETs are rapidly switched off (tdDOFF) and

the supply input P-MOSFET ideal diode provides the load

current. When forward biased, the ideal diodes regulate

their forward voltage drop to 20mV typical (VFR) when the

MOSFET is sufﬁciently sized. If the voltage input falls and

results in a forward voltage below 20mV, then the ideal

diode will begin turning off at a slow rate. Should the ideal

diode see a –18mV typical (VREV) or lower reverse voltage,

the ideal diode will turn off quickly (tdDOFF).

While in backup, the battery’s average cell voltage is moni-

tored to protect the battery from excessive discharge. If

the cell voltage drops below the value programmed by the

VDIS pin (Li-Ion default = 2.75V/cell, NiMH/NiCd default

= 0.95V/cell, lead acid default = 1.93V/cell), the battery

P-MOSFETs are rapidly turned off and the battery is dis-

connected from the load. If DCIN is above UVLO, the load

and the LTC4110 will be powered from the supply input. If

DCIN is below UVLO, the LTC4110 enters the micropower

shutdown mode (see the Shutdown Mode section for more

details). Also, the SMBus accessible BKUP_FLT fault bit

is set and maintained as long as sufﬁcient battery voltage

is present (VBAT ≥ 2.7V). This fault bit can be read after

DCIN returns to a voltage level exceeding the internal

UVLO threshold (see VUVI) and DCOUT has regained suf-

ﬁcient voltage (see DCOUT) to provide internal power. If

the GPIO2 port is programmed as the BKUP_FLTb status

output after DCIN returns, it will be forced low to repre-

sent an inverted BKUP_FLT bit. When DCIN returns, as

sensed by the UVLO, the shutdown mode is automatically

cancelled and normal operation can resume, however, the

BKUP_FLT bit remains set until either the SHDN pin is set

high (all registers reset) or register bits POR_RESET or

BUFLT_RST are set. See the Shutdown Mode section for

details. During backup, the external thermistor network

is monitored for battery presence.

BATTERY CHARGE MODE

Figure 2 shows the charge path to charge a battery. Cur-

rent is pulled from the supply input to charge the battery.

At the same time, the input supply provides power to

both the system load and the backup load. The battery

is isolated from the load at all times so it cannot affect

charger terminations algorithms.

If we ignore battery chemistry for a moment, as far as the

LTC4110 charger is concerned, there are only two basic

charge modes. When the TYPE pin selects a standard bat-

tery mode, charge termination is controlled by the LTC4110

OPERATION

Figure 1. Backup Mode Operation

LTC4110

INID

BATID

DCDIV

DCHFET

CHGFET

UVLO

SET POINT

BACKUP LOAD (DCOUT)

CURRENT FLOW

SYSTEM LOAD

BATTERY

DCIN

OFF

4110 F01

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