LTC1960 datasheet(14/24 Pages) LINER

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LTC1960

1960fa

Figure 4. PowerPath Driver Equivalent Circuit

Dual Charging

Note that the charge MUX switch drivers will operate

together to allow both batteries to be charged simulta-

neously. If both charge MUX switch drivers are enabled,

only the battery with the lowest voltage will be charged

until its voltage rises to equal the higher voltage battery.

The charge current will then share between the batteries

according to the capacity of each battery.

If both batteries are selected for charging, only batteries

with voltages above VCHMIN are allowed to charge. Dual

charging is not allowed when the low current mode of the

IDAC is selected. If dual charging is enabled when the IDAC

enters low current mode, then only BAT1 will be charged.

Charger Start-Up

When the charger controller is enabled by the SPI Inter-

face block, the charger output CSN will ramp from 0V until

it exceeds the selected battery voltage. The clamp error

amp is used to prevent the charger output from exceeding

the selected battery voltage by more than 0.7V during the

start-up transient while the charge MUX switches, have

yet to close. Once the charge MUX switches have closed,

the clamp releases ITH to allow control by another loop.

PowerPath Controller

The PowerPath switches are turned on and off via the SPI

interface, in any combination. The external P-MOSFETs

are usually connected as an input switch and an output

switch. The output switch PFET is connected in series with

the input PFET and the positive side of the short-circuit

sensing resistor, RSC. The input switch is connected in

series between the power source and the output PFET. The

PowerPath switch driver equivalent circuit is shown in

Figure 4. The output PFET is driven high and low by the

output side driver controlling pin GXXO, the PFET is either

on or off. The gate of the input PFET is driven by an error

amplifier which monitors the voltage between the input

power source (BAT1 in this case) and SCP. If the switch is

turned off, the two outputs are driven to the higher of the

two voltages present across the input/output terminals of

the switch. When the switch is instructed to turn on, the

output side driver immediately drives the gate of the

output PFET approximately 6V below the highest of the

voltages present at the input/output. When the output

PFET turns on, the voltage at SCP will be pulled up to a

diode drop below the source voltage by the bulk diode of

the input PFET. If the source voltage is more than 25mV

above SCP, EAP will drive the gate of the input PFET low

until the input PFET turns on and reduces the voltage

across the input/output to the EAP set point, or until the

Zener clamp engages to limit the voltage applied to the

input PFET. If the source voltage drops more than 20mV

below SCP, then comparator CP turns on SWP to quickly

prevent large reverse current in the switch. This operation

mimics a diode with a low forward voltage drop.

Figure 3. Charge MUX Switch Driver Equivalent Circuit

OPERATIO

–

GCH1

SCH1

BATTERY

FROM

CHARGER

BAT1

CSN

35mV

20mV

OFF

DCIN + 10V

(CHARGE PUMPED)

10k

1960 F03

EAC

–

GB1I

GB1O

FROM

BATTERY

BAT1

SCP

25mV

20mV

OFF

1960 F04

EAP

SWP

LOAD

RSC

reverse current from flowing in the switches. In essence,

this system performs as a low forward voltage diode.

Operation is identical for BAT2.

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