LTC4100 datasheet(33/48 Pages) LINER

33 / 48 page

LTC1760

1760fa

is turned off. As VIN decreases towards the selected bat-

tery voltage, the converter will attempt to turn on the top

MOSFET continuously (“dropout’’). A dropout timer detects

this condition and forces the top MOSFET to turn off, and

the bottom MOSFET on, for about 200ns at 40μs intervals

to recharge the bootstrap capacitor.

7.1 Charge MUX Switches

The equivalent circuit of a charge MUX switch driver is

shown in Figure 5. If the charger controller is not enabled,

the charge MUX drivers will drive the gate and source of

the series-connected MOSFETs to a low voltage and the

switch is off. When the charger controller is on, the charge

MUX driver will keep the MOSFETs off until the voltage at

CSN rises at least 35mV above the battery voltage. GCH1

is then driven with an error amplifier EAC until the volt-

age between BAT1 and CSN satisfies the error amplifier

or until GCH1 is clamped by the internal Zener diode.

The time required to close the switch could be quite long

(many ms) due to the small currents output by the error

amp and depending upon the size of the MOSFET switch.

If the voltage at CSN decreases below VBAT1 – 20mV a

comparator CC quickly turns off the MOSFETs to prevent

reverse current from flowing in the switches. In essence,

this system performs as a low forward voltage diode.

Operation is identical for BAT2.

7.2 Dual Charging

Note that the charge MUX switch drivers will operate

together to allow both batteries to be charged simultane-

ously. 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.

When batteries are controlled charging, only batteries with

voltages above VCHMIN are allowed to charge. When a bat-

tery is wake-up charging this restriction does not apply.

8 PowerPath Controller

The PowerPath switches are turned on and off by the power

management algorithm. The external PFETs 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 resis-

tor, 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 6. The

output PFET is driven ON or OFF by the output side driver

controlling pin GB10. 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/SCP

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/SCP. 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/SCP 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.

OPERATION

–

GCH1

SCH1

BATTERY

FROM

CHARGER

BAT1

CSN

35mV

20mV

OFF

DCIN + 10V

(CHARGE PUMPED)

10k

1760 F05

EAC

Figure 5. Charge MUX Switch Driver Equivalent Circuit

LTC4100 数据表(PDF) 33 Page - Linear Technology

LTC4100 数据表(HTML) 33 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

数据表下载