LTC1760CFW datasheet(30/44 Pages) LINER

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LTC1760

1760f

at hand. The average current control loop converts the

voltage between CSP and BAT to a representative current.

Error amp CA2 compares this current against the desired

current programmed by the IDAC at the ISET pin and adjusts

ITH for the desired voltage across RSENSE.

The voltage at BAT is divided down by an internal resistor

divider set by the VDAC and is used by error amp EA to

decrease ITH if the divider voltage is above the 0.8V

reference.

The amplifier CL1 monitors and limits the input current,

normally from the AC adapter, to a preset level (100 mV/

RCL). At input current limit, CL1 will decrease the ITH

voltage and thus reduce battery charging current.

An over-voltage comparator, OV, guards against transient

overshoots (>7.5%). In this case, the top MOSFET is

turned off until the over-voltage condition is cleared. This

feature is useful for batteries which “load dump” them-

selves by opening their protection switch to perform

functions such as calibration or pulse-mode charging.

The top MOSFET driver is powered from a floating boot-

strap capacitor C4. This capacitor is normally recharged

from VCC through an external diode when the top MOSFET

is turned off. As VIN decreases towards the selected

battery 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 voltage

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 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.

When batteries are controlled charging, only batteries

with voltages above VCHMIN are allowed to charge. When

a battery is wake-up charging this restriction does not

apply.

OPERATIO

–

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

TGATE

BGATE

OFF

tOFF

TRIP POINT SET BY ITH VOLTAGE

INDUCTOR

CURRENT

Figure 4.

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