LTC1760 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1760' PDF : 44 Pages View PDF
LTC1760
OPERATION
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.
TO
BATTERY
1
FROM
CHARGER
BAT1
CSN
DCIN + 10V
(CHARGE PUMPED)
35mV
–
EAC
+
GCH1
Q3
+
20mV
CC
–
OFF
SCH1
10k
Q4
1760 F05
Figure 5. Charge MUX Switch Driver Equivalent Circuit
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.
1760fa
33
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