Datasheet

LTC1760
42
1760fa
PowerPath
MUX
LTC1760
CHARGE
MUX
LTC1623
SMBus
SMBus
TO/FROM
HOST
TO
LOAD
CONDITIONING
LOAD
1760 F10
Figure 10. Large Load Conditioning Circuit
The LTC1760 has two transient conditions that force the
discharge path P-channel MOSFETs to have two additional
parameters to consider. The parameters are gate charge
Q
GATE
and single pulse power capability.
When the LTC1760 senses a LOW_POWER event, all
the P-channel MOSFETs are turned on simultaneously
to allow voltage recovery due to a loss of a given power
source. However, there is a delay in the time it takes to
turn on all the MOSFETs. Slow MOSFETs will require more
bulk capacitance to hold up all the system’s power sup-
ply function during the transition and fast MOSFET will
require less bulk capacitance. The transition speed of a
MOSFET to an on or off state is a direct function of the
MOSFET gate charge.
t = Q
GATE
/I
DRIVE
I
DRIVE
is the fixed drive current into the gate from the
LTC1760 and “t” is the time it takes to move that charge
to a new state and change the MOSFET conduction mode.
Hence time is directly related to Q
GATE
. Since Q
GATE
goes up
with MOSFETs of lower R
DS(ON)
, choosing such MOSFETs
has a counterproductive increase in gate charge making
the MOSFET slower. Please note that the LTC1760 recovery
time specification only refers to the time it takes for the
voltage to recover to the level just prior to the LOW_POWER
event as opposed to full voltage.
The single pulse current rating of MOSFET is important
when a short-circuit takes place. The MOSFET must
survive a 15ms overload. MOSFETs of lower R
DS(ON)
or MOSFETs that use more powerful thermal packages
will have a high power surge rating. Using too small of a
pulse rating will allow the MOSFET to blow to the open
circuit condition instantly like a fuse. Typically there is no
outward sign of failure because it happens so fast. Please
measure the surge current for all discharge power paths
under worse case conditions and consult the MOSFET
data sheet for the limitations. Voltage sources with the
highest voltage and the most bulk capacitance are often
the biggest risk. Specifically the MOSFETs in the wall
adapter path with wall adapters of high voltage, large
bulk capacitance and low resistance DC cables between
the adapter and device are the most common failures.
APPLICATIONS INFORMATION
Remember to only use the real wall adapter with a produc-
tion DC power cord when performing the wall adapter path
test. The use of a laboratory power supply is unrealistic
for this test and will force you to over specify the MOSFET
ratings. A battery pack usually has enough series resistance
to limit the peak current or are too low in voltage to create
enough instantaneous power to damage their respective
power path MOSFETs.
Conditioning Systems With Large Loads
In systems where the load is too large to be used for
conditioning a single battery it may be necessary to
bypass the built in calibrate function and simply switch
in an external load. A convenient way to accomplish this
task is by using an SMBus based LTC1623 load switch
controller. See Figure 10.
Unique Configuration Information
This section summarizes unique LTC1760 configurations
that allow some LTC1760 features to be eliminated. These
configurations may be selected in any combination with-
out adversely affecting LTC1760 operation. Refer to the
Typical Application circuit diagram located at the back of
this data sheet.