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LTC4412HV Datasheet(PDF) 7 Page - Linear Technology

Part No. LTC4412HV
Description  36V, Low Loss PowerPath Controller in ThinSOT
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LTC4412HV Datasheet(HTML) 7 Page - Linear Technology

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LTC4412HV
sn4412hv 4412hvfs
APPLICATIO S I FOR ATIO
Introduction
The system designer will find the LTC4412HV useful in a
variety of cost and space sensitive power control applica-
tions that include low loss diode OR’ing, fully automatic
switchover from a primary to an auxiliary source of power,
microcontroller controlled switchover from a primary to
an auxiliary source of power, load sharing between two or
more batteries, charging of multiple batteries from a
single charger and high side power switching.
External P-Channel MOSFET Transistor Selection
Important parameters for the selection of MOSFETs are
the maximum drain-source voltage VDS(MAX), threshold
voltage VGS(VT) and on-resistance RDS(ON).
The maximum allowable drain-source voltage, VDS(MAX),
must be high enough to withstand the maximum drain-
source voltage seen in the application.
The maximum gate drive voltage for the primary MOSFET
is set by the smaller of the VINsupply voltage or the internal
clamping voltage VG(ON). A logic level MOSFET is com-
monly used, but if a low supply voltage limits the gate
voltage, a sub-logic level threshold MOSFET should be
considered. The maximum gate drive voltage for the
auxiliary MOSFET, if used, is determined by the external
resistor connected to the STAT pin and the STAT pin sink
current.
As a general rule, select a MOSFET with a low enough
RDS(ON) to obtain the desired VDS while operating at full
load current and an achievable VGS. The MOSFET normally
operates in the linear region and acts like a voltage
controlled resistor. If the MOSFET is grossly undersized,
it can enter the saturation region and a large VDS may
result. However, the drain-source diode of the MOSFET, if
forward biased, will limit VDS. A large VDS, combined with
the load current, will likely result in excessively high
MOSFET power dissipation. Keep in mind that the
LTC4412HV will regulate the forward voltage drop across
the primary MOSFET at 20mV if RDS(ON) is low enough.
The required RDS(ON) can be calculated by dividing 0.02V
by the load current in amps. Achieving forward regulation
will minimize power loss and heat dissipation, but it is not
a necessity. If a forward voltage drop of more than 20mV
is acceptable then a smaller MOSFET can be used, but
must be sized compatible with the higher power dissipa-
tion. Care should be taken to ensure that the power
dissipated is never allowed to rise above the manufacturer’s
recommended maximum level. The auxiliary MOSFET
power switch, if used, has similar considerations, but its
VGS can be tailored by resistor selection. When choosing
the resistor value consider the full range of STAT pin
current (IS(SNK) ) that may flow through it.
VIN and SENSE Pin Bypass Capacitors
Many types of capacitors, ranging from 0.1
µF to 10µF and
located close to the LTC4412HV, will provide adequate VIN
bypassing if needed. Voltage droop can occur at the load
during a supply switchover because some time is required
to turn on the MOSFET power switch. Factors that deter-
mine the magnitude of the voltage droop include the
supply rise and fall times, the MOSFET’s characteristics,
the value of COUT and the load current. Droop can be made
insignificant by the proper choice of COUT, since the droop
is inversely proportional to the capacitance. Bypass ca-
pacitance for the load also depends on the application’s
dynamic load requirements and typically ranges from 1
µF
to 47
µF. In all cases, the maximum droop is limited to the
drain source diode forward drop inside the MOSFET.
Caution must be exercised when using multilayer ceramic
capacitors. Because of the self resonance and high Q
characteristics of some types of ceramic capacitors, high
voltage transients can be generated under some start-up
conditions such as connecting a supply input to a hot
power source. To reduce the Q and prevent these tran-
sients from exceeding the LTC4412HV’s absolute maxi-
mum voltage rating, the capacitor’s ESR can be increased
by adding up to several ohms of resistance in series with
the ceramic capacitor. Refer to Application Note 88.
The selected capacitance value and capacitor’s ESR can be
verified by observing VIN and SENSE for acceptable volt-
age transitions during dynamic conditions over the full
load current range. This should be checked with each
power source as well. Ringing may indicate an incorrect
bypass capacitor value and/or too low an ESR.


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