LTC4412
8
4412fb
For more information www.linear.com/LTC4412
applicaTions inForMaTion
Introduction
The system designer will find the LTC4412 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 V
DS(MAX),
threshold
voltage V
GS(VT)
and on-resistance R
DS(ON)
.
The maximum allowable drain-source voltage, V
DS(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 V
IN
supply voltage or the internal
clamping voltage V
G(ON).
A logic level MOSFET is commonly
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
R
DS(ON)
to obtain the desired V
DS
while operating at full
load current and an achievable V
GS
. The MOSFET nor-
mally 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 V
DS
may
result. However, the drain-source diode of the MOSFET,
if forward biased, will limit V
DS
. A large V
DS
, combined
with the load current, will likely result in excessively high
MOSFET power dissipation. Keep in mind that the LTC4412
will regulate the forward voltage drop across the primary
MOSFET at 20mV if R
DS(ON)
is low enough. The required
R
DS(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 accept-
able then a smaller MOSFET can be used, but must be
sized compatible with
the higher power dissipation. Care
should be taken to ensure that
the power dissipated is
never allowed to
rise above the manufacturer’s recom-
mended maximum
level. The
auxiliary MOSFET power
switch, if used, has similar considerations, but its V
GS
can be tailored by resistor selection. When choosing the
resistor value consider the full range of STAT pin current
(I
S(SNK)
) that may flow through it.
V
IN
and SENSE Pin Bypass Capacitors
Many types of capacitors, ranging from 0.1µF to 10µF and
located close to the LTC4412, will provide adequate V
IN
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 determine
the magnitude of the voltage droop include the supply rise
and fall times, the MOSFET’s characteristics, the value of
C
OUT
and the load current. Droop can be made insignificant
by the proper choice of C
OUT
,
since the droop is inversely
proportional to the capacitance. Bypass capacitance for
the load also depends on the application’s dynamic load
requirements and typically ranges from 1µF to 47µF. 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 transients
from exceeding the LTC4412’s absolute maximum 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 V
IN
and SENSE for acceptable
voltage 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.
