7
LTC1473
The LTC1473 is responsible for low-loss switching and
isolation at the “front end” of the power management
system, where up to two battery packs can be connected
and disconnected seamlessly. Smooth switching between
input power sources is accomplished with the help of
lowloss N-channel switches. They are driven by special
gate drive circuitry which limits the inrush current in and
out of the battery packs and the system power supply
capacitors.
All N-Channel Switching
The LTC1473 drives external back-to-back N-channel
MOSFET switches to direct power from two sources: the
primary battery and the secondary battery or a battery and
a wall unit. (N-channel MOSFET switches are more cost
effective and provide lower voltage drops than their P-
channel counterparts.)
Gate Drive (V
GG
) Power Supply
The gate drive for the low-loss N-channel switches is
supplied by an internal micropower boost regulator which
is regulated at approximately 8.5V above V
+
, up to 37V
maximum. In two battery systems, the LTC1473 V
+
pin is
diode ORed through three external diodes connected to
the three main power sources, DCIN, BAT1 and BAT2.
Thus, V
GG
is regulated at 8.5V above the highest power
source and will provide the overdrive required to fully
enhance the MOSFET switches.
For maximum efficiency the top of the boost regulator
inductor is connected to V
+
as shown in Figure 1. C1
provides filtering at the top of the 1mH switched inductor,
L1, which is housed in a small surface mount package. An
internal diode directs the current from the 1mH inductor to
the V
GG
output capacitor C2.
Inrush and Short-Circuit Current Limiting
The LTC1473 uses an adaptive inrush current limiting
scheme to reduce current flowing in and out of the two
main power sources and the following system’s input
capacitor during switch-over transitions. The voltage across
a single small valued resistor, R
SENSE
, is measured to
ascertain the instantaneous current flowing through the
Figure 2. SW A1/B1 Inrush Current Limiting
V
SENSE
+
V
SENSE
–
GA1
GB1
SAB1
SW A1
SW B1
R
SENSE
1473 F02
BAT1
+
OUTPUT
LOAD
C
OUT
V
GG
LTC1473
6V
6V
± 200mV
THRESHOLD
SW A/B
GATE
DRIVERS
BIDIRECTIONAL
INRUSH CURRENT
SENSING AND
LIMITING
BAT1 BAT2DCIN
V
+
SW
GND
1473 F01
V
GG
L1
1mH
C1
1µF
50V
TO GATE
DRIVERS
(8.5V + V
+
)
LTC1473
C2
1µF
50V
V
GG
SWITCHING
REGULATOR
Figure 1. V
GG
Switching Regulator
two switch pairs, SW A1/B1 and SW A2/B2, during the
transitions.
Figure 2 shows a block diagram of a switch driver pair, SW
A1/B1. A bidirectional current sensing and limiting circuit
determines when the voltage drop across R
SENSE
reaches
±200mV. The gate-to-source voltage, V
GS
, of the appro-
priate switch is limited during the transition period until
the inrush current subsides, generally within a few milli-
seconds, depending upon the value of the following
system’s input capacitor.
This scheme allows capacitors and MOSFET switches of
differing sizes and current ratings to be used in the same
system without circuit modifications.
OPERATION
U
