Datasheet

10
LTC1479
OPERATION
U
The LTC1479 is responsible for low-loss switching at the
“front end” of the power management system, where up
to two battery packs and a DC power source can be
indiscriminately connected and disconnected. Smooth
switching between input power sources is accomplished
with the help of low-loss N-channel switches 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 LTC1479 drives external back-to-back N-channel
MOSFET switches to direct power from the three main
power sources: the external DC power source, the pri-
mary battery and the secondary battery connected to the
main supply pins—DCIN, BAT1 and BAT2 respectively.
(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 a micropower boost regulator which is regu-
lated at approximately 36.5V. The V
GG
supply provides
sufficient headroom above the maximum 28V operating
voltage of the three main power sources to ensure that the
MOSFET switches are fully enhanced.
The power for this inductor based regulator is taken from
three internal diodes as shown in Figure 1. The three
Figure 1. V
GG
Switching Regulator
BAT1
BAT2
DCIN
V
+
SW
GND
1479 F01
V
GG
+
+
L1
1mH
C1
1µF
35V
C2
1µF
50V
TO GATE
DRIVERS
(36.5V)
LTC1479
V
GG

SWITCHING
REGULATOR
diodes are connected to each of the three main power
sources, DCIN, BAT1 and BAT2. The highest voltage
potential is directed to the top of the boost regulator
inductor to maximize regulator efficiency. C1 provides
filtering at the top of the 1mH switched inductor, L1, which
is housed in a small surface mount package.
A fourth internal diode directs the current from the 1mH
inductor to the V
GG
output capacitor, C2, further reducing
the external parts count. In fact, as demonstrated in Figure
1, only three external components are required by the V
GG
regulator, L1, C1 and C2.
Inrush Current Limiting
The LTC1479 uses an adaptive inrush current limiting
scheme to reduce current flowing in and out of the three
main power sources and the DC/DC converter input ca-
pacitor during switch-over transitions. The voltage across
a single small-valued resistor, R
SENSE
, is measured to
ascertain the instantaneous current flowing through the
three main switch pairs, SW A/B, SW C/D, and SW E/F
during the transitions.
Figure 2 is a block diagram showing only the DCIN switch
pair, SW A/B. (The gate drive circuits for switch pairs SW
C/D and SW E/F are identical). A bidirectional current
sensing and limiting circuit determines when the voltage
drop across R
SENSE
reaches plus or minus 200mV. The
gate-to-source voltage, V
GS
, of the appropriate switch is
limited during the transition period until the inrush current
subsides, generally within a few milliseconds, depending
upon the value of the DC/DC converter input capacitor.
V
SENSE
+
V
SENSE
–
GA GBSAB
SW A SW B R
SENSE
V
GG
LTC1479
1479 F02
DCIN
+
OUTPUT
TO DC/DC
CONVERTER
C
OUT
6V 6V
±200mV
THRESHOLD
SW A/B
GATE
DRIVERS
BIDIRECTIONAL
INRUSH CURRENT
SENSING AND
LIMITING
Figure 2. SW A/B Inrush Current Limiting