LTC1479 LINER [Linear Technology], LTC1479 Datasheet - Page 10

• Manufacturer Part Number: LTC1479
• Description: PowerPath Controller for Dual Battery Systems
• Manufacturer: LINER [Linear Technology]

OPERATION
LTC1479
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
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
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
10
LTC1479
DRIVERS
TO GATE
Figure 1. V
GG
REGULATOR
SWITCHING
) Power Supply
DCIN
V
U
GG
(36.5V)
BAT1 BAT2
GG
Switching Regulator
GND
V
V
SW
GG
+
GG
L1
1mH
+
supply provides
C2
1 F
50V
1479 F01
+
C1
1 F
35V
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
the external parts count. In fact, as demonstrated in Figure
1, only three external components are required by the V
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
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
gate-to-source voltage, V
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.
DCIN
LTC1479
V
GG
SW A
GA
6V
DRIVERS
Figure 2. SW A/B Inrush Current Limiting
SW A/B
GATE
SAB
SENSE
6V
GG
SW B
GB
output capacitor, C2, further reducing
reaches plus or minus 200mV. The
INRUSH CURRENT
BIDIRECTIONAL
GS
V
SENSING AND
THRESHOLD
SENSE +
LIMITING
, of the appropriate switch is
R
200mV
SENSE
SENSE
V
SENSE –
, is measured to
1479 F02
+
C
OUT
OUTPUT
TO DC/DC
CONVERTER
GG