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.)

The gate drive for the low-loss N-channel switches is

supplied by an internal micropower boost regulator which

diode ORed through three external diodes connected to

the three main power sources, DCIN, BAT1 and BAT2.

source and will provide the overdrive required to fully

enhance the MOSFET switches.

For maximum efficiency the top of the boost regulator

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

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

ascertain the instantaneous current flowing through the

Figure 2. SW A1/B1 Inrush Current Limiting

GA1

GB1

SAB1

SW A1

SW B1

1473 F02

BAT1

+

OUTPUT

LOAD

LTC1473

6V

6V

± 200mV

THRESHOLD

SW A/B

GATE

DRIVERS

BIDIRECTIONAL

INRUSH CURRENT

SENSING AND

LIMITING

BAT1

BAT2

DCIN

SW

GND

1473 F01

L1

1mH

C1

1

µF

50V

TO GATE

DRIVERS

LTC1473

C2

1

µF

50V

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

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.