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LTC1473LCGN

Dual Low Voltage PowerPath™ Switch Driver

Linear Technology 

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OPERATIO

The LTC1473L is responsible for low-loss switching and

isolation for a dual supply system, where during a power

backup situation, a battery pack can be connected or

disconnected seamlessly. Smooth switching between in-

put power sources is accomplished with the help of

low-loss 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 LTC1473L 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 DC power supply. (N-channel MOSFET switches are

more cost effective and provide lower voltage drops than

their P-channel counterparts.)

Gate Drive (VGG) 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 20V

maximum. In a DC supply and backup battery system, the

LTC1473L V+ pin is diode ORed through two external

Schottky diodes connected to the two main power sources,

DCIN and BAT1. Thus, VGG is regulated at 8.5V above the

higher power source and will provide the overdrive

required to fully enhance the MOSFET switches.

For maximum efficiency the input to the boost regulator

inductor is connected to V+ as shown in Figure 1. C1

provides filtering to the input of the 1mH switched induc-

tor, L1, which is housed in a small surface mount package.

An internal diode directs the current from the 1mH induc-

tor to the VGG output capacitor C2.

Inrush and Short-Circuit Current Limiting

The LTC1473L uses an adaptive inrush current limiting

scheme to reduce current flowing in and out of the battery

and the following system’s input capacitor during switch-

over transitions. The voltage across a single small valued

resistor, RSENSE, is measured to ascertain the instanta-

neous current flowing through either of the two switch

pairs, SW A1/B1 and SW A2/B2, during the transitions.

LTC1473L

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 RSENSE reaches

±200mV. The gate-to-source voltage, VGS, of the appro-

priate switch is limited during the transition period until

the inrush current subsides.

This scheme allows capacitors and MOSFET switches of

differing sizes and current ratings to be used in the same

system without circuit modifications.

LTC1473L

DCIN BAT1

V+

TO GATE

DRIVERS

(8.5V + V+)

VGG

SWITCHING

REGULATOR

L1

1mH

VGG

C1

1µF

SW

25V

C2

1µF

25V

GND

1473 F01

Figure 1. VGG Switching Regulator

SW A1

BAT1

SW B1

GA1 SAB1 GB1

6V 6V

RSENSE

VSENSE+

+

VSENSE–

OUTPUT

LOAD

COUT

± 200mV

THRESHOLD

VGG

SW A/B

GATE

DRIVERS

BIDIRECTIONAL

INRUSH CURRENT

SENSING AND

LIMITING

LTC1473L

1473 F02

Figure 2. SW A1/B1 Inrush Current Limiting

7

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