15

LT1511

off instantly when the input is dead shorted to avoid large

current surges from the battery back through the charger

into the FET. Gate capacitance slows turn-off, so a small

P-channel (Q2) is to discharge the gate capacitance quickly

in the event of an input short. The body diode of Q2 creates

the necessary pumping action to keep the gate of Q1 low

Optional Connection of Input Diode and

Current Sense Resistor

The typical application shown in Figure 1 on the first page

pin from the adapter input. This simple connection may be

unacceptable in situations where the main system power

must be disconnected from both the battery

and the

adapter under some conditons. In particular, if the adapter

is disconnected or turned off and it is desired to also

Figure 13. High Speed Switching Path

LT1511 • F13

L1

HIGH

FREQUENCY

CIRCULATING

PATH

BAT

SWITCH NODE

D1

SW

L1

CLP

CLN

ADAPTER

IN

TO

SYSTEM

POWER

R7

500

Ω

C1

1

µF

D3

LT1511

PARASITIC

INTERNAL

DIODE

1511 F12a

+

+

Figure 12a. Standard Connection

1511 F12b

SW

L1

CLP

CLN

ADAPTER

IN

TO

SYSTEM

POWER

R7

500

Ω

C1

1

µF

D4

D3

LT1511

PARASITIC

INTERNAL

DIODE

+

+

Figure 12b. Modified Input Diode Connection

disconnect the system load from the battery, the system

will remain powered through the parasitic diode from the

The circuit in Figure 12b allows system power to go to 0V

without drawing battery current by adding an additional

diode, D4. To ensure proper operation, the LT1511 current

sense amplifier inputs (CLP and CLN) were designed to

inputs are pulled to ground by a powered-down adapter.

Layout Considerations

Switch rise and fall times are under 10ns for maximum

efficiency. To prevent radiation, the catch diode, SW pin

and input bypass capacitor leads should be kept as short

as possible. A ground plane should be used under the

switching circuitry to prevent interplane coupling and to

act as a thermal spreading path. All ground pins should be

connected to expanded traces for low thermal resistance.

The fast-switching high current ground path, including the

switch, catch diode and input capacitor, should be kept

very short. Catch diode and input capacitor should be

close to the chip and terminated to the same point. This

path contains nanosecond rise and fall times with several

amps of current. The other paths contain only DC and/or

200kHz tri-wave and are less critical. Figure 13 indicates

the high speed, high current switching path. Figure 14

shows critical path layout. Contact Linear Technology for

an actual LT1511 circuit PCB layout or Gerber file.

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.