15

LT1769

Lower Dropout Voltage

For even lower dropout and/or reducing heat on the board,

the input diode D3 can be replaced with a FET (see Figure

11). Connect a P-channel FET in place of the input diode

with its gate connected to the battery causing the FET to

turn off when the input voltage goes low. The problem is

that the gate must be pumped low so that the FET is fully

turned on even when the input is only a volt or two above

the battery voltage. Also there is a turn-off speed issue.

The FET should turn 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 added to

discharge the gate capacitance quickly in the event of an

input short. The Q2 body diode creates the necessary

pumping action to keep the gate of Q1 low during normal

Optional Diode Connections

The typical application in Figure 1 shows a single diode

block reverse input voltage (both steady state and tran-

sient). This simple connection may be unacceptable in

situations where the system load must be powered from

the battery when the adapter input power is removed. As

shown in Figure 12, a parasitic diode exists from the SW

removed, this diode will become forward biased and will

provide a current path from the battery to the system load.

Because of diode power limitations, it is not recom-

mended to power the system load through the internal

parasitic diode. To safely power the system load from the

battery, an additional Schottky diode (D4) is needed. For

MOSFET which is turned on when the adapter power is

removed.

Layout Considerations

Switch rise and fall times are under 10ns for maximum

efficiency. To minimize 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

the LT1769 circuit PCB layout or Gerber file.

SW

L1

CLP

CLN

ADAPTER

IN

TO

SYSTEM

LOAD

R7

500

Ω

C1

1

µF

D3

LT1769

INTERNAL

PARASITIC

DIODE

1769 F12a

D4

+

+

+

Figure 12. Modified Diode Connection

Figure 13. High Speed Switching Path

1769 F13

L1

HIGH

FREQUENCY

CIRCULATING

PATH

BAT

SWITCH NODE

D1

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.