13

LT1571 Series

Figure 13. Critical Electrical and Thermal Path Layer for LT1571-5

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 (SENSE pin)

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-FET (Q2) dis-

charges 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 during normal

operation.

Figure 12. High Speed Switching Path

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 expand 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 or 500kHz triwave and are less critical. Figure 12

indicates the high speed, high current switching path.

Figure 13 shows critical path layout.

Figure 11. Replacing the Input Diode

3V TO 6V

HIGH DUTY CYCLE

CONNECTION

1571 F11

C3

L1

D2

D1

Q2

Q1

50k

Q1: Si4435DY

Q2: TP0610L

10

µF

BOOST

SW

SENSE

LT1571

BAT

+

+

APPLICATIO S I FOR ATIO

1571 F12

L1

HIGH

FREQUENCY

CIRCULATING

PATH

BAT

SWITCH NODE

D1

L1

GND

1571 F13

LT1571-5

GND

CAP

PROG

BAT

GND

GND

SW

BOOST

BAT2

FLAG

SELECT

SENSE

GND