LT1507 datasheet(13/20 Pages) LINER

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LT1507

APPLICATIONS INFORMATION

This is safe for short periods of time, but it would be

prudent to check with the diode manufacturer if continu-

ous operation under these conditions must be tolerated.

BOOST PIN CONSIDERATIONS

For most applications, the boost components are a 0.22

µF

capacitor and an MBR0520 or BAT85 Schottky diode. This

capacitor value is twice that suggested for the LT1376

because the lower voltages commonly found in LT1507

applications may require lower ripple voltage across the

capacitor to ensure adequate boost voltage under worst-

case conditions. Efficiency is not affected by the capacitor

value, but the capacitor should have an ESR of less than

Ω toensurethatitcanberechargedfullyundertheworst-

case condition of minimum input voltage. Almost any type

of film or ceramic capacitor will work fine.

Figure 3. Suggested Layout

The anode of the diode can be connected to the regulated

output voltage or the unregulated input voltage. The

“boost voltage” generated across the boost capacitor is

then nearly identical to the anode voltage. The input

connection minimizes start-up problems and gives plenty

of boost voltage, but efficiency is slightly lower, especially

with input voltages above 10V. For 5V to 3.3V operation,

or any output voltage less than 3.3V, the diode should be

connected to the input. With input voltage more than 3V

above the output and an output voltage of at least 3.3V the

output connection will give better efficiency. Use the

BAT85 Schottky diode for 3.3V applications where the

anode is connected to the output.

LAYOUT CONSIDERATIONS

Suggested layout for the LT1507 is shown in Figure 3. The

main concern for layout is to minimize the length of the

INPUT

SYNC

LT1507 • F03

BOOST

SHDN

GND

SYNC

OUTPUT

MINIMIZE AREA OF

CONNECTIONS TO THE

SWITCH NODE AND

BOOST NODE, BUT OBSERVE

CURRENT DENSITY LIMITATIONS

IN PATH TO L1

KEEP INPUT CAPACITOR

AND CATCH DIODE CLOSE

TO REGULATOR AND

TERMINATE THEM

TO SAME POINT

CONNECT OUTPUT CAPACITOR

DIRECTLY TO HEAVY GROUND

TAKE OUTPUT DIRECTLY FROM END OF OUTPUT

CAPACITOR TO AVOID PARASITIC RESISTANCE

AND INDUCTANCE (KELVIN CONNECTION)

GROUND RING NEED

NOT BE AS SHOWN.

(NORMALLY EXISTS AS

INTERNAL PLANE)

CF AND RC ARE OPTIONAL.

SEE FREQUENCY

COMPENSATION

TERMINATE GND PIN

DIRECTLY TO GROUND

PLANE WITH VIA TO

MINIMIZE EMI. (MINIMIZE

DISTANCE TO INPUT

CAPACITOR C3). CONNECT

FEEDBACK RESISTORS AND

COMPENSATION

COMPONENTS DIRECTLY

TO GROUND PLANE OR TO

SWITCHER GND PIN.

LT1507 Datasheet(PDF) 13 Page - Linear Technology

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