LT1931 Просмотр технического описания (PDF) - Linear Technology

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LT1931

1.2MHz/2.2MHz Inverting DC/DC Converters in ThinSOT

Linear Technology 

LT1931/LT1931A

APPLICATIO S I FOR ATIO

VOUT

2V/DIV

IIN

0.5A/DIV

AC COUPLED

VSS

5V

0V

200µs/DIV

1931 F08

Figure 8. RSS = 15k, CSS = 33nF; VOUT Reaches – 5V in 1ms;

Input Current Peaks at 500mA

VOUT

2V/DIV

IIN

0.5A/DIV

AC COUPLED

VSS

5V

0V

500µs/DIV

1931 F09

Figure 9. RSS = 15k, CSS = 68nF; VOUT Reaches – 5V in 1.6ms;

Input Current Peaks at 350mA

increased to 68nF, resulting in a lower peak input current

of 350mA with a VOUT ramp time of 1.6ms. CSS or RSS can

be increased further for an even slower ramp, if desired.

Diode D2 serves to quickly discharge CSS when VSS is

driven low to shut down the device. D2 can be omitted,

resulting in a “soft-stop” slow discharge of the output

capacitor.

DIODE SELECTION

A Schottky diode is recommended for use with the LT1931/

LT1931A. The Motorola MBR0520 is a very good choice.

Where the input to output voltage differential exceeds 20V,

use the MBR0530 (a 30V diode). These diodes are rated to

handle an average forward current of 0.5 A. In applications

where the average forward current of the diode exceeds

0.5A, a Microsemi UPS5817 rated at 1A is recommended.

LAYOUT HINTS

The high-speed operation of the LT1931/LT1931A de-

mands careful attention to board layout. You will not get

advertised performance with careless layout. Figure 10

shows the recommended component placement. The

ground cut at the cathode of D1 is essential for low noise

operation.

–VOUT

L1B

D1 C2

C3

GND

L1A

C1

+

1

5

2

3

4

R2

R1

1931 F10

VIN

SHUTDOWN

Figure 10. Suggested Component Placement.

Note Cut in Ground Copper at D1’s Cathode

1931fa

8

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