lt1931a Linear Technology Corporation, lt1931a Datasheet - Page 8

• Manufacturer Part Number: lt1931a
• Description: Lt1931a - 1.2mhz/2.2mhz Inverting Dc/dc Converters In Thinsot
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1931/LT1931A
increased to 68nF, resulting in a lower peak input current
of 350mA with a V
be increased further for an even slower ramp, if desired.
Diode D2 serves to quickly discharge C
driven low to shut down the device. D2 can be omitted,
resulting in a “soft-stop” slow discharge of the output
capacitor.
8
AC COUPLED
AC COUPLED
Figure 8. R
Input Current Peaks at 500mA
Figure 9. R
Input Current Peaks at 350mA
0.5A/DIV
0.5A/DIV
V
V
2V/DIV
SS
2V/DIV
SS
V
V
OUT
OUT
5V
0V
I
5V
0V
I
IN
IN
SS
SS
= 15k, C
= 15k, C
OUT
U
SS
ramp time of 1.6ms. C
SS
= 33nF; V
= 68nF; V
200µs/DIV
500µs/DIV
U
OUT
OUT
W
Reaches – 5V in 1ms;
Reaches – 5V in 1.6ms;
SS
1931 F08
1931 F09
SS
when V
or R
U
SS
SS
can
is
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.
–V
OUT
Figure 10. Suggested Component Placement.
Note Cut in Ground Copper at D1’s Cathode
GND
+
C3
D1
C2
L1B
R2
L1A
1
2
3
1931 F10
R1
C1
5
4
+
SHUTDOWN
V
IN
1931fa