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