LT1931/LT1931A
W U U
U
APPLICATIO S I FOR ATIO
DIODE SELECTION
VOUT
2V/DIV
ASchottkydiodeisrecommendedforusewiththeLT1931/
LT1931A. The Motorola MBR0520 is a very good choice.
Wheretheinputtooutputvoltagedifferentialexceeds20V,
use the MBR0530 (a 30V diode). These diodes are rated to
handleanaverageforwardcurrentof0.5A. Inapplications
where the average forward current of the diode exceeds
0.5A, a Microsemi UPS5817 rated at 1A is recommended.
IIN
0.5A/DIV
AC COUPLED
5V
0V
VSS
200µs/DIV
1931 F08
Figure 8. R = 15k, C = 33nF; V Reaches –5V in 1ms;
OUT
Input Current Peaks at 500mA
SS
SS
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
2V/DIV
IIN
0.5A/DIV
AC COUPLED
5V
0V
VSS
500µs/DIV
1931 F09
Figure 9. R = 15k, C = 68nF; V Reaches –5V in 1.6ms;
OUT
SS
SS
L1A
L1B
C2
Input Current Peaks at 350mA
C1
–V
OUT
+
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.
D1
V
IN
C3
+
1
2
3
5
4
SHUTDOWN
R2
R1
GND
1931 F10
Figure 10. Suggested Component Placement.
Note Cut in Ground Copper at D1’s Cathode
1931fa
8
Linear [ Linear ]
