Datasheet

LT1946A
9
sn1946a 1946afs
Diode Selection
A Schottky diode is recommended for use with the LT1946A.
The Microsemi UPS120 is a very good choice. Where the
input to output voltage differential exceeds 20V, use the
UPS140 (a 40V diode). These diodes are rated to handle an
average forward current of 1A. For applications where the
average forward current of the diode is less than 0.5A, an
ON Semiconductor MBR0520 diode can be used.
Setting Output Voltage
To set the output voltage, select the values of R1 and R2
(see Figure 1) according to the following equation:
RR
V
V
OUT
12
125
1=






.
–
A good range for R2 is from 5k to 30k.
Layout Hints
The high speed operation of the LT1946A demands careful
attention to board layout. You will not get advertised
performance with careless layouts. Figure 6 shows the
recommended component placement for a boost con-
verter.
1
2
8
7
3
4
6
5
L1
C2
LT1946A
V
OUT
V
IN
GND
SHUTDOWN
R1
R2
MULTIPLE
VIAs
GROUND PLANE
19949 F04
C1
+
R
C
C
C
CSS
Figure 6. Recommended Component
Placement for Boost Converter
NOTE: DIRECT HIGH CURRENT PATHS USING WIDE PC TRACES. MINIMIZE TRACE AREA AT
PIN 1(V
C
) AND PIN 2(F
B
). USE MULTIPLE VIAS TO TIE PIN 4 COPPER TO GROUND PLANE. USE
VIAS AT ONE LOCATION ONLY TO AVOID INTRODUCING SWITCHING CURRENTS INTO THE
GROUND PLANE.
FREQUENCY (Hz)
GAIN (f)
100
50
0
–50
100 10k 100k 1M
1946A FO5a
1k
FREQUENCY (Hz)
PHASE (f)
0
–100
–200
100 10k 100k 1M
1946A FO5b
1k
–180
58°
Figure 5. Gain and Phase Plots of Figure 1 Circuit
APPLICATIO S I FOR ATIO
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