lt3479efe-trpbf Linear Technology Corporation, lt3479efe-trpbf Datasheet - Page 7

• Manufacturer Part Number: lt3479efe-trpbf
• Description: 3a, Full Featured Dc/dc Converter With Soft-start And Inrush Current Protection
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast switching speed, are ideal for LT3479 applications.
Table 3 lists several Schottky diodes that work well with
the LT3479. The diode’s average current rating must ex-
ceed the average output current. The diode’s maximum
reverse voltage must exceed the output voltage. The diode
conducts current only when the power switch is turned off
(typically less than 50% duty cycle), so a 3A diode is suf-
ficient for most designs. The companies below also offer
Schottky diodes with high voltage and current ratings.
Table 3. Suggested Diodes
MANUFACTURER
PART NUMBER
UPS340
UPS315
B220
B230
B240
B320
B330
B340
SBM340
Setting Positive Output Voltages
To set a positive output voltage, select the values of R1 and
R2 (see Figure 2) according to the following equation:
V
Figure 2. Positive Output Voltage Feedback Connections
OUT
=
1 235 1
.
CURRENT (A)
V
MAX
⎛
⎜
⎝
U
3
3
2
2
2
3
3
3
3
LT3479
+
R
R
2
1
U
V
FBN
⎞
⎟
⎠
FBP
REF
MAX REVERSE
VOLTAGE (V)
40
15
20
30
40
20
30
40
40
V
OUT
W
R1
R2
3479 F02
MANUFACTURER
Microsemi
www.microsemi.com
Diodes, Inc
www.diodes.com
U
Setting Negative Output Voltages
To set a negative output voltage, select the values of R3
and R4 (see Figure 3) according to the following equation:
Board Layout
As with all switching regulators, careful attention must be
paid to the PCB board layout and component placement.
To maximize efficiency, switch rise and fall times are made
as short as possible. To prevent radiation and high fre-
quency resonance problems, proper layout of the high
frequency switching path is essential. Minimize the length
and area of all traces connected to the SW pin and always
use a ground plane under the switching regulator to
minimize interplane coupling. The signal path including
the switch, output diode D1 and output capacitor C
contains nanosecond rise and fall times and should be
kept as short as possible. Recommended component
placement is shown in Figure 4.
Soft-Start
For many applications, it is necessary to minimize the
inrush current at start-up. The built-in soft-start circuit
significantly reduces the start-up current spike and output
voltage overshoot. A typical value is 10nF for 1.65ms.
Figure 5 shows the start-up output voltage and inductor
current waveforms in a typical application without a soft-
start capacitor. Notice the output voltage overshoot and
the large initial current. The addition of a 22nF capacitor
eliminates the output overshoot and reduces the peak
inductor current (Figure 6).
V
Figure 3. Negative Output Voltage Feedback Connections
OUT
=
– . 1 235
V
⎛
⎜
⎝
R
R
LT3479
3
4
⎞
⎟
⎠
V
FBN
FBP
REF
–V
OUT
R3
R4
3479 F03
LT3479
OUT
3479fa
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