LT3479
9
3479fc
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:
VV
R
R
OUT =⎛
⎝
⎜⎞
⎠
⎟
–.1 235 3
4
Figure 2. Positive Output Voltage Feedback Connections
R2
R1
3479 F02
FBN
VREF
VOUT
LT3479
FBP
Figure 3. Negative Output Voltage Feedback Connections
R4
R3
3479 F03
FBP
VREF
–VOUT
LT3479
FBN
APPLICATIONS INFORMATION
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 exceed
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 suffi cient for
most designs. The companies below also offer Schottky
diodes with high voltage and current ratings.
Table 3. Suggested Diodes
MANUFACTURER
PART NUMBER
MAX
CURRENT (A)
MAX REVERSE
VOLTAGE (V) MANUFACTURER
UPS340
UPS315
3
3
40
15
Microsemi
www.microsemi.com
B220
B230
B240
B320
B330
B340
SBM340
2
2
2
3
3
3
3
20
30
40
20
30
40
40
Diodes, Inc
www.diodes.com
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:
VV
R
R
OUT =+
⎛
⎝
⎜⎞
⎠
⎟
1 235 1 1
2
.
Board Layout
As with all switching regulators, careful attention must
be paid to the PCB board layout and component place-
ment. To maximize effi ciency, switch rise and fall times
are made as short as possible. To prevent radiation and
high frequency 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 COUT,
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
signifi cantly 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 induc-
tor 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 capaci-
tor eliminates the output overshoot and reduces the peak
inductor current (Figure 6).