Datasheet LT3479 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection |
| Pages / Page | 20 / 9 — APPLICATIONS INFORMATION. Diode Selection. Setting Negative Output … |
| File Format / Size | PDF / 256 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Diode Selection. Setting Negative Output Voltages. Table 3. Suggested Diodes. MANUFACTURER. MAX
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LT3479
APPLICATIONS INFORMATION Diode Selection Setting Negative Output Voltages
Schottky diodes, with their low forward voltage drop and To set a negative output voltage, select the values of R3 and fast switching speed, are ideal for LT3479 applications. R4 (see Figure 3) according to the following equation: Table 3 lists several Schottky diodes that work well with the ⎛ R3⎞ LT3479. The diode’s average current rating must exceed V – . 1 235V OUT = ⎜ ⎟ the average output current. The diode’s maximum reverse ⎝ R4⎠ voltage must exceed the output voltage. The diode conducts –VOUT current only when the power switch is turned off (typically R3 less than 50% duty cycle), so a 3A diode is suffi cient for FBP most designs. The companies below also offer Schottky LT3479 R4 diodes with high voltage and current ratings. VREF
Table 3. Suggested Diodes
FBN 3479 F03
MANUFACTURER MAX MAX REVERSE PART NUMBER CURRENT (A) VOLTAGE (V) MANUFACTURER Figure 3. Negative Output Voltage Feedback Connections
UPS340 3 40 Microsemi UPS315 3 15 www.microsemi.com B220 2 20 Diodes, Inc
Board Layout
B230 2 30 www.diodes.com B240 2 40 As with all switching regulators, careful attention must B320 3 20 be paid to the PCB board layout and component place- B330 3 30 B340 3 40 ment. To maximize effi ciency, switch rise and fall times SBM340 3 40 are made as short as possible. To prevent radiation and high frequency resonance problems, proper layout of the
Setting Positive Output Voltages
high frequency switching path is essential. Minimize the To set a positive output voltage, select the values of R1 and length and area of all traces connected to the SW pin and R2 (see Figure 2) according to the following equation: always use a ground plane under the switching regulator to minimize interplane coupling. The signal path including ⎛ R ⎞ 1 V 1 235 . V the switch, output diode D1 and output capacitor C OUT = ⎜1+ ⎟ OUT, ⎝ R2⎠ contains nanosecond rise and fall times and should be kept as short as possible. Recommended component placement is shown in Figure 4. FBP LT3479 VOUT
Soft-Start
VREF R1 For many applications, it is necessary to minimize the FBN inrush current at start-up. The built-in soft-start circuit R2 signifi cantly reduces the start-up current spike and output 3479 F02 voltage overshoot. A typical value is 10nF for 1.65ms.
Figure 2. Positive Output Voltage Feedback Connections
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). 3479fc 9
APPLICATIONS INFORMATION Diode Selection Setting Negative Output Voltages
Schottky diodes, with their low forward voltage drop and To set a negative output voltage, select the values of R3 and fast switching speed, are ideal for LT3479 applications. R4 (see Figure 3) according to the following equation: Table 3 lists several Schottky diodes that work well with the ⎛ R3⎞ LT3479. The diode’s average current rating must exceed V – . 1 235V OUT = ⎜ ⎟ the average output current. The diode’s maximum reverse ⎝ R4⎠ voltage must exceed the output voltage. The diode conducts –VOUT current only when the power switch is turned off (typically R3 less than 50% duty cycle), so a 3A diode is suffi cient for FBP most designs. The companies below also offer Schottky LT3479 R4 diodes with high voltage and current ratings. VREF
Table 3. Suggested Diodes
FBN 3479 F03
MANUFACTURER MAX MAX REVERSE PART NUMBER CURRENT (A) VOLTAGE (V) MANUFACTURER Figure 3. Negative Output Voltage Feedback Connections
UPS340 3 40 Microsemi UPS315 3 15 www.microsemi.com B220 2 20 Diodes, Inc
Board Layout
B230 2 30 www.diodes.com B240 2 40 As with all switching regulators, careful attention must B320 3 20 be paid to the PCB board layout and component place- B330 3 30 B340 3 40 ment. To maximize effi ciency, switch rise and fall times SBM340 3 40 are made as short as possible. To prevent radiation and high frequency resonance problems, proper layout of the
Setting Positive Output Voltages
high frequency switching path is essential. Minimize the To set a positive output voltage, select the values of R1 and length and area of all traces connected to the SW pin and R2 (see Figure 2) according to the following equation: always use a ground plane under the switching regulator to minimize interplane coupling. The signal path including ⎛ R ⎞ 1 V 1 235 . V the switch, output diode D1 and output capacitor C OUT = ⎜1+ ⎟ OUT, ⎝ R2⎠ contains nanosecond rise and fall times and should be kept as short as possible. Recommended component placement is shown in Figure 4. FBP LT3479 VOUT
Soft-Start
VREF R1 For many applications, it is necessary to minimize the FBN inrush current at start-up. The built-in soft-start circuit R2 signifi cantly reduces the start-up current spike and output 3479 F02 voltage overshoot. A typical value is 10nF for 1.65ms.
Figure 2. Positive Output Voltage Feedback Connections
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). 3479fc 9
