Datasheet LT3431 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | High Voltage, 3A, 500kHz Step-Down Switching Regulator |
| Pages / Page | 28 / 9 — APPLICATIO S I FOR ATIO. Figure 2. Frequency and Current Limit Foldback. … |
| File Format / Size | PDF / 263 Kb |
| Document Language | English |
APPLICATIO S I FOR ATIO. Figure 2. Frequency and Current Limit Foldback. CHOOSING THE INDUCTOR
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LT3431
U U W U APPLICATIO S I FOR ATIO
LT3431 VSW TO FREQUENCY L1 SHIFTING OUTPUT 5V 1.4V Q1 ERROR AMPLIFIER + 1.2V R1 R3 R4 1k 2k FB + – C1 BUFFER Q2 R2 TO SYNC CIRCUIT VC GND 3431 F02
Figure 2. Frequency and Current Limit Foldback
will increase and the protection accorded by frequency and current foldback will decrease. VOUT USING 47µF CERAMIC OUTPUT CAPACITOR
CHOOSING THE INDUCTOR
10mV/DIV For most applications, the output inductor will fall into the VOUT USING range of 5µH to 33µH. Lower values are chosen to reduce 100µF, 0.08Ω TANTALUM physical size of the inductor. Higher values allow more OUTPUT CAPACITOR output current because they reduce peak current seen by the LT3431 switch, which has a 3A limit. Higher values 1µs/DIV also reduce output ripple voltage. VIN = 12V 3431 F03 VOUT = 5V When choosing an inductor you will need to consider L = 10µH output ripple voltage, maximum load current, peak induc-
Figure 3. LT3431 Output Ripple Voltage Waveforms.
tor current and fault current in the inductor. In addition,
Ceramic vs Tantalum Output Capacitors
other factors such as core and copper losses, allowable component height, EMI, saturation and cost should also Output ripple voltage is determined by ripple current be considered. The following procedure is suggested as a (ILP-P) through the inductor and the high frequency way of handling these somewhat complicated and con- impedance of the output capacitor. At high frequencies, flicting requirements. the impedance of the tantalum capacitor is dominated by its effective series resistance (ESR).
Output Ripple Voltage Tantalum Output Capacitor
Figure 3 shows a comparison of output ripple voltage for the LT3431 using either a tantalum or ceramic output The typical method for reducing output ripple voltage capacitor. It can be seen from Figure 3 that output ripple when using a tantalum output capacitor is to increase the voltage can be significantly reduced by using the ceramic inductor value (to reduce the ripple current in the induc- output capacitor; the significant decrease in output ripple tor). The following equations will help in choosing the voltage is due to the very low ESR of ceramic capacitors. required inductor value to achieve a desirable output ripple voltage level. If output ripple voltage is of less sn3431 3431fs 9
U U W U APPLICATIO S I FOR ATIO
LT3431 VSW TO FREQUENCY L1 SHIFTING OUTPUT 5V 1.4V Q1 ERROR AMPLIFIER + 1.2V R1 R3 R4 1k 2k FB + – C1 BUFFER Q2 R2 TO SYNC CIRCUIT VC GND 3431 F02
Figure 2. Frequency and Current Limit Foldback
will increase and the protection accorded by frequency and current foldback will decrease. VOUT USING 47µF CERAMIC OUTPUT CAPACITOR
CHOOSING THE INDUCTOR
10mV/DIV For most applications, the output inductor will fall into the VOUT USING range of 5µH to 33µH. Lower values are chosen to reduce 100µF, 0.08Ω TANTALUM physical size of the inductor. Higher values allow more OUTPUT CAPACITOR output current because they reduce peak current seen by the LT3431 switch, which has a 3A limit. Higher values 1µs/DIV also reduce output ripple voltage. VIN = 12V 3431 F03 VOUT = 5V When choosing an inductor you will need to consider L = 10µH output ripple voltage, maximum load current, peak induc-
Figure 3. LT3431 Output Ripple Voltage Waveforms.
tor current and fault current in the inductor. In addition,
Ceramic vs Tantalum Output Capacitors
other factors such as core and copper losses, allowable component height, EMI, saturation and cost should also Output ripple voltage is determined by ripple current be considered. The following procedure is suggested as a (ILP-P) through the inductor and the high frequency way of handling these somewhat complicated and con- impedance of the output capacitor. At high frequencies, flicting requirements. the impedance of the tantalum capacitor is dominated by its effective series resistance (ESR).
Output Ripple Voltage Tantalum Output Capacitor
Figure 3 shows a comparison of output ripple voltage for the LT3431 using either a tantalum or ceramic output The typical method for reducing output ripple voltage capacitor. It can be seen from Figure 3 that output ripple when using a tantalum output capacitor is to increase the voltage can be significantly reduced by using the ceramic inductor value (to reduce the ripple current in the induc- output capacitor; the significant decrease in output ripple tor). The following equations will help in choosing the voltage is due to the very low ESR of ceramic capacitors. required inductor value to achieve a desirable output ripple voltage level. If output ripple voltage is of less sn3431 3431fs 9
