Datasheet LT3503 (Analog Devices) - 9
| Hersteller | Analog Devices |
| Beschreibung | 1A, 2.2MHz Step-Down Switching Regulator in 2mm x 3mm DFN |
| Seiten / Seite | 20 / 9 — APPLICATIO S I FOR ATIO. Figure 3. Figure 4. Inductor Selection and … |
| Dateiformat / Größe | PDF / 438 Kb |
| Dokumentensprache | Englisch |
APPLICATIO S I FOR ATIO. Figure 3. Figure 4. Inductor Selection and Maximum Output Current
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LT3503
U U W U APPLICATIO S I FOR ATIO
Now the required on time has decreased below the mini- VSW mum on time of 130ns. Instead of the switch pulse width 10V/DIV becoming narrower to accommodate the lower duty cycle IL requirement, the switch pulse width remains fixed at 1A/DIV 130ns. In Figure 2 the inductor current ramps up to a value VOUT 20mV/DIV exceeding the load current and the output ripple increases to ~40mV. The part then remains off until the output COUT = 47µF 1µs/DIV 3503 F03 VOUT = 0.78V voltage dips below 100% of the programmed value before VIN = 20V I it begins switching again. LOAD = 1.1A L = 1.1µH
Figure 3
Provided that the output remains in regulation and that the inductor does not saturate, operation above VIN(PS) is safe and will not damage the part. Figure 3 illustrates the VSW 10V/DIV switching waveforms when the input voltage is increased IL to its absolute maximum rating of 20V. 1A/DIV The part is robust enough to survive prolonged operation VOUT 20mV/DIV under these conditions as long as the peak inductor C 1µs/DIV 3503 F04 current does not exceed 2.2A. In Figure 3 the peak inductor OUT = 2 × 47µF VOUT = 0.78V current of 2A suggests that the saturation current rating of VIN = 20V ILOAD = 1.1A the inductor should be ~2.6A, which may require an L = 2.7µH inductor of large physical size. The peak inductor current
Figure 4
value can be reduced by simultaneously increasing the where V inductance and output capacitance. In Figure 4 the peak D is the voltage drop of the catch diode (~0.4V) and L is in µH. With this value there will be no subharmonic inductor current is reduced to 1.3A by doubling the output oscillation for applications with 50% or greater duty cycle. capacitor and inductor values. Now the required inductor The inductor’s RMS current rating must be greater than current saturation rating is ~1.7A, so that even though the your maximum load current and its saturation current inductance value has increased, it may be possible to should be about 30% higher. For robust operation in fault achieve a physically smaller inductor size. conditions, the saturation current should be above 2.2A. Note that inductor current saturation ratings often de- To keep efficiency high, the series resistance (DCR) should crease with temperature and that inductor current satura- be less than 0.1Ω. Table 1 lists several vendors and types tion may further limit performance in this operating regime. that are suitable.
Inductor Selection and Maximum Output Current
Of course, such a simple design guide will not always result in the optimum inductor for your application. A A good first choice for the inductor value is: larger value provides a higher maximum load current and L = 0.6 (V reduces output voltage ripple at the expense of slower OUT + VD) 3503f 9
U U W U APPLICATIO S I FOR ATIO
Now the required on time has decreased below the mini- VSW mum on time of 130ns. Instead of the switch pulse width 10V/DIV becoming narrower to accommodate the lower duty cycle IL requirement, the switch pulse width remains fixed at 1A/DIV 130ns. In Figure 2 the inductor current ramps up to a value VOUT 20mV/DIV exceeding the load current and the output ripple increases to ~40mV. The part then remains off until the output COUT = 47µF 1µs/DIV 3503 F03 VOUT = 0.78V voltage dips below 100% of the programmed value before VIN = 20V I it begins switching again. LOAD = 1.1A L = 1.1µH
Figure 3
Provided that the output remains in regulation and that the inductor does not saturate, operation above VIN(PS) is safe and will not damage the part. Figure 3 illustrates the VSW 10V/DIV switching waveforms when the input voltage is increased IL to its absolute maximum rating of 20V. 1A/DIV The part is robust enough to survive prolonged operation VOUT 20mV/DIV under these conditions as long as the peak inductor C 1µs/DIV 3503 F04 current does not exceed 2.2A. In Figure 3 the peak inductor OUT = 2 × 47µF VOUT = 0.78V current of 2A suggests that the saturation current rating of VIN = 20V ILOAD = 1.1A the inductor should be ~2.6A, which may require an L = 2.7µH inductor of large physical size. The peak inductor current
Figure 4
value can be reduced by simultaneously increasing the where V inductance and output capacitance. In Figure 4 the peak D is the voltage drop of the catch diode (~0.4V) and L is in µH. With this value there will be no subharmonic inductor current is reduced to 1.3A by doubling the output oscillation for applications with 50% or greater duty cycle. capacitor and inductor values. Now the required inductor The inductor’s RMS current rating must be greater than current saturation rating is ~1.7A, so that even though the your maximum load current and its saturation current inductance value has increased, it may be possible to should be about 30% higher. For robust operation in fault achieve a physically smaller inductor size. conditions, the saturation current should be above 2.2A. Note that inductor current saturation ratings often de- To keep efficiency high, the series resistance (DCR) should crease with temperature and that inductor current satura- be less than 0.1Ω. Table 1 lists several vendors and types tion may further limit performance in this operating regime. that are suitable.
Inductor Selection and Maximum Output Current
Of course, such a simple design guide will not always result in the optimum inductor for your application. A A good first choice for the inductor value is: larger value provides a higher maximum load current and L = 0.6 (V reduces output voltage ripple at the expense of slower OUT + VD) 3503f 9