Datasheet LT3493-3 (Analog Devices) - 9

LT3493-3
U U W U APPLICATIO S I FOR ATIO Minimum On Time
As the input voltage increases, the inductor current ramps up quicker, the number of skipped pulses increases and The part will still regulate the output at input voltages that the output voltage ripple increases. For operation above exceed VIN(PS) (up to 40V), however, the output voltage V ripple increases as the input voltage is increased. Figure 1 IN(MAX) the only component requirement is that the components be adequately rated for operation at the illustrates switching waveforms in continuous mode for a intended voltage levels. 3V output application near VIN(PS) = 33V. The part is robust enough to survive prolonged operation As the input voltage is increased, the part is required to under these conditions as long as the peak inductor switch for shorter periods of time. Delays associated with current does not exceed 2.2A. Inductor current saturation turning off the power switch dictate the minimum on time may further limit performance in this operating regime. of the part. The minimum on time for the LT3493-3 is 130ns. Figure 2 illustrates the switching waveforms when
Inductor Selection and Maximum Output Current
the input voltage is increased to VIN = 35V. A good first choice for the inductor value is: Now the required on time has decreased below the mini- mum on time of 130ns. Instead of the switch pulse width L = 1.6 (VOUT + VD) becoming narrower to accommodate the lower duty cycle where VD is the voltage drop of the catch diode (~0.4V) and requirement, the switch pulse width remains fixed at L is in µH. With this value there will be no subharmonic 130ns. In Figure 2 the inductor current ramps up to a value oscillation for applications with 50% or greater duty cycle. exceeding the load current and the output ripple increases The inductor’s RMS current rating must be greater than to ~200mV. The part then remains off until the output your maximum load current and its saturation current voltage dips below 100% of the programmed value before should be about 30% higher. For robust operation in fault it begins switching again. conditions, the saturation current should be above 2.2A. Provided that the output remains in regulation and that the To keep efficiency high, the series resistance (DCR) should inductor does not saturate, operation above V be less than 0.1Ω. Table 1 lists several vendors and types IN(PS) is safe and will not damage the part. Figure 3 illustrates the that are suitable. switching waveforms when the input voltage is increased Of course, such a simple design guide will not always to its absolute maximum rating of 40V. result in the optimum inductor for your application. A V V SW SW 20V/DIV 20V/DIV IL IL 0.5A/DIV 0.5A/DIV VOUT VOUT 200mV/DIV 200mV/DIV AC COUPLED AC COUPLED COUT = 10µF 2µs/DIV 3493-3 F02 COUT = 10µF 2µs/DIV 3493-3 F03 VOUT = 3V VOUT = 3V VIN = 35V VIN = 40V ILOAD = 0.75A ILOAD = 0.75A L = 10µH L = 10µH
Figure 2 Figure 3
3493-3f 9