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LM2590HV-AQ Datasheet(PDF) 16 Page - National Semiconductor (TI) |
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LM2590HV-AQ Datasheet(HTML) 16 Page - National Semiconductor (TI) |
16 / 22 page ![]() Application Information INDUCTOR SELECTION PROCEDURE Application Note AN-1197 titled "Selecting Inductors for Buck Converters" provides detailed information on this topic. For a quick-start the designer may refer to the nomographs pro- vided in Figure 4 to Figure 6. To widen the choice of the Designer to a more general selection of available inductors, the nomographs provide the required inductance and also the energy in the core expressed in microjoules (µJ), as an alternative to just prescribing custom parts. The following points need to be highlighted: 1. The Energy values shown on the nomographs apply to steady operation at the corresponding x-coordinate (rated maximum load current). However under start-up, without soft-start, or a short-circuit on the output, the current in the inductor will momentarily/repetitively hit the current limit I CLIM of the device, and this current could be much higher than the rated load, I LOAD. This represents an overload situation, and can cause the Inductor to saturate (if it has been designed only to handle the energy of steady operation). However most types of core structures used for such applications have a large inherent air gap (for example powdered iron types or ferrite rod inductors), and so the inductance does not fall off too sharply under an overload. The device is usually able to protect itself by not allowing the current to ever exceed I CLIM. But if the DC input voltage to the regulator is over 40V, the current can slew up so fast under core saturation, that the device may not be able to act fast enough to restrict the current. The cur- rent can then rise without limit till destruction of the device takes place. Therefore to ensure reliability, it is recommended, that if the DC Input Voltage exceeds 40V, the inductor must ALWAYS be sized to handle an instantaneous current equal to I CLIM without saturating, irrespective of the type of core structure/material. 2. The Energy under steady operation is where L is in µH and I PEAK is the peak of the inductor current waveform with the regulator delivering I LOAD. These are the energy values shown in the nomographs. See Example 1 below. 3. The Energy under overload is If V IN > 40V, the inductor should be sized to handle e CLIM instead of the steady energy values. The worst case I CLIM for the LM2590HV-AQ is 3A. The Energy rating depends on the Inductance. See Example 2 be- low. 4. The nomographs were generated by allowing a greater amount of percentage current ripple in the Inductor as the maximum rated load decreases (see Figure 7). This was done to permit the use of smaller inductors at light loads. Figure 7 however shows only the ’median’ value of the current ripple. In reality there may be a great spread around this because the nomographs approxi- mate the exact calculated inductance to standard avail- able values. It is a good idea to refer to AN-1197 for detailed calculations if a certain maximum inductor cur- rent ripple is required for various possible reasons. Also consider the rather wide tolerance on the nominal induc- tance of commercial inductors. 5. Figure 6 shows the inductor selection curves for the Adjustable version. The y-axis is ’Et’, in Vµsecs. It is the applied volts across the inductor during the ON time of the switch (V IN-VSAT-VOUT) multiplied by the time for which the switch is on in µsecs. See Example 3 below. Example 1: (V IN ≤ 40V) LM2590HV-5.0-AQ, V IN = 24V, Output 5V @ 0.8A 1. A first pass inductor selection is based upon Inductance and rated max load current. We choose an inductor with the Inductance value indicated by the nomograph (Figure 5) and a current rating equal to the maximum load current. We therefore quick-select a 100µH/0.8 A inductor (designed for 150 kHz operation) for this application. 2. We should confirm that it is rated to handle 50 µJ (see Figure 5) by either estimating the peak current or by a detailed calculation as shown in AN-1197, and also that the losses are acceptable. Example 2: (V IN > 40V) LM2590HV-5.0-AQ, VIN = 48V, Output 5V @ 1A 1. A first pass inductor selection is based upon Inductance and the switch currrent limit. We choose an inductor with the Inductance value indicated by the nomograph (Figure 5) and a current rating equal to I CLIM. We therefore quick-select a 100µH/3A inductor (designed for 150 kHz operation) for this application. 2. We should confirm that it is rated to handle e CLIM by the procedure shown in AN-1197 and that the losses are accept- able. Here e CLIM is: Example 3: (V IN ≤ 40V) LM2590HV-ADJ-AQ, V IN = 20V, Output 10V @ 1A 1. Since input voltage is less than 40V, a first pass inductor selection is based upon Inductance and rated max load current. We choose an inductor with the Inductance value indicated by the nomograph Figure 6 and a current rating equal to the maximum load. But we first need to calculate Et for the given application. The Duty cycle is where V D is the drop across the Catch Diode ( ) 0.5V for a Schottky) and V SAT the drop across the switch ( )1.5V). So And the switch ON time is where f is the switching frequency in Hz. So www.national.com 16 |
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