Datenblatt-Suchmaschine für elektronische Bauteile |
|
ADP1173AR Datenblatt(PDF) 7 Page - Analog Devices |
|
ADP1173AR Datenblatt(HTML) 7 Page - Analog Devices |
7 / 16 page ADP1173 –7– REV. 0 For example, assume that +5 V at 300 mA is required from a 12 V to +24 V input. Deriving the peak current from Equation 6 yields: IPEAK = 2 ×300 mA 0.55 5 + 0.5 12 – 1.5 + 0.5 = 545 mA The peak current can then be inserted into Equation 7 to calcu- late the inductor value: L = 12 –1.5 – 5 545 mA ×23 µs = 232 µH Since 232 µH is not a standard value, the next lower standard value of 220 µH would be specified. To avoid exceeding the maximum switch current when the input voltage is at +24 V, an RLIM resistor should be specified. Using the step-down curve of Figure 5, a value of 180 Ω will limit the switch current to 600 mA. Inductor Selection—Positive-to-Negative Converter The configuration for a positive-to-negative converter using the ADP1173 is shown in Figure 17. As with the step-up converter, all of the output power for the inverting circuit must be supplied by the inductor. The required inductor power is derived from the formula: PL = |VOUT|+V D ()× I OUT () (8) The ADP1173 power switch does not saturate in positive-to- negative mode. The voltage drop across the switch can be modeled as a 0.75 V base-emitter diode in series with a 0.65 Ω resistor. When the switch turns on, inductor current will rise at a rate determined by: IL (t) = V L R' 1– e _R't L (9) where R' = 0.65 Ω + R L(DC) where VL = VIN – 0.75 V For example, assume that a –5 V output at 50 mA is to be generated from a +4.5 V to +5.5 V source. The power in the inductor is calculated from Equation 8: PL = |−5V|+ 0.5V ()×(50mA)=275mW During each switching cycle, the inductor must supply the following energy: PL f OSC = 275 mW 24 kHz =11.5µJ Using a standard inductor value of 220 µH, with 0.2 Ω dc resistance, will produce a peak switch current of: IPEAK = 4.5V –0.75V 0.65 Ω+0.2 Ω 1– e –0.85 Ω× 23 µs 220 µH = 375 mA Once the peak current is known, the inductor energy can be calculated from Equation 5: EL = 1 2 (220 µH)×(375 mA)2 =15.5µJ The inductor energy of 15.5 µJ is greater than the P L/fOSC requirement of 11.5 µJ, so the 220 µH inductor will work in this application. The input voltage only varies between 4.5 V and 5.5 V in this example. Therefore, the peak current will not change enough to require an RLIM resistor and the ILIM pin can be connected directly to VIN. Care should be taken to ensure that the peak current does not exceed 650 mA. CAPACITOR SELECTION For optimum performance, the ADP1173’s output capacitor must be carefully selected. Choosing an inappropriate capacitor can result in low efficiency and/or high output ripple. Ordinary aluminum electrolytic capacitors are inexpensive, but often have poor Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). Low ESR aluminum ca- pacitors, specifically designed for switch mode converter appli- cations, are also available, and these are a better choice than general purpose devices. Even better performance can be achieved with tantalum capacitors, although their cost is higher. Very low values of ESR can be achieved by using OS-CON* capacitors (Sanyo Corporation, San Diego, CA). These devices are fairly small, available with tape-and-reel packaging, and have very low ESR. The effects of capacitor selection on output ripple are demon- strated in Figures 11, 12, and 13. These figures show the output of the same ADP1173 converter, which was evaluated with three different output capacitors. In each case, the peak switch current is 500 mA and the capacitor value is 100 µF. Figure 11 shows a Panasonic HF-series* radial aluminum electrolytic. When the switch turns off, the output voltage jumps by about 90 mV and then decays as the inductor discharges into the capacitor. The rise in voltage indicates an ESR of about 0.18 Ω. In Figure 12, the aluminum electrolytic has been replaced by a Sprague 593D-series* tantalum device. In this case the output jumps about 35 mV, which indicates an ESR of 0.07 Ω. Figure 13 shows an OS-CON SA series capacitor in the same circuit, and ESR is only 0.02 Ω. *All trademarks are properties of their respective holders. |
Ähnliche Teilenummer - ADP1173AR |
|
Ähnliche Beschreibung - ADP1173AR |
|
|
Link URL |
Privatsphäre und Datenschutz |
ALLDATASHEETDE.COM |
War ALLDATASHEET hilfreich? [ DONATE ] |
Über Alldatasheet | Werbung | Kontakt | Privatsphäre und Datenschutz | Linktausch | Hersteller All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |