Datasheet LT1110 (Linear Technology) - 9
| Hersteller | Linear Technology |
| Beschreibung | Micropower DC-DC Converter Adjustable and Fixed 5V, 12V |
| Seiten / Seite | 16 / 9 — APPLICATI. S I FOR ATIO. Figure 1. Aluminum. Capacitor Selection. Figure … |
| Dateiformat / Größe | PDF / 323 Kb |
| Dokumentensprache | Englisch |
APPLICATI. S I FOR ATIO. Figure 1. Aluminum. Capacitor Selection. Figure 2. Tantalum. Figure 3. OS-CON
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LT1110
O U U W U APPLICATI S I FOR ATIO
switch closes, current in the inductor builds according to capacitors provide still better performance at more ex- pense. We recommend OS-CON capacitors from Sanyo V R – t ' Corporation (San Diego, CA). These units are physically I L (+ = 1 – e ) L L (1 ) 5 R' quite small and have extremely low ESR. To illustrate, Figures 1, 2 and 3 show the output voltage of an LT1110 where R' = 0.65Ω + DCRL based converter with three 100µF capacitors. The peak VL = VIN – 0.75V switch current is 500mA in all cases. Figure 1 shows a Sprague 501D, 25V aluminum capacitor. V As an example, suppose –5V at 75mA is to be generated OUT jumps by over 120mV when the switch turns off, followed by a drop from a 4.5V to 5.5V input. Recalling Equation (14), in voltage as the inductor dumps into the capacitor. This P | 5V | . 0 5V 7 m 5 A = m 413 W ( )( ) works out to be an ESR of over 240mΩ. Figure 2 shows the L = − + . ( ) 16 same circuit, but with a Sprague 150D, 20V tantalum Energy required from the inductor is capacitor replacing the aluminum unit. Output jump is PL mW = 413 = now about 35mV, corresponding to an ESR of 70mΩ. 5 9 . J µ . 1 ( 7) f kHz Figure 3 shows the circuit with a 16V OS-CON unit. ESR is OSC 70 now only 20mΩ. Picking an inductor value of 56µH with 0.2Ω DCR results in a peak switch current of 4 5 . V – 0 7 . 5V –0 8 . 5Ω • 10 s ( ) µ I = 1 – e m 621 A PEAK 56 H ( . 18 ( ) µ 0 6 . 5Ω + 0 2 . Ω) = 50mV/DIV Substituting IPEAK into Equation (04) results in 1 5 s/DIV µ E 56 H 0. A 621 2 µ 10 8 . J LT1110 • TA19 L = ( )( ) = µ . 1 ( 9) 2
Figure 1. Aluminum
Since 10.8µJ > 5.9µJ, the 56µH inductor will work. With this relatively small input range, RLIM is not usually necessary and the ILIM pin can be tied directly to VIN. As in the step-down case, peak switch current should be limited 50mV/DIV to ~800mA.
Capacitor Selection
5 s/DIV µ LT1110 • TA20 Selecting the right output capacitor is almost as important
Figure 2. Tantalum
as selecting the right inductor. A poor choice for a filter capacitor can result in poor efficiency and/or high output ripple. Ordinary aluminum electrolytics, while inexpensive and readily available, may have unacceptably poor Equiva- lent Series Resistance (ESR) and ESL (inductance). There 50mV/DIV are low ESR aluminum capacitors on the market specifi- cally designed for switch mode DC-DC converters which work much better than general-purpose units. Tantalum 5 s/DIV µ LT1110 • TA21
Figure 3. OS-CON
9
O U U W U APPLICATI S I FOR ATIO
switch closes, current in the inductor builds according to capacitors provide still better performance at more ex- pense. We recommend OS-CON capacitors from Sanyo V R – t ' Corporation (San Diego, CA). These units are physically I L (+ = 1 – e ) L L (1 ) 5 R' quite small and have extremely low ESR. To illustrate, Figures 1, 2 and 3 show the output voltage of an LT1110 where R' = 0.65Ω + DCRL based converter with three 100µF capacitors. The peak VL = VIN – 0.75V switch current is 500mA in all cases. Figure 1 shows a Sprague 501D, 25V aluminum capacitor. V As an example, suppose –5V at 75mA is to be generated OUT jumps by over 120mV when the switch turns off, followed by a drop from a 4.5V to 5.5V input. Recalling Equation (14), in voltage as the inductor dumps into the capacitor. This P | 5V | . 0 5V 7 m 5 A = m 413 W ( )( ) works out to be an ESR of over 240mΩ. Figure 2 shows the L = − + . ( ) 16 same circuit, but with a Sprague 150D, 20V tantalum Energy required from the inductor is capacitor replacing the aluminum unit. Output jump is PL mW = 413 = now about 35mV, corresponding to an ESR of 70mΩ. 5 9 . J µ . 1 ( 7) f kHz Figure 3 shows the circuit with a 16V OS-CON unit. ESR is OSC 70 now only 20mΩ. Picking an inductor value of 56µH with 0.2Ω DCR results in a peak switch current of 4 5 . V – 0 7 . 5V –0 8 . 5Ω • 10 s ( ) µ I = 1 – e m 621 A PEAK 56 H ( . 18 ( ) µ 0 6 . 5Ω + 0 2 . Ω) = 50mV/DIV Substituting IPEAK into Equation (04) results in 1 5 s/DIV µ E 56 H 0. A 621 2 µ 10 8 . J LT1110 • TA19 L = ( )( ) = µ . 1 ( 9) 2
Figure 1. Aluminum
Since 10.8µJ > 5.9µJ, the 56µH inductor will work. With this relatively small input range, RLIM is not usually necessary and the ILIM pin can be tied directly to VIN. As in the step-down case, peak switch current should be limited 50mV/DIV to ~800mA.
Capacitor Selection
5 s/DIV µ LT1110 • TA20 Selecting the right output capacitor is almost as important
Figure 2. Tantalum
as selecting the right inductor. A poor choice for a filter capacitor can result in poor efficiency and/or high output ripple. Ordinary aluminum electrolytics, while inexpensive and readily available, may have unacceptably poor Equiva- lent Series Resistance (ESR) and ESL (inductance). There 50mV/DIV are low ESR aluminum capacitors on the market specifi- cally designed for switch mode DC-DC converters which work much better than general-purpose units. Tantalum 5 s/DIV µ LT1110 • TA21
Figure 3. OS-CON
9