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LT1317BCMS8 Datasheet(PDF) 10 Page - Linear Technology |
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LT1317BCMS8 Datasheet(HTML) 10 Page - Linear Technology |
10 / 16 page 10 LT1317/LT1317B FREQUENCY COMPENSATION The LT1317 has an external compensation pin (VC) which allows the frequency response to be optimized for the circuit configuration. In most cases, the values used in Figure 1 will work. Some circuits may need additional compensation and a simple trial and error method for determining the necessary component values is given. Figure 6 shows the test setup. A load step is applied and the resulting output voltage waveform is observed. Fig- ures 7 through 10 detail the response for various values of R and C in the compensation network. The circuit of Figure 7 starts with a large C and small R giving a highly overdamped system. This system will always be stable but the output voltage displays a long settling time of >5ms. Figure 8’s circuit has reduced C giving a shorter settling time but still overdamped. Figure 9 shows the results when C is reduced to the point where the system becomes underdamped. The output voltage responds quickly ( ≈200µs to 300µs) but some ringing exists. Figure 10 has APPLICATIONS INFORMATION optimum R and C values giving the best possible settling time with adequate phase margin. An additional 100pF capacitor (CC2) is connected to the VC pin and is necessary if the LT1317 is operated near current limit. Also, CC2 should be present when higher ESR output capacitors are used. Figure 6. Frequency Response Test Setup VIN SW FB LT1317 10 µH MBR0520L SHDN R 604k 50 Ω VOUT 3.3V 1M 15 Ω 2W C CC2 100pF 1317 F06 VC GND VIN 2V 47 µF 47 µF + + 1ms/DIV 1317 F09 ILOAD 2mA TO 200mA VOUT 100mV/DIV AC COUPLED 1ms/DIV 1317 F10 ILOAD 2mA TO 200mA VOUT 100mV/DIV AC COUPLED 5ms/DIV 1317 F07 ILOAD 2mA TO 200mA VOUT 100mV/DIV AC COUPLED 5ms/DIV 1317 F08 ILOAD 2mA TO 200mA VOUT 100mV/DIV AC COUPLED Figure 8. Reducing C to 22nF Speeds Up the Response. (R = 33k) Figure 10. 3.3nF and 33k Gives the Shortest Settling Time with No Ringing. Figure 9. Using 680pF for C Results in an Underdamped System with Ringing. (R = 33k) Figure 7. With C = 56nF and R = 33k, the System is Highly Overdamped. |
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