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LTC1705 Datasheet(PDF) 19 Page - Linear Technology |
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LTC1705 Datasheet(HTML) 19 Page - Linear Technology |
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19 / 28 page  LTC1705 19 APPLICATIO S I FOR ATIO gain falls to 0dB. The simplest strategy is to set up the feedback amplifier as an inverting integrator, with the 0dB frequency lower than the LC pole (Figure 6). This “Type 1” configuration is stable but transient response is less than exceptional if the LC pole is at a low frequency. Figure 7 shows an improved “Type 2” circuit that uses an additional pole-zero pair to temporarily remove 90 ° of phase shift. This allows the loop to remain stable with 90 ° more phase shift in the LC section, provided the loop reaches 0dB gain near the center of the phase “bump.” Type 2 loops work well in systems where the ESR zero in the LC roll-off happens close to the LC pole, limiting the total phase shift due to the LC. The additional phase compensation in the feedback amplifier allows the 0dB point to be at or above the LC pole frequency, improving loop bandwidth substantially over a simple Type 1 loop. It has limited ability to compensate for LC combinations where low capacitor ESR keeps the phase shift near 180 ° for an extended frequency range. LTC1705 circuits using conventional switching grade electrolytic output capaci- tors can often get acceptable phase margin with Type 2 compensation. 1705 F07 0 PHASE –6dB/OCT –6dB/OCT GAIN FREQ –90 –180 –270 –360 RB VREF R1 R2 FB C2 IN OUT + – C1 1705 F08 0 PHASE –6dB/OCT +6dB/OCT –6dB/OCT GAIN FREQ –90 –180 –270 –360 RB VREF R1 R2 FB C2 IN OUT + – C1 C3 R3 1705 F06 0 PHASE –6dB/OCT GAIN FREQ –90 –180 –270 –360 RB R1 FB C1 IN OUT + – VREF Figure 6. Type 1 Schematic and Transfer Function Figure 7. Type 2 Schematic and Transfer Function Figure 8. Type 3 Schematic and Transfer Function “Type 3” loops (Figure 8) use two poles and two zeros to obtain a 180 ° phase boost in the middle of the frequency band. A properly designed Type 3 circuit can maintain acceptable loop stability even when low output capacitor ESR causes the LC section to approach 180 ° phase shift well above the initial LC roll-off. As with a Type 2 circuit, the loop should cross through 0dB in the middle of the phase bump to maximize phase margin. Many LTC1705 circuits using low ESR tantalum or OS-CON output capaci- tors need Type 3 compensation to obtain acceptable phase margin with a high bandwidth feedback loop. Feedback Component Selection Selecting the R and C values for a typical Type 2 or Type 3 loop is a nontrivial task. The applications shown in this data sheet show typical values, optimized for the power components shown. They should give acceptable perfor- mance with similar power components, but can be way off if even one major power component is changed signifi- cantly. Applications that require optimized transient re- sponse will need to recalculate the compensation values specifically for the circuit in question. The underlying mathematics are complex, but the component values can be calculated in a straightforward manner if we know the gain and phase of the modulator at the crossover fre- quency. Modulator gain and phase can be measured directly from a breadboard or can be simulated if the appropriate parasitic values are known. Measurement will give more accurate results, but simulation can often get close enough to give a working system. To measure the modulator gain and phase directly, wire up a breadboard with an LTC1705 |
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