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ISL95873HRUZ-T Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL95873HRUZ-T Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 17 page ISL95873 9 FN8390.0 December 10, 2012 R4TM Modulator The R4™ modulator is an evolutionary step in R3™ technology. Like R3™, the R4™ modulator allows variable frequency in response to load transients and maintains the benefits of current-mode hysteretic controllers. However, in addition, the R4™ modulator reduces regulator output impedance and uses accurate referencing to eliminate the need for a high-gain voltage amplifier in the compensation loop. The result is a topology that can be tuned to voltage-mode hysteretic transient speed while maintaining a linear control model and removes the need for any compensation. This greatly simplifies the regulator design for customers and reduces external component cost. Stability The removal of compensation derives from the R4™ modulator’s lack of need for high DC gain. In traditional architectures, high DC gain is achieved with an integrator in the voltage loop. The integrator introduces a pole in the open-loop transfer function at low frequencies. Thus, combined with the double-pole from the output L/C filter, creates a three pole system that must be compensated to maintain stability. Classic control theory requires a single-pole transition through unity gain to ensure a stable system. Current-mode architectures (includes peak, peak-valley, current-mode hysteretic, R3™ and R4™) generate a zero at or near the L/C resonant point, effectively canceling one of the system’s poles. The system still contains two poles, one of which must be canceled with a zero before unity gain crossover to achieve stability. Compensation components are added to introduce the necessary zero. Figure 8 illustrates the classic integrator configuration for a voltage loop error-amplifier. While the integrator provides the high DC gain required for accurate regulation in traditional technologies, it also introduces a low-frequency pole into the control loop. Figure 9 shows the open-loop response that results from the addition of an integrating capacitor in the voltage loop. The compensation components found in Figure 8 are necessary to achieve stability. Because R4™ does not require a high-gain voltage loop, the integrator can be removed, reducing the number of inherent poles in the loop to two. The current-mode zero continues to cancel one of the poles, ensuring a single-pole crossover for a wide range of output filter choices. The result is a stable system with no need for compensation components or complex equations to properly tune the stability. Figure 10 shows the R4™ error-amplifier that does not require an integrator for high DC gain to achieve accurate regulation. The result to the open loop response can be seen in Figure 11. FIGURE 7. ISL95873 VOLTAGE PROGRAMMING CIRCUIT SREF VSET + - VREF EA + - FB RFB VOUT VCOMP FIGURE 8. INTEGRATOR ERROR-AMPLIFIER CONFIGURATION V INTEGRATOR FOR HIGH DC GAIN COMPENSATION TO COUNTER INTEGRATOR POLE VOUT VDAC VCOMP FIGURE 9. UNCOMPENSATED INTEGRATOR OPEN-LOOP RESPONSE f (Hz) p1 p2 p3 L/C DOUBLE-POLE INTEGRATOR POLE z1 ZERO -20dB CROSSOVER REQUIRED FOR STABILITY COMPENSATOR TO ADD z2 IS NEEDED R3TM LOOP GAIN (dB) CURRENT-MODE |
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