<p>Figure 9: Amplifier output voltage with calculated Zobel component values.</p>

The Idea behind doing an equivalence check between SDC files of the top level and block level is that these SDC files are typically generated from each other depending on the top-down or bottom-up flow. In a bottom-up flow, if a block meets timing when analyzed independently, and doesn't meet timing when integrated into the top level, it is because there is some inconsistency in the way constrains get applied at the top level. Equivalence checking will ensure that constraints at the block level will result in the same timing at the top level. The same argument holds for a top-down flow.

variable condenser

Consider the following example (Figure 4 ). If any constant (applied using a set_case_analysis) reaches the block's output port, and if the fan-out of this port is connected to glue logic at the top level, then there should be a corresponding set_case_analysis on the next logic level in the top. If no such set_case_analysis is found, then it should be flagged by the equivalence tool. If the set_case_analysis is found but the values are conflicting, then that should be flagged, too.

4. Inconsistent case analysis between top and block levels..

variable condenser

Consider the following example in Figure 5 and the corresponding SDC files in Figure 6 for each block and the top level sub-system.

variable condenser

5. A sub-system with two blocks, B1 and B2.

In peak current mode control, the impedance of the load has a strong effect on both the DC gain and the low-frequency pole of the control-to-output transfer function. For voltage regulators the load impedance is determined by dividing output voltage by output current. LEDs are diodes, with a dynamic resistance. This dynamic resistance can only be determined by plotting the VF versus IF curve and then taking the tangent line to find the slope at the desired forward current. As shown in Fig. 1, the current regulator uses the load itself as a feedback divider to close the control loop. This reduces the DC gain by a factor of (RSNS / (RSNS + rD )). It is tempting to compensate a boost LED driver with a simple integrator, sacrificing bandwidth for stability. The reality is that many, if not most LED driver applications require dimming. Whether dimming is done by linear adjustment of IF (analog dimming) or by turning the output on and off at high frequency (digital, or PWM dimming) the system requires high bandwidth and fast transient response just as a voltage regulator does.

The buck-boost challenge LEDs for lighting are being adopted much faster than the standards for solid state illumination have developed. A wide variety of input voltages power a wide variety of LEDs. The number of LEDs in series, the type of LEDs, and the variation of VF with both process and die temperature all contribute to a wide range of output voltage. For example, high-end automobiles are converting to LEDs for their daytime running lamps. Three 3-watt white LEDs present a load of about 12 volts at a current of 1 amp. Automotive voltage systems usually require continuous operation over a range of 9 to 16 volts, with an extended range of 6 to 42 volts where performance is reduced but the system can operate without suffering damage. In general, the buck regulator makes the best LED driver, followed by the boost, but neither is appropriate for this case. If a buck-boost regulator must be used, the most difficult decision to make is often which topology to use.

One fundamental difference between buck-boost regulators of any topology and the buck regulator or the boost regulator is that the buck-boosts never connect the input power supply directly to the output. Both the buck and the boost regulator connect VIN to VO (across the inductor and switch/diode) during a portion of their switching cycles, and this direct connection gives them better efficiency. All buck-boost regulators store the entire energy delivered to the load in either a magnetic field (inductor or transformer) or in an electric field (in a capacitor), which results in higher peak currents or higher voltage in the power switches. In particular, evaluation of the converter at the corners of both input voltage and output voltage is necessary because peak switch current occurs at VIN-MIN and VO-MAX , but peak switch voltage occurs at VIN-MAX and VIN-MAX and VO-MAX . In general this means that a buck-boost regulator of a certain output power will be larger and less efficient than a buck or boost regulator of equal output power.

The single inductor buck-boost can be built with the same parts count as a buck regulator or boost regulator, making it attractive from a system cost standpoint. One disadvantage of this topology is that the polarity of Vo is inverted (Figure 2a) or regulated with respect to VIN (Figure 2b). Level-shifting or polarity inverting circuitry must be employed in these converters. Like the boost converter, they have a discontinuous output current, and require an output capacitor to maintain a continuous LED current. The power MOSFET suffers a peak current of IIN plus IF and a peak voltage of VIN plus VO .

Analog FastSPICE Results This section provides results of the simulation of three complex analog/RF blocks using our traditional SPICE tool, a digital fastSPICE tool, and Berkeley Design Automation Analog FastSPICE. Table 1 summarizes the results.

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