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LTC3114-1 Arkusz danych(PDF) 25 Page - Analog Devices |
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LTC3114-1 Arkusz danych(HTML) 25 Page - Analog Devices |
25 / 34 page LTC3114-1 25 Rev. D For more information www.analog.com APPLICATIONS INFORMATION transconductance gain of 1.97A/V. Similar to peak current mode control, the inner average current mode control loop effectively turns the inductor into a current source over the frequency range of interest, resulting in a fre- quency response from the power stage that exhibits a single pole (–20dB/decade) roll off. The output capacitor (COUT) and load resistance (RLOAD) form the normally dominant low frequency pole and the effective series resistance of the output capacitor and its capacitance form a zero, usually at a high enough frequency to be ignored. A potentially troublesome right half plane zero (RHPZ) is also encountered if the LTC3114-1 is operated in boost mode. The RHPZ causes an increase in gain, like a zero, but a decrease in phase, like a pole. This will ulti- mately limit the maximum converter bandwidth that can be achieved with the LTC3114-1. The RHPZ is not present when operating in buck mode. The overall open loop gain at DC is the product of the following terms: Voltage Error Amp Gain: gm • RO = 120µs • 3.6M = 432V/V (not adjustable) Voltage Divider Gain: VFB VOUT = 1V VOUT (determined by the application, VFB is the reference voltage for the voltage error amplifier) Current Loop Transconductance: GC = 1.7A 0.865V = 1.97A/V (not adjustable) Load Resistance (RLOAD) (determined by the application) The frequency dependent terms that affect the loop gain include: Output Load Pole(P1): 1 2 π •RLOAD •COUT (application dependent) Error Amplifier Compensation (2 Poles and 1 Zero): These are the design variables available Right Half Plane Zero (RHPZ): boost mode only (deter- mined by maximum load, VIN, VOUT and inductor) Current Amplifier Compensation Components (Fixed Internal to the LTC3114-1) The internal current amplifier and inner current loop have a much higher bandwidth than the overall loop, however, unlike an ideal VCCS with a flat gain versus frequency characteristic, the inner loop exhibits gain peaking in the range of approximately 2kHz to 20kHz that is an artifact of the fixed current amplifier compen- sation. This gain peaking has the effect of pushing out the overall loop crossover frequency, while providing some phase margin boost as well. As long as there is sufficient margin between the loop crossover frequency and the worst-case RHPZ frequency, then stable operation over all conditions is relatively easy to achieve. The design parameters for compensation design will focus on the series resistor and capacitors connected from VC to ground (RZ, CP1 and CP2). The general goal is to provide a phase boost using the compensation net- work zero in order to maximize the bandwidth and phase margin of the converter. Being a buck-boost converter, the target loop crossover frequency for the compensation design will be dictated by the highest boost ratio and load current that is expected as this will result in the lowest RHPZ frequency. An illustrative example is provided next that will derive the compensation components for a typical LTC3114-1 application. Compensation Example This section will demonstrate how to derive and select the compensation components for a typical LTC3114-1 appli- cation. Designing compensation for other applications gm 1V 1V gm = 1.7A/0.865V + – FB VOLTAGE ERROR AMP VOLTAGE CONTROLLED CURRENT SOURCE VC GND RZ RTOP 1.7M RBOT 100k RCOSER 0.01 RLOAD 18 CP1 COUT 22µF VOUT CP2 31141 F06 Figure 6. Simplified Representation of Average Current Mode Control Loop |
Podobny numer części - LTC3114-1 |
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Podobny opis - LTC3114-1 |
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