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LTC4400-2EMS8 Datasheet(PDF) 10 Page - Linear Technology |
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LTC4400-2EMS8 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LTC4400-1/LTC4400-2 sn4400 4400fas Determining External Loop Gain and Bandwidth The external loop voltage gain contributed by the RF chan- nel and RF feedback coupling network should be mea- sured in a closed-loop configuration. A voltage step is applied to PCTL and the change in VPCA/B is measured. The detected voltage is K • PCTL, where K is the internal gain between PCTL and the RF pin, and the external voltage gain contributed by the RF power amplifier and RF feedback coupling network is K • ∆VPCTL/∆VVPC. Measuring voltage gain in the closed-loop configuration accounts for the nonlinear detector gain that is dependent on RF input voltage and frequency. The LTC4400-X unity gain bandwidth specified in the data sheet assumes that the net voltage gain contributed by the RF power amplifier and RF feedback coupler is unity. The bandwidth is calculated by measuring the rise time be- tween 10% and 90% of the voltage change at VPCA/B for a small step in voltage applied to PCTL. BW1 = 0.35/rise time The LTC4400-X control amplifier unity gain bandwidth (BW1) is typically 450kHz below a PCTL voltage of 80mV. For PCTL voltages < 80mV, the RF detected voltage is 0.6PCTL. For PCTL voltages >160mV, RF detected voltage is 1.22PCTL – 0.1. This change in gain is due to an internal compression circuit designed to extend the detector range. For example, to determine the external RF channel loop voltage gain with the loop closed, apply a 100mV step to PCTL from 300mV to 400mV. VPCA/B will increase to APPLICATIO S I FOR ATIO supply enough feedback voltage to the RF pin to cancel this 100mV step which would be the required detected voltage step of 122mV. VPCA/B changed from 1.5V to 1.561V to create the RF output power change required. The net external voltage gain contributed by the RF power amplifier and RF feedback coupling network can be calcu- lated by dividing the 122mV change at the RF pin by the 61mV change at the VPCA/B pin. The net external voltage gain would then be approximately 2. The loop bandwidth extends to 2 • BW1. If BW1 is 230kHz, the loop bandwidth increases to approximately 460kHz. The phase margin can be determined from Figures 2 and 3. Repeat the above voltage gain measurement over the full power and fre- quency range. External pole frequencies within the loop will further reduce phase margin. The phase margin degradation, due to external and internal pole combinations, is difficult to determine since complex poles are present. Gain peaking may occur, resulting in higher bandwidth and lower phase margin than predicted from the open-loop Bode plot. A low frequency AC SPICE model of the LTC4400-X power controller is included to better determine pole and zero interactions. The user can apply external gains and poles to determine bandwidth and phase margin. DC, transient and RF information cannot be extracted from the present model. The model is suitable for external gain evaluations up to 6 ×. The 270kHz PCTL input filter limits the band- width, therefore, use the RF input as demonstrated in the model. Gain compression is not modeled. Figure 2. Measured Open-Loop Gain and Phase, PCTL < 80mV Figure 3. Measured Open-Loop Gain and Phase, PCTL > 160mV FREQUENCY (Hz) 100 –20 –10 0 20 10 40 30 1k 10k 100k 1M 10M 4400 F02 –30 –40 –50 –60 60 50 80 70 –20 0 20 60 40 100 80 –40 –60 –80 –100 –120 140 120 160 PHASE GAIN RLOAD = 2k CLOAD = 33pF FREQUENCY (Hz) 100 –20 –10 0 20 10 40 30 1k 10k 100k 1M 10M 4400 F03 –30 –40 –50 –60 60 50 80 70 –20 0 20 60 40 100 80 –40 –60 –80 –100 140 120 180 160 PHASE RLOAD = 2k CLOAD = 33pF GAIN |
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