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ADP1871 Datasheet(PDF) 25 Page - Analog Devices |
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ADP1871 Datasheet(HTML) 25 Page - Analog Devices |
25 / 44 page ![]() ADP1870/ADP1871 Rev. 0 | Page 25 of 44 Ceramic capacitors are known to have low ESR. However, the trade-off of using X5R technology is that up to 80% of its capaci- tance might be lost due to derating as the voltage applied across the capacitor is increased (see Figure 80). Although X7R series capacitors can also be used, the available selection is limited to only up to 22 μF. 20 10 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 0 5 10 15 20 25 30 DC VOLTAGE (VDC) X7R (50V) X5R (25V) X5R (16V) 10µF TDK 25V, X7R, 1210 C3225X7R1E106M 22µF MURATA 25V, X7R, 1210 GRM32ER71E226KE15L 47µF MURATA 16V, X5R, 1210 GRM32ER61C476KE15L Figure 80. Capacitance vs. DC Voltage Characteristics for Ceramic Capacitors Electrolytic capacitors satisfy the bulk capacitance requirements for most high current applications. Because the ESR of electrolytic capacitors is much higher than that of ceramic capacitors, when using electrolytic capacitors, several MLCCs should be mounted in parallel to reduce the overall series resistance. COMPENSATION NETWORK Due to their current-mode architecture, the ADP1870/ADP1871 require Type II compensation. To determine the component values needed for compensation (resistance and capacitance values), it is necessary to examine the converter’s overall loop gain (H) at the unity gain frequency (fSW/10) when H = 1 V/V: FILT COMP REF OUT CS M Z Z V V G G H × × × × = = V/V 1 Examining each variable at high frequency enables the unity- gain transfer function to be simplified to provide expressions for the RCOMP and CCOMP component values. Output Filter Impedance (ZFILT) Examining the filter’s transfer function at high frequencies simplifies to OUT FILTER sC Z 1 = at the crossover frequency (s = 2πfCROSS). Error Amplifier Output Impedance (ZCOMP) Assuming that CC2 is significantly smaller than CCOMP, CC2 can be omitted from the output impedance equation of the error amplifier. The transfer function simplifies to CROSS ZERO CROSS COMP COMP f f f R Z ) ( + = and SW CROSS f f × = 12 1 where fZERO, the zero frequency, is set to be 1/4th of the crossover frequency for the ADP1870. Error Amplifier Gain (GM) The error amplifier gain (transconductance) is GM = 500 μA/V Current-Sense Loop Gain (GCS) The current-sense loop gain is ON CS CS R A G × = 1 (A/V) where: ACS (V/V) is programmable for 3 V/V, 6 V/V, 12 V/V, and 24 V/V (see the Programming Resistor (RES) Detect Circuit and Valley Current-Limit Setting sections). RON is the channel impedance of the lower-side MOSFET. Crossover Frequency The crossover frequency is the frequency at which the overall loop (system) gain is 0 dB (H = 1 V/V). For current-mode converters, such as the ADP1870, it is recommended that the user set the crossover frequency between 1/10th and 1/15th of the switching frequency. SW CROSS f f 12 1 = The relationship between CCOMP and fZERO (zero frequency) is as follows: COMP COMP ZERO C R f × × π = 2 1 ) The zero frequency is set to 1/4th of the crossover frequency. Combining all of the above parameters results in REF OUT CS M OUT CROSS ZERO CROSS CROSS COMP V V G G C f f f f R × π × + = 2 ZERO COMP COMP f R C × × π × = 2 1 |
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