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MAX8725ETI データシート(PDF) 22 Page - Maxim Integrated Products |
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MAX8725ETI データシート(HTML) 22 Page - Maxim Integrated Products |
22 / 30 page Multichemistry Battery Chargers with Automatic System Power Selector 22 ______________________________________________________________________________________ In discontinuous mode, a new cycle is not started until the LVC voltage rises above 0.15V. Discontinuous- mode operation can occur during conditioning charge of overdischarged battery packs, when the charge cur- rent has been reduced sufficiently by the CCS control loop, or when the charger is in constant voltage mode with a nearly full battery pack. Compensation The charge voltage, charge current, and input current- limit regulation loops are compensated separately and independently at the CCV, CCI, and CCS pins. CCV Loop Compensation The simplified schematic in Figure 5 is sufficient to describe the operation of the MAX1909/MAX8725 when the voltage loop (CCV) is in control. The required com- pensation network is a pole-zero pair formed with CCV and RCV. The pole is necessary to roll off the voltage loop’s response at low frequency. The zero is necessary to compensate the pole formed by the output capacitor and the load. RESR is the equivalent series resistance (ESR) of the charger output capacitor (COUT). RL is the equivalent charger output load, where RL = ∆VBATT / ∆ICHG. The equivalent output impedance of the GMV amplifier, ROGMV, is greater than 10MΩ. The voltage loop transconductance (GMV = ICCV / VBATT) depends on the MODE input, which determines the number of cells. GMV = 0.125mA/mV for 4 cells and GMV = 0.167mA/mV for 3 cells. The DC-DC converter transcon- ductance is dependent upon the charge current-sense resistor RS2: where ACSI = 20, and RS2 = 0.015Ω in the Typical Operating Circuits (Figures 1 and 2), so GMOUT = 3.33A/V. The loop transfer function is: LTF GM RsC R sC R R sC R GsC R OUT OGMV CV CV CV OGMV L OUT L MV OUT ESR =× ×+ × () +× () × +× () +× () 1 1 1 1 GM ARS OUT CSI = × 1 2 CCV COUT RCV RL RESR ROGMV CCV BATT GMV REF GMOUT Figure 5. CCV Loop Diagram NO. NAME CALCULATION DESCRIPTION 1 CCV pole Lowest frequency pole created by CCV and GMV’s finite output resistance. Since ROGMV is very large and not well controlled, the exact value for the pole frequency is also not well controlled (ROGMV > 10MΩ). 2 CCV zero Voltage-loop compensation zero. If this zero is at the same frequency or lower than the output pole fP_OUT, then the loop transfer function approximates a single pole response near the crossover frequency. Choose CCV to place this zero at least one decade below crossover to ensure adequate phase margin. 3 Output pole Output pole formed with the effective load resistance RL and the output capacitance COUT. RL influences the DC gain but does not affect the stability of the system or the crossover frequency. 4 Output zero Output ESR Zero. This zero can keep the loop from crossing unity gain if fZ_OUT is less than the desired crossover frequency; therefore, choose a capacitor with an ESR zero greater than the crossover frequency. Table 1. Poles and Zeros of the Voltage-Loop Transfer Function f RC PCV OGMV CV _ = × 1 2π f RC ZCV CV CV _ = × 1 2π f RC P OUT L OUT _ = × 1 2π f RC Z OUT ESR OUT _ = × 1 2π |
同様の部品番号 - MAX8725ETI |
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同様の説明 - MAX8725ETI |
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