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ADP1621ARMZ-R7 数据表(PDF) 18 Page - Analog Devices |
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ADP1621ARMZ-R7 数据表(HTML) 18 Page - Analog Devices |
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18 / 32 page  ADP1621 Data Sheet Rev. D | Page 18 of 32 For low ESR output capacitors, such as ceramic capacitors, C2 is small, generally in the range of 10 pF to 400 pF. Because of the parasitic inductance, resistance, and capacitance of the PCB layout, the RCOMP, CCOMP, and C2 values might need to be adjusted by observing the load transient response of the ADP1621 to establish a stable operating system and achieve optimal transient performance. For most applications, RCOMP is in the range of 5 kΩ to 100 kΩ, and CCOMP is in the range of 100 pF to 30 nF. COMP CCOMP RCOMP C2 REF gm 2 3 Figure 31. Compensation Components SLOPE COMPENSATION The ADP1621 includes a circuit that allows adjustable slope compensation. Slope compensation is required by current- mode regulators to stabilize the current-control loop when operating in continuous conduction and the switching duty cycle is greater than 50%. Slope compensation is achieved by internally forcing a ramping current source out of the CS current-sense pin. By placing a resistor between the CS pin and the current sensing device (the drain of the external MOSFET in the case of lossless current sensing or the source of the MOSFET if a current-sense resistor is used), a voltage is developed across the resistor that is proportional to the slope-compensation current. To ensure stability of the current-mode control loop, use a compensation voltage slope that is equal to or greater than one- half of the current-sense representation of the inductor current downslope. Therefore, it follows that L V V V R f t f I R IN D OUT CS SW MIN OFF, SW SC,PK S − + × > × − × × × 1 2 (33) where: RS is the slope-compensation resistor. ISC,PK is the peak slope-compensation current. fSW is the switching frequency. RCS is the current-sense resistor. VOUT is the regulated output voltage. VD is the forward-voltage drop of the diode. VIN is the input voltage. tOFF,MIN is the minimum off time. L is the power-stage inductor. In the case of lossless current sensing, RCS is equal to the on resistance, RDSON, of the external power MOSFET. Otherwise, RCS represents the external current- sense resistor. Solving for RS gives the following slope-compensation criterion: ( ) ( ) L f I f t V V V R R SW PK SC SW MIN OFF IN D OUT CS S × × × × − × − + × > , , 2 1 (34) Keep in mind that the above inequality is a function of both ADP1621 parameters and off-chip components, the values of which vary from device to device and with temperature. Select RS to ensure current-loop stability for all possible variations. After accounting for parameter variations, use values of RS that are as close to the calculated limit as possible because excessive slope compensation reduces the benefits of current-mode control and increases the “softness” of the current limit, as discussed in the Current Limit section. Given a typical peak slope-compensation current of 70 µA, RS must not exceed 1.6 kΩ because the voltage at the CS pin is typically clamped at 116 mV. It is also recommended that RS be greater than 20 Ω. If the calculated RS is greater than 1.6 kΩ, the parameters in Equation 34, such as RCS, fSW, and L, can be adjusted such that RS is less than 1.6 kΩ. In conclusion, the value of RS should be 20 Ω ≤ RS ≤ 1.6 kΩ. CURRENT LIMIT The current limit in the ADP1621 limits the peak inductor current and is achieved by the COMP voltage clamp. The peak inductor current, IL,PK, is given by CS SW MIN OFF S PK SC ZCT COMP CLAMP COMP PK L R f t D R I n V V I × − × × − − = , , , , , 1 (35) where: VCOMP,CLAMP is the COMP clamp voltage (typically 2.0 V). VCOMP,ZCT is the COMP zero-current threshold (typically 1.0 V). n is the current-sense amplifier gain (typically 9.5). ISC,PK is the peak slope-compensation current (typically 70 µA). RS is the slope-compensation resistor. D is the duty cycle. fSW is the switching frequency. tOFF,MIN is the minimum off time (typically 190 ns). RCS is the current-sense resistor. In the case of lossless current sensing, RCS is equal to the on resistance, RDSON, of the external power MOSFET. Otherwise, RCS represents the external current- sense resistor. The current limit in the ADP1621 is a soft current limit. When the inductor current reaches the IL,PK limit given in Equation 35, the duty cycle decreases, and the output voltage drops below the desired voltage. The IL,PK limit in Equation 35 then increases in response to the smaller duty cycle, D. The larger the slope- compensation resistor, RS, the larger the effect on IL,PK for an incremental decrease in D. This behavior results in a soft current limit for the ADP1621. Use values of RS that are as close as possible to the calculated limit derived from Equation 34. If high-precision current limiting is required, consider inserting a fuse in series with the inductor. |
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