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CS5212EDR14 数据表(PDF) 11 Page - ON Semiconductor |
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CS5212EDR14 数据表(HTML) 11 Page - ON Semiconductor |
11 / 13 page CS5212 http://onsemi.com 11 VFFB Feedback Selection To take full advantage of the V2 control scheme, a small amount of output ripple must be fed back to the VFFB pin, typically 50 mV. For most application, this requirement is simple to achieve and the VFFB can be connected directly to the VFB pin. There are some application that have to meet stringent load transient requirements. One of the key factor in achieving tight dynamic voltage regulation is low ESR. Low ESR at the regulator output results in low output voltage ripple. This situation could result in increase noise sensitivity and a potential for loop instability. In applications where the output ripple is not sufficient, the performance of the CS5212 can be improved by adding a fixed amount external ramp compensation to the VFFB pin. Refer to Figure 7, the amount of ramp at the VFFB pin depends on the switch node Voltage, Feedback Voltage, R1 and C2. Vramp + (Vsw * VFB) ton (R1 C2) where: Vramp = amount of ramp needed; Vsw = switch note voltage; VFB = voltage feedback, 1 V; ton = switch on−time. To minimize the lost in efficiency R1 resistance should be large, typically 100 k or larger. With R1 chosen, C2 can be determined by the following; C2 + (Vsw * VFB) ton (R1 Vramp) C1 is used as a bypass capacitor and its value should be equal to or greater than C2. Figure 7. Small RC Filter Providing the Proper Voltage Ramp at the Beginning of Each On−Time Cycle Vsw R2 1.0 k VFFB VFB R1 C1 C2 Maximum Frequency Operation The minimum pulse width may limit the maximum operating frequency. The duty factor, given by the output/input voltage ratio, multiplied by the period determines the pulse width during normal operation. This pulse width must be greater than 200 ns, or duty cycle jitter could become excessive. For low pulse widths below 300 ns, external slope compensation should be added to the VFFB pin to increase the PWM ramp signal and improve stability. 50 mV of added ramp at the VFFB pin is typically enough. Current Sense Component Selection The current limit threshold is set by sensing a 60 mV voltage differential between the IS+ and IS− pins. Referring to Figure 8, the time constant of the R2,C1 filter should be set larger than the L/R1 time constant under worst case tolerances, to prevent overshoot in the sensed voltage and tripping the current limit too low. Resistor R3 of value equal to R2 is added for bias current cancellation. R2 and R3 should not be made too large, to reduce errors from bias current offsets. For typical L/R time constants, a 0.1 mF capacitor for C1 will allow R2 to be between 1.0 k and 10 k W. The current limit without R4 and R5, which are optional, is given by 60 mV/R1, where R1 is the internal resistance of the inductor, obtained from the manufacturer. The addition of R5 can be used to decrease the current limit to a value given by: ILIM + (60 mV * (VOUT R3 (R3 ) R5)) R1 where VOUT is the output voltage. Similiarly, omitting R5 and adding R4 will increase the current limit to a value given by: ILIM + 60 mV R1 (1 ) R2 R4) Essentially, R4 or R5 are used to increase or decrease the inductor voltage drop which corresponds to 60 mV at the IS+ and IS− pins. Figure 8. Current Limit R5 R3 IS− IS+ R2 60 mV Trip R4 C1 R1 L1 L VOUT Switching Node Boost Component Selection for Upper and Lower FET Gate Drive The boost (BST) pin provides for application of a higher voltage to drive the upper FET. This voltage may be provided by a fixed higher voltage or it may be generated with a boost capacitor and charging diodes, as shown in Figure 1. The voltage in the boost configuration would be the summation of the voltage from the charging diodes and the output voltage swing. Care must be taken to keep the peak voltage with respect to ground less than 20 V peak. The capacitor value should be ten times larger than the capacitance of the top FET. The boost circuit requires a modification to achieve startup. See Rpullup Selection for boost circuit startup. |
类似零件编号 - CS5212EDR14 |
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类似说明 - CS5212EDR14 |
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