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CS5211 Fiches technique(PDF) 7 Page - ON Semiconductor |
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CS5211 Fiches technique(HTML) 7 Page - ON Semiconductor |
7 / 16 page CS5211 http://onsemi.com 7 COMP voltage. Since the COMP voltage increases gradually, the Soft–Start can be achieved. The start–up period ends when the output voltage reaches the level set by the external resistor divider. Output Enable Since there can be no switching until the COMP pin exceeds the 0.5 V offset built into the PWM comparator, the COMP pin can also be used for an enable function. Hold the COMP pin below 0.4 V with an open collector circuit to disable the output. When the COMP pin is released to enable start–up, the user must ensure there is no leakage current from the enable circuit into COMP. During normal operation the COMP output is driven with only 5.0 µA to 30 µA internally. Hiccup Mode Overcurrent Protection Under normal load conditions, the voltage across the IS+ and IS– pins is less than the 60 mV overcurrent threshold. If the threshold is exceeded, the overcurrent fault latch is set, the high side gate driver is forced low, and the COMP pin is discharged with 5.0 µA. There is no switching until the COMP voltage drops below a 0.25 V threshold. Then, the fault latch is cleared and a soft–start is initiated. The low effective duty cycle during hiccup overcurrent greatly reduces component stress for an extended fault. Inductor Current Sensing Besides using a current sense resistor to sense inductor current, CS5211 provides the users with the possibility of using loss–less inductor sensing technique. This sensing technique utilizes the Equivalent Series Resistance (ESR) of the inductor to sense the current. The output current is sensed through an RC network in parallel with the inductor as shown in Figure 4. The voltage across the small capacitor is then fed to the OC comparator. R RL C Q1 IS– IS+ CO VIN Figure 4. Inductor Current Sensing Q2 L If the values of R and C are chosen such that: L RL + RC Then the voltage across the capacitor C will be: VC + RLIL Therefore, if the time constant of the RC network is equal to that of the inductor, the voltage across the capacitor is proportional to the inductor current by a factor of the inductor ESR. In practice, the user should ensure that under all component tolerances, the RC time constant is larger than the L/R time constant. This will keep the high frequency gain for VC(s)/IL(s) less than the low frequency gain, and avoid unnecessary OCP tripping during short duration overcurrent situations. Compared with conventional resistor sensing, the inductor ESR current sensing technique is lossless, but is not as accurate due to variation in the ESR from inductor to inductor and over temperature. For typical inductor ESR, the 0.39%/ °C positive temperature coefficient will reduce the current limit at high temperature, and will help prevent thermal runaway, but will force an increased design target at room temperature. This technique can be more accurate than using a PCB trace, since PCB copper thickness can vary 10–20%, compared to 1% variation in wire diameter thickness typical of inductors. Remote Voltage Sensing The CS5211 has the capability to sense the voltage when the load is located far away from the regulator. The SGND pin is dedicated to the differential remote sensing. The negative remote sense line is connected to SGND pin directly, while the positive remote sense line is usually connected to the top of the feedback voltage divider. To prevent over–voltage condition caused by open remote sense lines, the divider should also be locally connected to the output of the regulator through a low value resistor. That resistor is used to compensate for the voltage drop across the output power cables. |
Numéro de pièce similaire - CS5211 |
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Description similaire - CS5211 |
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