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MIC2130-4YTSE Datasheet(PDF) 10 Page - Micrel Semiconductor |
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MIC2130-4YTSE Datasheet(HTML) 10 Page - Micrel Semiconductor |
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10 / 20 page  Micrel, Inc. MIC2130/1 April 2008 10 M9999-042108-C At startup, the Soft-start MOSFET (SSFET) is released and CSS starts to charge at the rate dVSS/dt–2µA/CSS. The PNP transistor’s emitter (COMP) starts to track VSS (plus a junction voltage ≈0.65). When (COMP) reaches the lower end of the PWM ramp voltage at 1.10V, switching pulses will begin to drive the power MOSFETs. This voltage rise continues on the COMP pin until the control loop reaches the regulation point. During this soft start period, the gate drive pulses to the MOSFET will start at the minimum pulse width and increase up to the duty cycle D required for regulation. The COMP voltage can be anywhere from 1.1V to 2.1V which corresponds to a duty cycle D of 0-85%. VSS will however, continue to rise as the PNP base-emitter junction becomes reverse biased. The SS pin is allowed to rise to 2.5V (four diode drops) max to allow fast response to fault conditions. During large over current or short circuit conditions, i.e., where current limit is detected and VOUT is <60% of nominal, the SSFET is momentarily switched on. This discharges CSS to ~150mV at which point, it re-starts the soft start cycle once again. During soft start, hysteretic comparators are disabled. Duty Cycle D can be written in terms of VCOMP D = (0.85) x VCOMP – (0.935) or VCOMP = (D + 0.935)/0.85 Example: VIN = 12V; VOUT = 3.3V; D = VOUT/VIN = 0.275 VCOMP = 1.424V; i.e. the steady-state DC Value of VCOMP when D = 0.275 T1 is the time for VCOMP to charge up to 1.1V, therefore, VSS is one diode drop below VCOMP. T2 is the time for VCOMP to charge up to (D + 0.935)/ 0.85 + 1.1V. Soft Start time = T1 + T2 Where T1 = (1.1-Vdiode) x CSS/2µA; the time until output pulsing starts at minimum duty. And, T2 = (1/0.85) x D x CSS/ 2µA; the time until output pulsing increases to D. The compensation capacitors at the COMP pin (CCOMP = Cc1+Cc2 in Figure 4) will also need to charge up to VCOMP. This charging time starts as soon as MOSFET (SSFET) is released. Depending upon the size of the CCOMP, the charging time could be greater than T1+T2. CCOMP could be used for the Soft Start cap by leaving SS pin open. TcCOMP = (1/0.85) x D x CCOMP/5µA: The time until output pulsing increases to D. Protection There exits four different types of output protection. 1. Output “hard short” over current 2. Output “soft short” over current 3. Output under voltage 4. Output over voltage Current Limit The MIC2130/31 uses the RDSON of the low-side MOSFET to sense overcurrent conditions. The lower MOSFET is used because it displays much lower parasitic oscillations during switching then the upper MOSFET. Using the low-side MOSFET RDSON as a current sense is an excellent method for circuit protection. This method will avoid adding cost, board space and power losses taken by discrete current sense resistors. Hard Short Generally, the MIC2130/31 current limit circuit acts to provide a fixed maximum output current until the resistance of the load is so low that the voltage across it is no longer within regulation limits. At this point (60% of nominal output voltage), the part employs Hiccup mode. During Hiccup mode, the output pulses stop and the soft start cap is discharged and soft start mode begins. After the soft start time, if the output voltage is still 60% low, then the process repeats again and continues until the short is removed. Hard short current mode is initiated to protect down stream loads from excessive current and also reduces overall power dissipation in the PWM converter components during a fault. Soft Short Before “Hard Short” mode (also called “hiccup mode”) occurs “soft short” current limiting is provided to prevent system shutdown or disturbance if the overload is only marginal. When the load current exceeds the current limit by only a few ma for a short time (milliseconds) then the hard short mode is not desired. Instead, the “Soft Short” loop is used. When the current limit comparator senses an over current it then starts to discharge the SS Cap with a 40µA current source. The current limit comparator gets reset every cycle so if the short still exist during the next cycle then the SS cap will continue to get discharged with the 40µA current source. The comp pin follows the SS pin (Figure 4) and the gm control loop will lower the output voltage accordingly for as long as the short exists. So, instead of shutting down the output as in a hard short, the output is gently and slightly reduced until the over current condition discontinues. If however the short increase to the point of lowering the output to 60%, then hard short will result. The fast hysteretic control loop (FHyCL) is initiated by a 6% drop in output voltage and it is not desired during an over current condition therefore, the FHyCL feature will |
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