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LM2636M 데이터시트(PDF) 8 Page - National Semiconductor (TI) |
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LM2636M 데이터시트(HTML) 8 Page - National Semiconductor (TI) |
8 / 14 page Applications Information (Continued) the error amplifier is then compared with an internally generated PWM ramp signal and the result of the comparison is a series of pulses with certain duty ratios. These pulses are used to control the turn-on and turn-off of the MOSFET gate drivers. In this way, the error in the output voltage gets “compensated” or cancelled by the change in the duty ratio of the FET switches. During a large load tran- sient, depending on the compensation design, the change in duty ratio can be as fast as less than one switching cycle. Refer to Design Considerations section for more details. Besides the usual voltage mode feedback control loop, the LM2636 also has a pair of fast comparators (the MIN and MAX comparators) to help maintain the output voltage dur- ing a large and fast load transient. The trip points of the com- parators are set to ±5% of the DAC output voltage. When the load transient is so large that the output voltage goes outside the ±5% window, the MIN or MAX comparator will bypass the primary voltage control loop and immediately set the duty ratio to either maximum value or to zero. This pro- vides the fastest possible way to react to such a large load transient in a classical buck converter. Power Good Signal The power good signal is used to indicate that the output voltage is within specified range. In the LM2636, the range is set to a ±10% window of the DAC output voltage. During soft start, the power good signal is always low. At the end of the soft start session,the output voltage is checked and the PWRGD pin will be asserted if the voltage is within specified range. Over Voltage Protection When the output voltage exceeds 115% of the DAC output voltage after the end of soft start, the LM2636 will enter over voltage protection mode in which it shuts itself down. The up- per gate driver is held low while the lower gate driver is held high. PWRGD will be low. For LM2636 to recover from OVP mode, either OUTEN or V CC voltage has to be toggled. An- other more subtle way to recover is to float all the VID pins and reapply the correct code. Current Limit Current limit is realized by sensing the V DS voltage of the high side MOSFET when it is on. Since the r DS_ON of a MOSFET is a known value, current through the MOSFET can be known by monitoring V DS. The relationship between the three parameters is: To implement the current limit function, an external resistor R IMAX is need. The resistor should be connected between the drain of the high side MOSFET and the IMAX pin. A con- stant current of around 180 µA is forced into the IMAX pin and causes a fixed voltage drop across the R IMAX resistor. This voltage drop is then compared with the V DS of the high side MOSFET and if the latter is higher, over current is reached. So the appropriate value of R IMAX for a pre- determined current limit level I LIM can be calculated by the following equation: For example, if we know that the r DS_ON of the MOSFET is 20 m Ω, and the current limit we want to set is 20A, then we should choose the value of R IMAX to be 2.2 kΩ. To provide the greatest protection over the high side MOS- FET, cycle by cycle protection is implemented. The sampling of the V DS starts as early as about 300 ns after the switch is turned on. Whenever an over current condition is detected, the high side switch is immediately turned off and the low side switch turned on, until the next switching cycle comes. The delay of 300 ns is to circumvent switching noise when the MOSFET is first turned on. DESIGN CONSIDERATIONS Control Loop Compensation A switching regulator should be properly compensated to achieve a stable condition. For a synchronous buck regula- tor that needs to meet stringent load transient requirement such as a Pentium II MPU core voltage supply, a simple 2-pole-1-zero compensation network should suffice, such as the one shown in Figure 4 (C 1,C2,R1 and R2). This is be- cause the ESR zero of the typical output capacitors is low enough to make the control-to-output transfer function a single-pole-roll-off. As an example, let us figure out the values of the compensa- tion network components in Figure 4. Assume the following parameters:R=20 Ω,R L =20mΩ,RC =9mΩ,L =2µH, C = 7.5 mF, V IN =5V, Vm = 2V and switching frequency = 300 kHz. These parameters are based on the typical appli- cation in Figure 1. Notice R L is the sum of the inductor DC re- sistance and the on resistance of the MOSFETs. DS100834-9 FIGURE 4. Buck Converter from a Control Point of View www.national.com 8 |
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