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ADP1055 데이터시트(PDF) 34 Page - Analog Devices |
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ADP1055 데이터시트(HTML) 34 Page - Analog Devices |
34 / 140 page ADP1055 Data Sheet Rev. A | Page 34 of 140 PWM edges to be adapted on the fly. The ADT feature is activated when the primary or secondary current (CS1 or CS2) falls below the threshold programmed in Register 0xFE1E. The software GUI allows the user to easily program the dead time values, and it is recommended that the GUI be used for this purpose. Figure 56. Adaptive Dead Time Window in the GUI Before ADT is configured, the primary current threshold must be programmed. Each individual PWM rising and falling edge (t1 to t12) can then be programmed to have a specific dead time offset at no load (zero current). This offset can be positive or negative and is relative to the nominal edge position. When the current is between zero and the threshold, the amount of dead time is linearly adjusted in steps of 5 ns. The averaging period of the CS1/CS2 current is selected using Register 0xFE1E[7], and the speed of the dead time adjustment can also be programmed to accommodate faster or slower adjustment in Register 0xFE1D[5:0]. For example, if the CS1 threshold is set to 2 A, t1 has a nominal rising edge of 100 ns. If the ADT setting for t1 is 40 ns at no load, t1 moves to 140 ns when the current is 0 A and to 120 ns when the current is 1 A. Similarly, ADT can be applied in the negative direction. The ADT feature is useful in quasi resonant topologies where an energy transfer occurs from the inductor (generally, from one or more of the leakage inductance, magnetizing inductance, and external inductance) to the capacitor (usually the drain-source capacitance of the MOSFET power switch) for the purpose of achieving zero voltage switching (ZVS). Generally, the condition for ensuring ZVS is that the energy in the inductor must exceed the energy in the capacitor. A resonant transition occurs when energy is dumped from the inductor to the capacitor (capacitor being charged with opposite polarity voltage). At one point, there is close to 0 V across the MOSFET, and at this point the power switch is turned on. If this energy is not sufficient, the MOSFET turns on without ZVS. In this case, ADT can be used to wait until the resonant transition reaches its peak value so that a near ZVS turn-on is achieved. SR DELAY The ADP1055 is well suited for dc-to-dc converters in isolated topologies. Each time a PWM signal crosses the isolation barrier, an additional propagation delay is added due to the isolating com- ponents. The ADP1055 allows programming of an adjustable delay (0 ns to 315 ns in steps of 5 ns) using Register 0xFE52[5:0]. This delay moves both SR1 and SR2 later in time with respect to OUTA to OUTD to compensate for the added delay due to the isolating components. In this way, the edges of all PWM outputs can be aligned, and the SR delay can be applied separately as a constant dead time. CURRENT SHARING (ISHARE PIN) The ADP1055 supports both analog current sharing and digital current sharing. The ADP1055 can use either the CS1 current information or the CS2 current information for current sharing. Analog Current Sharing Analog current sharing uses the internal current sensing circuitry to provide a current reading to an external current error amplifier. Therefore, an additional differential current amplifier is not necessary. The current reading from CS1 or CS2 can be output to the ISHARE pin in the form of a digital bit stream, which is the output of the current sense ADC (see Figure 57). The bit stream is proportional to the current delivered by this unit to the load. By filtering this digital bit stream using an external RC filter, the current infor- mation is turned into an analog voltage that is proportional to the current delivered by this unit to the load. This voltage can be compared to the share bus voltage. If the unit is not supplying enough current, an error signal can be applied to the VS± feed- back point. This signal causes the unit to increase its output voltage and, in turn, its current contribution to the load. Figure 57. Analog Current Share Configuration CURRENT SENSE ADC SHARE BUS LPF BIT STREAM BIT STREAM ISHARE VOLTAGE CURRENT CS2– CS2+ |
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