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LM25115SDX Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM25115SDX Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 17 page  Leading Edge Pulse Width Modulation Unlike conventional voltage mode controllers, the LM25115 implements leading edge pulse width modulation. A current source equal to 3 times the I SYNC current is used to charge the capacitor connected to the RAMP pin as shown in Figure 4. The ramp signal and the output of the error amplifier (COMP) are combined through a resistor network to produce a voltage ramp with variable dc offset (CRMIX in Figure 4). The high side MOSFET which drives the HS pin is held in the off state at the beginning of the phase signal. When the voltage of CRMIX exceeds the internal threshold voltage CV, the PWM comparator turns on the high side MOSFET. The HS pin rises and the MOSFET delivers current from the main converter phase signal to the output of the auxiliary regula- tor. The PWM cycle ends when the phase signal falls and power is no longer supplied to the drain of the high side MOSFET. Leading edge modulation of the auxiliary PWM controller is required if the main converter is implemented with peak current mode control. If trailing edge modulation were used, the additional load on the transformer secondary from the auxiliary channel would be drawn only during the first portion of the phase signal pulse. Referring to Figure 5, the turn off the high side MOSFET of the auxiliary regulator would cre- ate a non-monotonic negative step in the transformer cur- rent. This negative current step would produce instability in a peak current mode controller. With leading edge modulation, the additional load presented by the auxiliary regulator on the transformer secondary will be present during the latter portion of the phase signal. This positive step in the phase signal current can be accommodated by a peak current mode controller without instability. 20172614 FIGURE 4. Synchronization and Leading Edge Modulation 20172620 FIGURE 5. Leading versus Trailing Edge Modulation www.national.com 11 |
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