8.1.2 Voltage Mode Control Using th

8.1.2 Voltage Mode Control Using the LM25037An external resistor (RFF) and capacitor (CFF) connected to VIN, AGND, and the RAMP pins is required to create a saw-tooth modulation ramp signal shown in Figure 18. The slope of the signal at RAMP will vary in proportion to the input line voltage. The varying slope provides line feed-forward information necessary to improve line transient response with voltage mode control. With a constant error signal, the on-time (tON) varies inversely with the input voltage (VIN) to stabilize the volt-second product of the transformer primary. Using a lie feed-forward ramp for PWM control requires very little change in the voltage regulation loop to compensate for changes in input voltage, as compared to a fixed slope oscillator ramp. Furthermore, voltage mode control is less susceptible to noise and does not require leading edge filtering, and is therefore a good choice for wide input range power converters. Voltage mode control requires a more complicated compensation network, due to the complex-conjugate poles of the L-C output filter.In push-pull and full-bridge topologies, any asymmetry in the volt-second product applied to primary in one phase may not be cancelled by subsequent phase, possibly resulting in a DC current build-up in the transformer, which pushes the transformer core towards saturation. Special care in the transformer design, such as gapping the core, or adding ballasting resistance in the primary is required to rectify this imbalance when using voltage mode control with these topologies. Current mode control naturally corrects for any volt-second asymmetry in the primary.The recommended capacitor value range for CFF is from 100 pF to 1500 pF. Referring to Figure 18, it can be seen that value CFF must be small enough such that the capacitor can be discharged within the clock (CLK) pulse width each cycle. The CLK pulse width is same as the dead-time set by RT2. The minimum possible dead-time for LM25037 is 50 ns and the internal discharge FET RDS(ON) is 5 Ω (typical),The value of RFF required can be calculated using Equation 3.