7.3.2 FB Pin (Feedback Pin)

The FB pin connects to the collector of an optocoupler output transistor through an external current-limiting resistor (R_FB). Depending on the operating mode, the controller uses different content of the collector current flowing out of the FB pin (I_FB) to regulate the output voltage. For the operating modes based on peak current control, I_FB is converted into an internal peak current threshold (V_CST) to modulate the amplitude of the current sense signal on the CS pin. For example, when the output voltage (V_O) is lower than the regulation level set by the shunt regulator, the “current level” of I_FB moves to lower value, so V_CST goes up to deliver more power to the output load.

As the burst control takes over the V_O regulation, where V_CST is clamped to V_CST(BUR), the “current ripple” of I_FB is used to modulate burst off time, as shown in Figure 13. Specifically, after a group of pulses stop bursting, the output load current starts to discharge the output capacitor, which makes V_O start to decay. A proper type-III compensation on the secondary side of V_O feedback loop minimizes the phase-delay between I_FB current ripple and output voltage ripple. For a detailed design guide on each passive component of the type-III compensator, please refer to Application and Implementation. When the decaying I_FB intersects with an internal reference current (I_TH(FB)), the ripple regulator generates a new set of grouped burst pulses to deliver more power, which makes V_O and I_FB ripples move upward.

The nature of ripple-based control in burst mode requires additional care on the noise level of I_FB to improve the consistency of burst off-time between burst cycles. Firstly, a high-quality ceramic-bypass capacitor between FB pin and REF pin (C_FB) is required for decoupling I_FB noise. A minimum of 100 pF is recommended. There is an internal 8-kΩ resistor (R_FBI) connected to the FB pin that in conjunction with an external C_FB forms an effective low-pass filter. On the other hand, too strong low-pass filtering with too large C_FB can attenuate the I_FB ripple creating slope distortion of the intersection point between I_FB and I_TH(FB), which can cause inconsistent burst off-times, even though V_O stays in regulation and the I_FB noise is low. Secondly, since ABM utilizes the falling-edge burst-ripple content of I_FB to determine the beginning of every burst packet, the operation is affected if the burst-ripple content of the output voltage is too small due to using a low-ESR output capacitor, or if there is an additional low-frequency ringing on the output ripple due to using a second-order output filter.

Compared with an electrolytic-type of output capacitor, the advantage of ACF using a low-ESR output capacitor such as a polymer capacitor is to minimize the switching-ripple content of the output voltage to meet the ripple specification, but the burst-ripple content is also reduced. Therefore, the switching ripple and noise on I_FB may be very close to I_TH(FB), which triggers the next burst event prematurely. For a converter using a second-order output filter, a π filter design as example, even though both switching-ripple and burst-ripple contents are further attenuated, additional low-frequency ringing caused by the resonance between the output filter inductor and one of the output filter capacitors is generated, which may trigger the next burst event too early as well. Therefore, applying an active ripple compensation (ARC) technique is recommended to generate a noise-free burst ripple artificially to stabilize the ABM operation of ACF using either a low-ESR output capacitor or a second-order output filter.

Figure 14 illustrates the implementation of ARC formed by a high-impedance resistor (R_COMP) in series with a small-signal enhancement MOSFET (Q_COMP) where its gate is controlled by the RUN pin of UCC28780. When RUN pin is in a high state which turns on Q_COMP, R_COMP connected to FB pin creates a compensation current (I_COMP), with a magnitude around V_FB / R_COMP. When RUN pin changes to a low state which turns off Q_COMP, R_COMP and the drain-source junction capacitor of Q_COMP creates a slow falling edge of I_COMP, with a ramp slope dependent on the RC time constant. Then, the summation of the current from the optocoupler (I_OPTO) and I_COMP becomes the total feedback current out of FB pin (I_FB) to compare with I_TH(FB). As the ARC operation in Figure 15 explains, the magnitude of I_COMP helps to push any switching and noise content of I_FB away from I_TH(FB), and the slow falling edge of I_COMP further pushes the undesirable ripple content away from I_TH(FB), especially the low-frequency ringing of the π output filter. The magnitude of I_COMP can be adjusted by R_COMP, and 1 MΩ to 2 MΩ is the recommended value which injects around 2 μA to 4 μA of compensation ripple current into the loop.