The FB pin usually connects to the collector of an optocoupler output transistor through an external current-limiting resistor (R_FB). A maximum of 20 kΩ for R_FB is recommended. The feedback network of UCC28782 is shown in Figure 8-3. A high-quality ceramic by-pass capacitor between FB pin and REF pin (C_FB) is required for decoupling I_FB from switching noise interference. A minimum of 220 pF is recommended for C_FB . An internal 8-kΩ resistor (R_FBI) at the FB pin in conjunction with the external C_FB forms an effective low-pass filter. Section 9 provides a detailed design guide on the secondary-side compensation network of V_O feedback loop, to improve the load transient response and also limit the I_FB ripple of ABM mode within the recommended range.

Figure 8-3 External Feedback Network Connected to the FB Pin

Depending on the operating mode, the controller interprets the current flowing out of the FB pin (I_FB) to regulate the output voltage. For AAM and LPM modes based on peak current control, I_FB is converted into an internal peak current-sense 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 absolute current level of I_FB reduces, causing a higher V_CST to increase more power to the output load. In ABM, the burst control loop takes over the V_O regulation, where V_CST is clamped to V_CST(BUR) and the ripple component of I_FB participates in the modulation of the burst off time.

Figure 8-4 illustrates the operating principle of the ABM. A burst of switching pulses raises the output voltage V_O which increases I_FB. At the end of the burst, the load current discharges the output capacitor, which decreases V_O and I_FB. UCC28782 injects a noise-free internal ramp compensation current (I_COMP) superimposed on I_FB in order to stabilize the ABM operation. When the RUN pin is high, I_COMP is reset to 0 μA. When the RUN pin goes low, I_COMP is gradually increased to 6 μA with a positive slope of 0.214 A/s. The summation of I_FB and I_COMP is compared with I_TH(FB) to trigger the next burst event. The magnitude and sharp slope of I_COMP help to push switching ripple and high-frequency noise component of I_OPTO away from I_TH(FB).

Figure 8-4 Concept of Burst Control with an Internal Ramp Compensation