Datasheet L6565 (STMicroelectronics) - 10
| Manufacturer | STMicroelectronics |
| Description | Quasi-Resonant SMPS Controller |
| Pages / Page | 17 / 10 — L6565. Figure 17. a) Overcurrent setpoint vs. VFF voltage; b) Line … |
| File Format / Size | PDF / 256 Kb |
| Document Language | English |
L6565. Figure 17. a) Overcurrent setpoint vs. VFF voltage; b) Line Feedforward function block. 1.5. 0.5. 2.5. 3.5
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Text Version of Document
L6565
It acts on the clamp level of the control voltage Vcsx, that is on the overcurrent setpoint, so that it is a function of the converter's input voltage sensed through a dedicated pin (#3, VFF): the higher the input voltage, the lower the setpoint. This is illustrated in the diagram of figure 17a that shows the relationship between the voltage at the pin VFF and Vcsx (with the error amplifier saturated high in the attempt of keeping output voltage regulation). The schematic in figure 17b shows also how the function is included in the control loop. With a proper selection of the external divider R1-R2 it is possible to achieve the optimum compensation described by the lower curve in the diagram of figure 16. In applications where this function is not wanted, e.g. because of a narrow input voltage range, the VFF pin can be simply grounded, thus saving the resistor divider. The overcurrent setpoint will be then fixed at the maximum value of about 1.4V (1.5V max.). Line Feedforward is also beneficial to other characteristics of quasi-resonant converters: it improves their input ripple rejection ability and limits the variation of the power stage's small-signal gain versus the line voltage.
Figure 17. a) Overcurrent setpoint vs. VFF voltage; b) Line Feedforward function block
Vcsx [V]
1.5
VCOMP = Upper clamp
1 a) 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5
VVFF [V] +Vin R1 R2 Rs COMP VFF CS ZCD 2 3 4 5 starter STOP STARTER S ZCD 7 + INV 1 VOLTAGE Q DRIVER - FEED - PWM R GD E/A FORWARD + (reset-dominant) + 2.5V DISABLE -Hiccup 2 V L6565
b)
10/17
It acts on the clamp level of the control voltage Vcsx, that is on the overcurrent setpoint, so that it is a function of the converter's input voltage sensed through a dedicated pin (#3, VFF): the higher the input voltage, the lower the setpoint. This is illustrated in the diagram of figure 17a that shows the relationship between the voltage at the pin VFF and Vcsx (with the error amplifier saturated high in the attempt of keeping output voltage regulation). The schematic in figure 17b shows also how the function is included in the control loop. With a proper selection of the external divider R1-R2 it is possible to achieve the optimum compensation described by the lower curve in the diagram of figure 16. In applications where this function is not wanted, e.g. because of a narrow input voltage range, the VFF pin can be simply grounded, thus saving the resistor divider. The overcurrent setpoint will be then fixed at the maximum value of about 1.4V (1.5V max.). Line Feedforward is also beneficial to other characteristics of quasi-resonant converters: it improves their input ripple rejection ability and limits the variation of the power stage's small-signal gain versus the line voltage.
Figure 17. a) Overcurrent setpoint vs. VFF voltage; b) Line Feedforward function block
Vcsx [V]
1.5
VCOMP = Upper clamp
1 a) 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5
VVFF [V] +Vin R1 R2 Rs COMP VFF CS ZCD 2 3 4 5 starter STOP STARTER S ZCD 7 + INV 1 VOLTAGE Q DRIVER - FEED - PWM R GD E/A FORWARD + (reset-dominant) + 2.5V DISABLE -Hiccup 2 V L6565
b)
10/17
