Electronic Components Datasheet Search |
|
UCC2858 Datasheet(PDF) 10 Page - Texas Instruments |
|
UCC2858 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 12 page 10 UCC1858 UCC2858 UCC3858 The discrete nature of the RMS voltage feedforward means that there are regions of operation where the in- put voltage changes, but the VRMS value fed into the mul- tiplier does not change. The voltage error amplifier compensates for this by changing its output to maintain the required multiplier output current. When the output of the ADC changes, there is a jump in the output of the er- ror amplifier. There is a resultant shift in the foldback fre- quency if the converter is at light load. However, the impact of this change is minimal on the overall converter operation. Another key consideration with the RMS voltage scheme is that it relies on the zero-crossing of the IAC signal to be effective. At very light loads and high line conditions, the rectified AC does not quite reach zero if a large capacitor is being used for filtering on the rectified side of the bridge. In such instances, the feedforward effect does not take place and the controller functionality is lost. For UCC3858, the IAC current should go below 10µA for the zero crossing detection to take place. It is recommended that the capacitor value be kept low enough for the light load operation or the feedforward be derived directly from the AC side of the input bridge as shown in the typical applications circuit. Gate Drive Considerations The gate drive circuit in UCC3858 is designed for high speed power switch drive. It consists of low impedance pull-up and pull-down DMOS output stages. When oper- ating with high bias voltages, in order to stay within the SOA of the DMOS output stages, it is recommended that the gate drive current be limited to 0.5A peak with the use of external gate resistor. Please see the characteris- tic curve in Fig. 8 for determining the required external re- sistance. Current Amplifier Set-up The multiplier is set-up first by choosing the VRMS range. The maximum multiplier output is at low line, full load conditions. The inductor peak current also occurs at the same point. The multiplier terminating resistor can be de- termined using equation 5. R IL R I MULT PK SENSE MULT PK = • (5) The peak current limiting function provided by the UCC3858 is integrated into MOUT. The signal on MOUT is normally maintained at 0V as the (IMULT •RMULT) can- cels the voltage drop across the sense resistor with closed loop operation. During short circuit or transient startup conditions, the multiplier current can not fully can- cel the voltage drop across RSENSE and the voltage at MOUT drops below 0V. The internal peak current limit is activated when MOUT drops below –0.5V. The peak cur- rent limit at any operating point is given by: I IR R LIM MULT MULT SENSE = •+ 05 . (6) The current amplifier can be compensated using previ- ously presented techniques, (Application Note U- 134), summarized here. A simplified high frequency model for inductor current to duty cycle transfer function is given by: Gs i d V sL id LO () = ∧ ∧ = (7) The gain of the current feedback path at the frequency of interest (crossover) is given by: d i R R RV L SENSE Z ISE ∧ ∧ =• • 1 (8) Where VSE is the ramp amplitude (p-p) which is 3.5V for UCC3858. Combining equations 7 and 8 yields the loop gain of the current loop and equating it to 1 at the de- sired crossover frequency can result in a design value for RZ. The current loop crossover frequency selected using conventional trade offs. However, it should be ensured that the current-loop is stable at the minimum switching frequency under foldback conditions. APPLICATION INFORMATION (cont.) 0 10 20 30 40 10 12 14 16 18 20 VDD(V) Figure 8. Reguired series gate resistance as a function of supply voltage. |
Similar Part No. - UCC2858 |
|
Similar Description - UCC2858 |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |