Moteur de recherche de fiches techniques de composants électroniques |
|
UC2848 Fiches technique(PDF) 6 Page - Texas Instruments |
|
UC2848 Fiches technique(HTML) 6 Page - Texas Instruments |
6 / 17 page 6 UC1848 UC2848 UC3848 Oscillator A capacitor from the CT pin to GND programs oscillator frequency, as shown in Fig. 2. Frequency is determined by: F = 1 / (10k · CT). The sawtooth wave shape is generated by a charging current of 200 A and a discharge current of 1800 A. The discharge time of the sawtooth is guaranteed dead time for the output driver. If the maximum duty cycle control is defeated by connecting DMAX to VREF, the maximum duty cycle is limited by the oscillator to 90%. If an adjust- ment is required, an additional trim resistor RT from CT to Ground can be used to adjust the oscillator frequency. RT should not be less than 40k . This will allow up to a 22% decrease in frequency. Inductor Current Waveform Synthesizer Average current mode control is a very useful technique to control the value of any current within a switching con- verter. Input current, output inductor current, switch cur- rent, diode current or almost any other current can be controlled. In order to implement average current mode control, the value of the current must be explicitly known at all times. To control output inductor current (IL) in a buck derived isolated converter, switch current provides inductor current information, but only during the on time of the switch. During the off time, switch current drops abruptly to zero, but the inductor current actually dimin- ishes with a slope dIL/dt = –VO/L. This down slope must be synthesized in some manner on the primary side to provide the entire inductor current waveform for the con- trol circuit. The patented current waveform synthesizer (Fig. 4) con- sists of a unidirectional voltage follower which forces the voltage on capacitor CI to follow the on time switch cur- rent waveform. A programmable discharge current syn- thesizes the off time portion of the waveform. ION is the input to the follower. The discharge current is pro- grammed at IOFF. The follower has a one volt offset, so that zero current corresponds to one volt at CI. The best utilization of the UC3848 is to translate maximum average inductor cur- rent to a 4V signal level. Given N and Ns (the turns ratio of the power and current sense transformers), proper scaling of IL to V(CI) requires a sense resistor Rs as cal- culated from: Rs = 4V · Ns · N / IL(max). Restated, the maximum average inductor current will be limited to: IL(max) = 4V · Ns · N/Rs. IOFF and CI need to be chosen so that the ratio of dV(CI)/dt to dIL/dt is the same during switch off time as on time. Recommended nominal off current is 100 A. This requires CI = (100 A · N · Ns · L) / (Rs · VO(nom)) where L is the output inductor value and VO(nom) is the converter regulated output voltage. There are several methods to program IOFF. If accurate average current control is required during short circuit op- eration, IOFF must track output voltage. The method shown in Fig. 4 derives a voltage proportional to VIN · D (Duty Cycle). (In a buck converter, output voltage is pro- portional to VIN · D.) A resistively loaded diode connec- tion to the bootstrap winding yields a square wave whose amplitude is proportional to VIN and is duty cycle modu- lated by the control circuit. Averaging this waveform with a filter generates a primary side replica of secondary reg- ulated VO. A single pole filter is shown, but in practice a two or three pole filter provides better transient response. Filtered voltage is converted by ROFF to a current to the IOFF pin to control CI down slope. APPLICATION INFORMATION (cont.) UDG-93008-1 |
Numéro de pièce similaire - UC2848 |
|
Description similaire - UC2848 |
|
|
Lien URL |
Politique de confidentialité |
ALLDATASHEET.FR |
ALLDATASHEET vous a-t-il été utile ? [ DONATE ] |
À propos de Alldatasheet | Publicité | Contactez-nous | Politique de confidentialité | Echange de liens | Fabricants 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 |