Electronic Components Datasheet Search
  English  ▼

Delete All
ON OFF
ALLDATASHEET.COM

X  

Preview PDF Download HTML

UCC28251 Datasheet(PDF) 32 Page - Texas Instruments

Part No. UCC28251
Description  UCC28251 Advanced PWM Controller With Prebias Operation
Download  55 Pages
Scroll/Zoom Zoom In 100% Zoom Out
Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
Logo 

UCC28251 Datasheet(HTML) 32 Page - Texas Instruments

Zoom Inzoom in Zoom Outzoom out
 32 / 55 page
background image
(
)
COMP( final)
MAX
V
SR _ D
0.5
3 V 2
=
-
´
´
IN
PRE BIAS
RAMP
SR(ramp)
PRE BIAS
V
V
2 n
V
V
2 V
-
-
-
´
=
´
´
BLANK
C
CS
R
CS
RAMP/CS
GND
UCC28251
V
IN 36 V to 75 V
BLANK
C
CS
R
CS
RAMP/CS
GND
UCC28251
V
CHARGE
UCC28251
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
www.ti.com
Application Information (continued)
To realize a fixed amplitude ramp, connect RCS to the constant voltage source, so that the ramp capacitor
charging voltage is fixed regardless of line and load condition. The RAMP/CS pin is clamped internally to 4 V for
internal device protection. Because the internal pull-down switch has about 40-
Ω on-resistance, the CCS value
must be small enough to discharge RAMP/CS from the peak to ground within TD(sp) + 70 ns (i.e. the pulse width
of BLANK signal).
To achieve the input voltage feedforward, the slope of the ramp needs to be proportional to the input voltage. Tie
RCS to the input line voltage. Because the ramp voltage is much lower than the input voltage, the ramp capacitor
charging current is considered to be proportional to the input voltage. With input voltage feedforward, the COMP
pin voltage should only move slightly even with large input voltage variation. This will provide much better line
transient response for the converter.
Figure 41. External Configuration Of RAMP/CS Pin With/Without Feed-Forward Operation
The input voltage feedforward also helps on pre-biased start up. When doing primary-side control to pre-biased
start up, three conditions need to be considered:
9.1.3.1 Condition 1
At initial start up the primary side needs to provide enough energy to prevent output voltage dip;
9.1.3.2 Condition 2
At the end of soft start, it is required to keep the SR duty cycle change to be as small as possible. With input
voltage feedforward, the COMP pin voltage is virtually fixed for different input voltages. Therefore, before the end
of soft start, the duty cycle is the same for different input voltages. Choose the RCS and CCS following the
procedure below.
Considering initial start up, the RAMP peak voltage should be:
(22)
In this equation, VIN is the input voltage because of the feedforward any input voltage should be fine; VPRE-BIAS is
the highest pre-bias start-up voltage required by the system; n is the tranformer primary to secondary turns ratio
and VSR(ramp) is the internal SR ramp peak voltage 3 V.
Another consideration is at the end of soft start, the SR duty cycle changes from controlled by the soft start, to
complimentary to the primary-side duty cycle. The design should keep the transition as smooth as possible.
Considering this, based on the output voltage and input voltage range, as well as the transformer turns ratio,
calculate the SR duty cycle at different line voltages.
Next, based on the maximum duty cycle on the SR_DMAX, and the internal fixed ramp amplitude 3 V, the COMP
voltage at regulation can be chosen as:
(23)
32
Submit Documentation Feedback
Copyright © 2013–2014, Texas Instruments Incorporated
Product Folder Links: UCC28251


Html Pages

1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55 


Datasheet Download




Link URL




Privacy Policy
ALLDATASHEET.COM
Does ALLDATASHEET help your business so far?  [ DONATE ]  

About Alldatasheet   |   Advertisement   |   Datasheet Upload   |   Contact us   |   Privacy Policy   |   Alldatasheet API   |   Link Exchange   |   Manufacturer List
All Rights Reserved© Alldatasheet.com


Mirror Sites
English : Alldatasheet.com  |   English : Alldatasheet.net  |   Chinese : Alldatasheetcn.com  |   German : Alldatasheetde.com  |   Japanese : Alldatasheet.jp
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