Datasheet

( )
d SP
SW
T
1
1
T
150ns
2 f
2 150kHz
R 94.9k R4 100k
33.2pF 33.2pF
-
-
´
´
= = = W Þ = W
IN
pre bias
RAMP SR _ RAMP
pre bias
V
36 V
V 3.0 V
2 n 2 4
V V 3.0 V 0.750 V
2 V 2 3.0 V
-
-
- -
´ ´
= ´ = ´ =
´ ´
O
IN
n V
1 4 3.3 V 1
D 0.176
V
2 75 V / 2 2
2
´
´
= ´ = ´ =
SR _ D 1 D 1 0.176 0.82= - = - =
COMP
V (SR _ D 0.5) 3.0 V 2 (0.824 0.5) 3.0 V 2 1.944 V= - ´ ´ = - ´ ´ =
COMP
RAMP
V
1.944 V
V 5.523 V
(D 2) (0.176 2)
= = =
´ ´
RAMP _1
CHARGE
RAMP sw
CHARGE RAMP
1 1
R 336.9k
36 V
V
2 ln( ) 470pF 150kHz
2 ln C f
36 V 0.750 V
V V
= = = W
æ ö
´ ´ ´
´ ´ ´
ç ÷
-
-
è ø
RAMP _ 2
CHARGE
RAMP sw
CHARGE RAMP
1 1
R 92.7k
75 V
V
2 ln( ) 470pF 150kHz
2 ln C f
75 V 5.523 V
V V
= = = W
æ ö
´ ´ ´
´ ´ ´
ç ÷
-
-
è ø
UCC28250
www.ti.com SLUSA29C –APRIL 2010– REVISED JULY 2011
Step 3, Programming the Device
Step 3-1
Equation 3 is used to determine RT based on switching frequency, 300 kHz and assumes the dead time of 150
ns.
(29)
Step 3-2, Determine RAMP Resistance and Capacitance
There are two-fold considerations to determine RAMP resistance and capacitance. Equation 23 provides RAMP
consideration for SR initial start up with prebias. The corresponding RAMP peak voltage is determined with input
voltage low line and maximum prebias output voltage. In the below T1 turns ratio n = 4.
(30)
Equation 24 and Equation 25 provides RAMP consideration for soft start completion to make duty cycle match
(1-D) = SR_D.
1. Calculate OUTA or OUTB duty cycle at 75-V input voltage, 3.3-V output.
(31)
2. Calculate SRA or SRB duty cycle.
(32)
3. Calculate the COMP voltage value in steady state (Equation 24).
(33)
4. Calculate the RAMP peak value (Equation 25).
(34)
5. Arbitrary select C
RAMP
470 pF, then C3 = 470 pF.
6. Calculate R
RAMP
.
(35)
(36)
As different RAMP resistor values are obtained, at this stage, we may take their average value for initial design.
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