Datasheet

SS
SS
COMP( final)
27 A T
27 A 15ms
C 0.101 F C8 0.1 F
V 4.0 V
m ´
m ´
= = = m Þ = m
HICC HICC
2.7 A 2.7 A
C T 0.8s 1.03 F C7 1.0 F
2.4 V 0.3 V 2.4 V 0.3 V
m m
= ´ = ´ = m Þ = m
- -
( )
( )
( )
( )
r f
2
r
0.7 V V V 0.7 V 73 V 72 V
R 880 R2 866
11 A V 0.7 V 11 A 73 V 0.7 V
´ - ´ -
£ = = W Þ = W
m ´ - m ´ -
R
1 2
V 0.7 V
73 V 0.7 V
R R 866 89.4k R1 88.7k
0.7 V 0.7 V
-
-
= ´ = ´ W = W Þ = W
( ) ( )
( ) ( )
R F 2 R
3
r
0.7 V V V 11 A R V 0.7 V
R
11 A V
0.7 V 73 V 72 V 11 A 866 73 V 0.7 V
14 R14 14
11 A 73 V
´ - - m ´ ´ -
= =
m ´
´ - - m ´ W´ -
= = W Þ = W
m ´
UCC28250
SLUSA29C –APRIL 2010– REVISED JULY 2011 www.ti.com
Step 3-3, Determine Soft-Start Capacitance
Determine soft-start capacitance with soft-start time 15 ms.
(37)
Step 3-4, Determine Dead-Time Resistance
Assuming the dead time is 150 ns, Select R7 = R8 = 121 kΩ based on Figure 9 and Figure 10.
Step 3-5, Determine OCP Hiccup Off-Time Capacitance
Assuming off time is 0.8 s (Equation 15).
(38)
Step 3-6, Determine Primary-Side OVP Resistance
Assuming OV_OFF = 73 V, OV_ON = 72 V (Equation 16 to Equation 18).
(39)
(40)
(41)
Step 3-7, Select Capacitance for VDD and VREF
As recommended by the datasheet, select C6 = C4 = 1.0 µF. The final design is shown in Figure 47.
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