Datasheet

( )
( )
( )
ON
IN max
DMAG min
PS OCV F
t V 2
t
N V V
´ ´
=
´ +
( )
( )
( ) ( )
( )
PP max CST min
P
ON min
CST max
IN max
I V
L
t
V
V 2
´
= ´
´
UCC28710, UCC28711
UCC28712, UCC28713
UCC28714, UCC28715
SLUSB86 –NOVEMBER 2012 www.ti.com
Transformer Parameter Verification
The transformer turns ratio selected affects the MOSFET V
DS
and secondary rectifier reverse voltage so these
should be reviewed. The UCC28710 family does require a minimum on time of the MOSFET (t
ON
) and minimum
D
MAG
time (t
DMAG
) of the secondary rectifier in the high line, minimum load condition. The selection of f
MAX
, L
P
and R
CS
affects the minimum t
ON
and t
DMAG
.
The secondary rectifier and MOSFET voltage stress can be determined by the equations below.
(18)
For the MOSFET V
DS
voltage stress, an estimated leakage inductance voltage spike (V
LK
) needs to be included.
(19)
Equation 20 and Equation 21 are used to determine if the minimum t
ON
target of 300 ns and minimum t
DMAG
target of 1.2 µs is achieved.
(20)
(21)
Output Capacitance
The output capacitance value is typically determined by the transient response requirement from no-load. For
example, in some USB charger applications there is a requirement to maintain a minimum V
O
of 4.1 V with a
load-step transient of 0 mA to 500 mA . The equation below assumes that the switching frequency can be at the
UCC28710 family's minimum of f
SW(min)
.
(22)
Another consideration of the output capacitor(s) is the ripple voltage requirement which is reviewed based on
secondary peak current and ESR. A margin of 20% is added to the capacitor ESR requirement in the equation
below.
(23)
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Product Folder Links: UCC28710, UCC28711 UCC28712, UCC28713 UCC28714, UCC28715