Datasheet

SA SB
R R=
( )
VCC OFF SA
OA OB
OFF
V V R
R R
V
-
= =
( )
2
0 1
0 1
VCC S OFF DA SA
TA TB
S OFF
V (V . V ) V R
R R
V . V
- ´ - +
= =
´ -
1
3
TA TB
TA S
C C
R f
= =
´ ´
UCC28070
www.ti.com SLUS794E –NOVEMBER 2007–REVISED APRIL 2011
When the inductor current becomes discontinuous the boost inductors ring with the parasitic capacitances in the
boost stages. This inductor current rings through the CTs causing a false current sense signal. Please refer to
the following graphical representation of what the current sense signal looks like when the inductor current goes
discontinuous.
NOTE
The inductor current and RS may vary from this graphical representation depending on
how much inductor ringing is in the design when the unit goes discontinuous.
Figure 27. False Current Sense Signal
To counter for the offset (V
OFF
) just requires adjusting resistors R
OA
and R
OB
to ensure that when the unit goes
discontinuous the current sense resistor is not seeing a positive current when it should be zero. Setting the offset
to 120 mV is a good starting point and may need to be adjusted based on individual design criteria.
(35)
(36)
A small PWM ramp that is equal to 10% of the maximum current sense signal (V
S
) less the offset can then be
added by properly selecting R
TA
, R
TB
, C
TA
and C
TB
.
(37)
(38)
Copyright © 2007–2011, Texas Instruments Incorporated 37