Datasheet

VS ring p-p
TSMPL
TLK RESET
TDM
UCC28700, UCC28701
UCC28702, UCC28703
SLUSB41 – JULY 2012 www.ti.com
The UCC28700 VS signal sampler includes signal discrimination methods to ensure an accurate sample of the
output voltage from the auxiliary winding. There are however some details of the auxiliary winding signal to
ensure reliable operation, specifically the reset time of the leakage inductance and the duration of any
subsequent leakage inductance ring. Refer to Figure 14 below for a detailed illustration of waveform criteria to
ensure a reliable sample on the VS pin. The first detail to examine is the duration of the leakage inductance reset
pedestal, T
LK_RESET
in Figure 14. Since this can mimic the waveform of the secondary current decay, followed by
a sharp downslope, it is important to keep the leakage reset time less than 500 ns for I
PRI
minimum, and less
than 1.5 µs for I
PRI
maximum. The second detail is the amplitude of ringing on the V
AUX
waveform following
T
LK_RESET
. The peak-to-peak voltage at the VS pin should be less than approximately 100 mV
p-p
at least 200 ns
before the end of the demagnetization time, t
DM
. If there is a concern with excessive ringing, it usually occurs
during light or no-load conditions, when t
DM
is at the minimum. The tolerable ripple on VS is scaled up to the
auxiliary winding voltage by R
S1
and R
S2
, and is equal to 100 mV x (R
S1
+ R
S2
) / R
S2
.
Figure 14. Auxiliary Waveform Details
During voltage regulation, the controller operates in frequency modulation mode and amplitude modulation mode
as illustrated in Figure 15 below. The internal operating frequency limits of the device are 130 kHz maximum and
1 kHz minimum. The transformer primary inductance and primary peak current chosen sets the maximum
operating frequency of the converter. The output preload resistor and efficiency at low power determines the
converter minimum operating frequency. There is no stability compensation required for the UCC28700
controller.
Figure 15. Frequency and Amplitude Modulation Modes
(during voltage regulation)
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Product Folder Link(s): UCC28700, UCC28701 UCC28702, UCC28703