Datasheet

( )
( )
DS(master )
I 25 1 D 5 A= - ´ - + m
( )
( )
DS(slave)
I 25 1 D A= - ´ - m
.
Soft Restart
OFF Time Before Restart
Cycle-by-Cycle I
LIM
Normal
Operation
Soft Start
I
HCC
= 2.5 mA
I
DS
= (-25 x (1-D)+5) mA
25 mA
Fast Pull Up
by 1 kW Switch
I
SS
=25 mA
4.65
3.70
3.60
0.55
0.00
SS Pin (V)
SS Clamp Voltage
Pull Up Threshold
Output Enable
Threshold
Output Pulses (D)
UCC28950
SLUSA16B –MARCH 2010– REVISED OCTOBER 2011 www.ti.com
Current Sensing (CS)
The signal from current sense pin is used for cycle-by-cycle current limit, peak-current mode control, light-load
efficiency management and setting the delay time for outputs OUTA, OUTB, OUTC, OUTD and delay time for
outputs OUTE, OUTF. Connect the current sense resistor R
CS
between CS and GND. Depending on layout, to
prevent a potential electrical noise interference, it is recommended to put a small R-C filter between R
CS
resistor
and CS pin.
Cycle-by-Cycle Current Limit Current Protection and Hiccup Mode
The cycle-by-cycle current limit provides peak current limiting on the primary side of the converter when the load
current exceeds its predetermined threshold. For peak current mode control, certain leading edge blanking time
is needed to prevent the controller from false tripping due to switching noise. In order to save external RC filter
for the blanking time, an internal 30-ns filter at CS input is provided. The total propagation delay TCS from CS
pin to outputs is 100 ns. An external RC filter is still needed if the power stage requires more blanking time. The
2.0-V ±3% cycle-by-cycle current limit threshold is optimized for efficient current transformer based sensing. The
duration when a converter operates at cycle-by-cycle current limit depends on the value of soft-start capacitor
and how severe the over current condition is. This is achieved by the internal discharge current I
DS
Equation 15
and Equation 16 at SS pin.
(15)
(16)
The soft-start capacitor value also determines the so called hiccup mode off-time duration. The behavior of the
converter during different modes of operation, along with related soft start capacitor charge/discharge currents
are shown in Figure 18.
Figure 18. Timing Diagram of Soft-Start Voltage V
SS
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