Datasheet

ILIM
HICC
UCC28250
C
HICC
C
S
R
S
CT
1:n
Transformer Primary
Side Current
S
PK
0.5 V n
R
I
´
=
S
S
100ns
C
R
=
UCC28250
SLUSA29C –APRIL 2010– REVISED JULY 2011 www.ti.com
ILIM (Current Limit for Cycle-By-Cycle Over-Current Protection) (17/4)
Cycle-by-cycle current limit is accomplished using the ILIM pin for current mode control or for voltage mode
control. The input to the ILIM pin represents the primary current information. If the voltage sensed at ILIM pin
exceeds 0.5 V, the current sense comparator terminates the pulse of output OUTA or OUTB. If the high current
condition persists, the controller operates in a cycle-by-cycle current limit mode with duty cycle determined by the
current sense comparator instead of the PWM comparator. ILIM pin is pulled down by an internal switch at the
rising edge of every clock cycle. This internal switch remains on for an additional 70 ns after OUTA or OUTB
goes high to blank leading edge transient noise in the current sensing loop. This reduces the filtering
requirements at the ILIM pin and improves the current sense response time.
Figure 15. Current Limit Circuit
Once the over current protection level I
PK
is selected, the current transformer turns ratio and the burden resistor
value can be decided as:
(11)
In this equation, current transformer turns ratio is 1:n and R
S
is the burden resistor value.
Some filtering capacitance is required to reduce the sensing noise. Choose the RC constant at about 100 ns,
and calculate the capacitor value as:
(12)
The cycle-by-cycle current limit operation time before all four outputs shut down can be programmed by external
capacitor C
HICC
at HICC pin. (See HICC pin description)
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