Datasheet

LC S1 CS D PA
LC
P
K R R T N
R
L
´ ´ ´ ´
=
UCC28700, UCC28701
UCC28702, UCC28703
www.ti.com SLUSB41 – JULY 2012
CS (Current Sense): The current-sense pin is connected through a series resistor (R
LC
) to the current-sense
resistor (R
CS
). The current-sense threshold is 0.75 V for I
PP(max)
and 0.25 V for I
PP(min)
. The series resistor R
LC
provides the function of feed-forward line compensation to eliminate change in I
PP
due to change in di/dt and the
propagation delay of the internal comparator and MOSFET turn-off time. There is an internal leading-edge
blanking time of 235 ns to eliminate sensitivity to the MOSFET turn-on current spike. It should not be necessary
to place a bypass capacitor on the CS pin. The value of R
CS
is determined by the target output current in
constant-current (CC) regulation. The values of R
CS
and R
LC
can be determined by the equations below. The
term η
XFMR
is intended to account for the energy stored in the transformer but not delivered to the secondary.
This includes transformer resistance and core loss, bias power, and primary-to-secondary leakage ratio.
Example: With a transformer core and winding loss of 5%, primary-to-secondary leakage inductance of 3.5%,
and bias power to output power ratio of 1.5%. The η
XFMR
value is approximately: 1 - 0.05 - 0.035 - 0.015 = 0.9.
where
• V
CCR
is a current regulation constant (see ELECTRICAL CHARACTERISTICS),
• N
PS
is the transformer primary-to-secondary turns ratio (a ratio of 13 to 15 is recommended for 5-V output),
• I
OCC
is the target output current in constant-current regulation,
• η
XFMR
is the transformer efficiency. (3)
where
• R
S1
is the VS pin high-side resistor value,
• R
CS
is the current-sense resistor value,
• T
D
is the current-sense delay including MOSFET turn-off delay, add ~50 ns to MOSFET delay,
• N
PA
is the transformer primary-to-auxiliary turns ratio,
• L
P
is the transformer primary inductance,
• K
LC
is a current-scaling constant (see ELECTRICAL CHARACTERISTICS). (4)
CBC (Cable Compensation), Pin 1 UCC28700: The cable compensation pin is connected to a resistor to
ground to program the amount of output voltage compensation to offset cable resistance. The cable
compensation block provides a 0-V to 3-V voltage level on the CBC pin corresponding to 0 to I
OCC
output current.
The resistance selected on the CBC pin programs a current mirror that is summed into the VS feedback divider
therefore increasing the output voltage as I
OUT
increases. There is an internal series resistance of 28 kΩ to the
CBC pin which sets a maximum cable compensation of a 5-V output to 400 mV when CBC is shorted to ground.
The CBC resistance value can be determined by the equation below.
where
• V
O
is the output voltage,
• V
F
is the diode forward voltage,
• V
OCBC
is the target cable compensation voltage at the output terminals,
• V
CBC(max)
is the maximum voltage at the cable compensation pin at the maximum converter output current (see
ELECTRICAL CHARACTERISTICS),
• V
VSR
is the CV regulating level at the VS input (see ELECTRICAL CHARACTERISTICS). (5)
NTC (NTC Thermistor Shut-down), Pin 1 UCC28701/2/3: These versions of the UCC28700 family utilize pin 1
for an external NTC thermistor to allow user-programmable external thermal shut-down. The shut-down threshold
is 0.95 V with an internal 105-µA current source which results in a 9.05-kΩ thermistor shut-down threshold.
These controllers have either zero or fixed internal cable compensation.
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Product Folder Link(s): UCC28700, UCC28701 UCC28702, UCC28703