Datasheet

( ) ( )
( )
f
IN OUT OUT
TRIP RIPPLE TRIP
OCP
IN
DS on DS on
V V V
V I V
1
I
R 2 R 2 L V
- ´
= + = + ´
´ ´
( )
( ) ( )
( )
TRIP TRIP
TRIP
R k I A
V mV 24 mV
9
W ´ m
= -
TPS51123
www.ti.com SLUS890E –DECEMBER 2008–REVISED JANUARY 2013
Current Protection
TPS51123 has cycle-by-cycle over current limiting control. The inductor current is monitored during the ‘OFF’
state and the controller keeps the ‘OFF’ state during the inductor current is larger than the over current trip level.
In order to provide both good accuracy and cost effective solution, TPS51123 supports temperature
compensated MOSFET R
DS(on)
sensing. The TRIPx pin should be connected to GND through the trip voltage
setting resistor, R
TRIP
. TRIPx terminal sources I
TRIP
current, which is 10 μA typically at room temperature, and the
trip level is set to the OCL trip voltage V
TRIP
as below. Note that the V
TRIP
is limited up to about 205 mV
internally.
(3)
Note that when TRIPx voltage is under a certain thershould (typically 0.4V), the switcher channel concerned is
shut down. The inductor current is monitored by the voltage between GND pin and LLx pin so that LLx pin should
be connected to the drain terminal of the bottom MOSFET properly. Itrip has 4500 ppm/°C temperature slope to
compensate the temperature dependency of the R
DS(on)
. GND is used as the positive current sensing node so
that GND should be connected to the proper current sensing device, i.e. the source terminal of the bottom
MOSFET.
As the comparison is done during the ‘OFF’ state, V
TRIP
sets valley level of the inductor current. Thus, the load
current at over current threshold, I
OCP
, can be calculated as follows;
(4)
In an over current condition, the current to the load exceeds the current to the output capacitor thus the output
voltage tends to fall down. Eventually, it ends up with crossing the under voltage protection threshold and
shutdown both channels.
Overvoltage and Undervoltage Protection
TPS51123 monitors a resistor divided feedback voltage to detect over and under voltage. When the feedback
voltage becomes higher than 115% of the target voltage, the OVP comparator output goes high and the circuit
latches as the top MOSFET driver OFF and the bottom MOSFET driver ON.
Also, TPS51123 monitors VOx voltage directly and if it becomes greater than 5.75 V the TPS51123 turns off the
top MOSFET driver.
When the feedback voltage becomes lower than 60% of the target voltage, the UVP comparator output goes
high and an internal UVP delay counter begins counting. After 32 μs, TPS51123 latches OFF both top and
bottom MOSFETs drivers, and shut off both drivers of another channel. This function is enabled after 2 ms
following ENC has become high.
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