Datasheet

TRIP TRIP TRIP
V (mV) R (k ) I ( A)= W ´ m
( )
f
IND ripple
IN OUT OUT
TRIP TRIP
OCP
DS(on) DS(on) SW IN
I
(V V ) V
V V
1
I
8 R 2 8 R 2 L V
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TPS51218
www.ti.com SLUS935B –MAY 2009– REVISED FEBRUARY 2012
POWER-GOOD
The TPS51218 has powergood output that indicates high when switcher output is within the target. The
powergood function is activated after soft-start has finished. If the output voltage becomes within +10%/–5% of
the target value, internal comparators detect power-good state and the power-good signal becomes high after a
1-ms internal delay. If the output voltage goes outside of +15%/–10% of the target value, the powergood signal
becomes low after a 2-μs internal delay. The powergood output is an open-drain output and must be pulled up
externally.
CURRENT SENSE AND OVER CURRENT PROTECTION
TPS51218 has cycle-by-cycle overcurrent 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 overcurrent trip level. To
provide both good accuracy and cost effective solution, the TPS51218 supports temperature compensated
MOSFET R
DS(on)
sensing. The TRIP pin should be connected to GND through the trip voltage setting resistor,
R
TRIP
. The TRIP terminal sources I
TRIP
current, which is 10μA typically at room temperature, and the trip level is
set to the OCL trip voltage V
TRIP
as shown in Equation 4. Note that V
TRIP
is limited up to approximately 3 V
internally.
(4)
The inductor current is monitored by the voltage between GND pad and SW pin so that the SW pin should be
connected to the drain terminal of the low-side MOSFET properly. I
TRIP
has 4700ppm/°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 low-side
MOSFET.
As the comparison is done during the OFF state, V
TRIP
sets valley level of the inductor current. Thus, the load
current at overcurrent threshold, I
OCP
, can be calculated in Equation 5
(5)
In an overcurrent condition, the current to the load exceeds the current to the output capacitor thus the output
voltage tends to fall down. Eventually, it crosses the undervoltage protection threshold and shuts down the
controller.
When the device is operating in the forced continuous conduction mode, the negative current limit (NCL) protects
the external FET from carrying too much current. The NCL detect threshold is set as the same absolute value as
positive OCL but negative polarity. Please be noted the threshold still represents the valley value of the inductor
current.
OVER/UNDER VOLTAGE PROTECTION
TPS51218 monitors a resistor divided feedback voltage to detect over and undervoltage. When the feedback
voltage becomes higher than 120% of the target voltage, the OVP comparator output goes high and the circuit
latches as the high-side MOSFET driver OFF and the low-side MOSFET driver ON.
When the feedback voltage becomes lower than 70% of the target voltage, the UVP comparator output goes
high and an internal UVP delay counter begins counting. After a 1-ms delay, TPS51218 latches OFF both
high-side and low-side MOSFETs drivers. This function is enabled after 1.2 ms following EN has become high.
UVLO PROTECTION
TPS51218 has V5IN undervoltage lockout protection (UVLO). When the V5IN voltage is lower than UVLO
threshold voltage, the switch mode power supply shuts off. This is non-latch protection.
THERMAL SHUTDOWN
TPS51218 monitors the die temperature. If the temperature exceeds the threshold value (typically 145°C), the
TPS51218 is shut off. This is non-latch protection.
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