TPS51211
www.ti.com
SLUSAA7 –NOVEMBER 2010
POWER-GOOD
The TPS51211 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-ms internal delay. The powergood output is an open-drain output and must be pulled up
externally.
CURRENT SENSE AND OVER CURRENT PROTECTION
TPS51211 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 TPS51211 supports temperature compensated
MOSFET RDS(on) sensing. The TRIP pin should be connected to GND through the trip voltage setting resistor,
RTRIP. The TRIP terminal sources ITRIP current, which is 10mA typically at room temperature, and the trip level is
set to the OCL trip voltage VTRIP as shown in Equation 4. Note that VTRIP is limited up to approximately 3 V
internally.
V
(mV) = R
(kW)´I
(mA)
TRIP
TRIP
TRIP
(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. ITRIP has 4700ppm/°C temperature slope to
compensate the temperature dependency of the RDS(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, VTRIP sets valley level of the inductor current. Thus, the load
current at overcurrent threshold, IOCP, can be calculated in Equation 5
I
æ
ö
IND ripple
(
(V - V
)´ V
V
V
TRIP
)
1
TRIP
IN
OUT OUT
I
= ç
ç
è
÷ +
÷
ø
=
+
´
OCP
8´R
2
8´R
2´L ´ f
V
DS(on)
DS(on)
SW
IN
(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.
OVER/UNDER VOLTAGE PROTECTION
TPS51211 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, TPS51211 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
TPS51211 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
TPS51211 monitors the die temperature. If the temperature exceeds the threshold value (typically 145°C), the
TPS51211 is shut off. This is non-latch protection.
Copyright © 2010, Texas Instruments Incorporated
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