Datasheet

DCR
Inductor L
R11
C11
V
BuckX
Sx2
2
Sx11
V
C
TPS43330-Q1
or
TPS43332-Q1
TPS43330-Q1
TPS43332-Q1
www.ti.com SLVSA82E –MARCH 2011–REVISED APRIL 2013
Current-Mode Operation
Peak-current-mode control regulates the peak current through the inductor to maintain the output voltage at its
set value. The error between the feedback voltage at FBx and the internal reference produces a signal at the
output of the error amplifier (COMPx) which serves as the target for the peak inductor current. The device
senses the current through the inductor as a differential voltage at Sx1–Sx2 and compares voltage with this
target during each cycle. A fall or rise in load current produces a rise or fall in voltage at FBx, causing V
COMPx
to
fall or rise respectively, thus increasing or decreasing the current through the inductor until the average current
matches the load. This process maintains the output voltage in regulation.
The top N-channel MOSFET turns on at the beginning of each clock cycle and stays on until the inductor current
reaches its peak value. Once this MOSFET turns off, and after a small delay (shoot-through delay) the lower N-
channel MOSFET turns on until the start of the next clock cycle. In dropout operation, the high-side MOSFET
stays on continuously. In every fourth clock cycle, there is a limit on the duty cycle of 95% in order to charge the
bootstrap capacitor at CBx. This allows a maximum duty cycle of 98.75% for the buck regulators. During dropout,
the buck regulator switches at one-fourth of its normal frequency.
Current Sensing and Current Limit With Foldback
Clamping of the maximum value of COMPx is such as to limit the maximum current through the inductor to a
specified value. When the output of the buck regulator (and hence the feedback value at FBx) falls to a low value
due to a short circuit or overcurrent condition, the clamped voltage at COMPx successively decreases, thus
providing current foldback protection, which protects the high-side external MOSFET from excess current
(forward-direction current limit).
Similarly, if a fault condition shorts the output to a high voltage and the low-side MOSFET turns fully on, the
COMPx node drops low. A clamp is on its lower end as well, in order to limit the maximum current in the low-side
MOSFET (reverse-direction current limit).
An external resistor senses the current through the inductor. Choose the sense resistor such that the maximum
forward peak current in the inductor generates a voltage of 75 mV across the sense pins. This specified value is
for low duty cycles only. At typical duty-cycle conditions around 40% (assuming 5 V output and 12 V input), 50
mV is a more reasonable value, considering tolerances and mismatches. The typical characteristics provide a
guide for using the correct current-limit sense voltage.
The current-sense pins Sx1 and Sx2 are high-impedance pins with low leakage across the entire output range,
thus allowing DCR current sensing using the dc resistance of the inductor for higher efficiency. Figure 20 shows
DCR sensing. Here, the series resistance (DCR) of the inductor is the sense element. Place the filter
components close to the device for noise immunity. Remember that while the DCR sensing gives high efficiency,
it is inaccurate due to the temperature sensitivity and a wide variation of the parasitic inductor series resistance.
Hence, it may often be advantageous to use the more-accurate sense resistor for current sensing.
Figure 20. DCR Sensing Configuration
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