Datasheet

 
 
 
SLUS507D − JANUARY 2002 − REVISED NOVEMBER 2005
13
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APPLICATION INFORMATION
setting the current limit
Connecting a resistor from VDD to ILIM sets the current limit. A 15-µA current sink internal to the device causes
a voltage drop at ILIM that is equal to the overcurrent threshold voltage. Ensure that (V
DD
−0.8 V) ≤ V
ILIM
≤ V
DD
.
The tolerance of the current sink is too loose to do an accurate current limit. The main purpose is for hard fault
protection of the power switches. Given the tolerance of the ILIM sink current, and the R
DS(on)
range for a
MOSFET, it is generally possible to apply a load that thermally damages the converter. This device is intended
for embedded converters where load characteristics are defined and can be controlled.
soft-start and shutdown
These two functions are common to the SS/SD pin. The voltage at this pin is the controlling voltage of the error
amplifier during startup. This reduces the transient current required to charge the output capacitor at startup,
and allows for a smooth startup with no overshoot of the output voltage if done properly. A shutdown feature
can be implemented as shown in Figure 5.
C
SS
UDG−01143
SS/SD
4
TPS40000
SHUTDOWN
3 µA
Figure 5. Shutdown Implementation
The device shuts down when the voltage at the SS/SD pin falls below 120 mV. Because of this limitation, it is
recommended that a MOSFET be used as the controlling device, as in Figure 5. An open-drain CMOS logic
output would work equally well.
rectifier zero-current
Both the TPS40000 and TPS40002 parts are source-only, thus preventing reverse current in the synchronous
rectifier. Synchronous rectification is terminated by sensing the voltage, SW with respect to ground, across the
low-side MOSFET. When SW node is greater than −7 mV, rectification is terminated and stays off until the next
PWM cycle. In order to filter out undesired noise on the SW node, the zero-current comparator is blanked for
75 ns from the time the rectifier is turned on.
The TPS40001 and TPS40003 parts enable the zero-current comparator, (and therefore prevent reverse
current), while soft-start is active. However, when the output reaches regulation; that is, at the end of the
soft-start time, this comparator is disabled to allow the synchronous rectifier to sink current.