Datasheet

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SLUS589B− NOVEMBER 2003 − REVISED FEBRUARY 2005
11
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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 becomes the short circuit threshold. 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.
A local capacitor (with a value 50 pF to 150 pF) placed across the resistor between VDD and ILIM may improve
coupling a common mode noise between VDD and ILIM.
SOFT-START AND SHUTDOWN
These two functions are combined on the SS/SD pin. There is a VBE offset (0.65-V) between the external SS/SD
pin and internal soft-start voltage at the error amplifier input, allowing the device to be enabled before starting
V
OUT
as shown in Figure 5. 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.
t − Time − 1 ms/div
FB
(200 mV/ div)
SS/SD
(200 mV/ div)
Figure 5. Offset Between SS/SD and FB at Startup