TPS40020 Datasheet, PDF (13/32 Pages) Texas Instruments – ENHANCED LOW INPUT VOLTAGE MODE SYNCHRONOUS BUCK CONTROLLER

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TPS40020
TPS40021
SLUS535C − MARCH 2003 − REVISED SEPTEMBER 2004
APPLICATION INFORMATION
Setting the Short Circuit Current Limit Threshold
Connecting a resistor from VDD to ILIM sets the current limit. A current sink in the chip causes a voltage drop
across the resistor connected to ILIM. This voltage drop is the short circuit current threshold for the part. The
current that the ILIM pin sinks is dependent on the value of the resistor connected to RT and is given by:
I_ILIM + 19.0
0.69 V
R_T
(1)
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 small capacitor can
be added between ILIM and VDD for filtering. However, capacitors should not be used if the synchronization
function is to be used.
Soft-Start and Shutdown
The soft-start and shutdown functions are common to the SS/SD pin. The voltage at this pin is the controlling
voltage sent to 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. A shutdown feature
can be implemented as shown in Figure 5.
3.3 µA
SHUTDOWN
6
CSS ^SS/SD
TPS4002x
Figure 5. Shutdown Implementation
C_SS
+
I_SS
V_FB
t_SS (F)
(2)
where
D t_SS is the start up time in seconds
Switching Frequency
The switching frequency is programmed by a resistor from RT to SGND. Nominal switching frequency can be
calculated by:
R_T
(kW)
+
37.736 10³
f_OSC(kHz)
*
5.09
(kW)
(3)
13