TPS40020

TPS40021

SLUS535C − MARCH 2003 − REVISED SEPTEMBER 2004

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13

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

0.69 V

R

T

The tolerance of the current sink is too loose to do an accurate current limit. The main purpose is for hard fault

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.

CSS SS/SD

6

TPS4002x

SHUTDOWN

3.3

µA

Figure 5. Shutdown Implementation

C

I

SS

V

FB

t

where

Switching Frequency

The switching frequency is programmed by a resistor from RT to SGND. Nominal switching frequency can be

calculated by:

R

37.736

f

OSC

(kHz)

* 5.09 (kW)

(1)

(2)

(3)