TPS40020

TPS40021

SLUS535D − MARCH 2003 − REVISED JULY 2007

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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

ILIM + 19.0

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

protection of the power switches. Given the tolerance of the ILIM sink current, and the RDS(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 over−rides the

reference voltage on the error amplifier during startup. This controls the output voltage slew rate and the surge

current required to charge the output capacitor at startup, allowing for a smooth startup with no overshoot of

the output voltage. Initial HDRV pulse widths during Soft−Start are typically very narrow, likely less than 150ns.

As a result, HDRV and LDRV can be on simultaneously, resulting in cross−conduction the MOSFETs of the

power stage. To minimize cross−conduction during soft−start, the soft−start time when the pulse widths are

less than 150ns should be kept to a minimum. A shutdown feature can be implemented by pulling SS/SD to

GND via a transistor as shown in Figure 5.

CSS SS/SD

6

TPS4002x

SHUTDOWN

3.3

µA

Figure 5. Shutdown Implementation

C

SS +

I

SS

V

FB

t

SS (F)

where

D tSS 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)

(1)

(2)

(3)

Not Recommended for New Designs