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Interaction Between Soft-Start and Tracking Startup

Overcurrent Protection

R

V

ILIM

gm

CSA

R

LDC

I

OC

W

(29)

t

C

ILIM

ln 1

*

1

n

(s)

(30)

TPS40101

SLUS726 – SEPTEMBER 2006

APPLICATION INFORMATION (continued)

Since the TPS40101 provides two means of controlling the startup (closed loop soft-start and tracking) care

must be taken to ensure that the two methods do not interfere with each other. The two methods should not be

allowed to try and control the output at the same time. If tracking is to be used, the reference input to the

tracking amplifier (TRKIN) should be held low until soft-start completes, or the voltage at the SS pin is at least

above 1.04 V. This ensures that the soft-start circuit is not trying to control the startup at the same time as

tracking circuit. If it is desired to have soft-start control the startup, then there are two options:

•

Disconnect the tracking amplifier output from the FB node (this is the recommended solution. The tracking

amplifier can then be used for other system purposes if desired)

•

Maintain the tracking amplifier output connection to the FB circuit - the reference to the tracking amplifier

should be tied to VDD pin in this case. This places the tracking amplifier output (TRKOUT) in a low state

continuously and therefore removes any influence the tracking circuit has on the converter startup.

Additionally, when tracking is allowed to control the startup, soft-start should not be set to an arbitrarily short

time. This causes the output voltage to bump up when power is applied to the converter as soft-start ramps up

quickly and the tracking loop (which is necessarily low bandwidth) cannot respond fast enough to control the

output to zero voltage. In other words, the soft start ramp rate must be within the capability of the tracking loop

to override.

Overcurrent characteristics are determined by connecting a parallel R-C network from the ILIM pin to GND. The

ILIM pin sources a current that is proportional to the current sense amplifier transconductance and the voltage

between ISNS and VO. This current produces a voltage on the R-C network at ILIM. If the voltage at the ILIM

pin reaches 1.48 V, an overcurrent condition is declared and the outputs stop switching for a period of time. This

time period is determined by the time is takes to discharge the soft-start capacitor with a controlled current sink.

To set the overcurrent level:

where

• V

• I

• gm

• R

• R

The response time of the overcurrent circuit is determined by the R-C time constant at the ILIM pin and the level

of the overcurrent. The response time is given by:

where

• t

• R

• n is the multiplier of the overcurrent. If the overcurrent is 2 times the programmed level, then n is 2.

By suitable manipulation of the time constant at ILIM, the overcurrent response can be tailored to ride out short

term transients and still provide protection for overloads and short circuits. The gm of the current sense amplifier

has a temperature coefficient of approximately -2000 ppm/

°C. This is to help offset the temperature coefficient of

resistance of the copper in the inductor, about +4000 ppm/

°C. The net is a +2000 ppm/°C temperature

coefficient. So, for a 100

°C increase in temperature, the overcurrent threshold decreases by 20%, assuming

good thermal coupling between the controller and the inductor. Temperature compensation can be done as

described earlier if desired.

When an overcurrent condition is declared, the controller stops switching and turns off both the high-side

25

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