1.07k

Rfbb

2.26k

Rfbt

107

Rtkb

226

Rtkt

SS/TRK

3.3V Master

FB

2.5Vout

50 2A

Int VCC

LMZ22005

SNVS686J – MARCH 2011 – REVISED AUGUST 2015

www.ti.com

8.2.2.4 Soft-start Capacitor Selection

Programmable soft-start permits the regulator to slowly ramp to its steady-state operating point after being

enabled, thereby reducing current inrush from the input supply and slowing the output voltage rise-time.

Upon turnon, after all UVLO conditions have been passed, an internal 2-ms circuit slowly ramps the SS/TRK

unpopulated. Longer soft-start periods are achieved by adding an external capacitor to this input.

Soft-start duration is given by the formula:

(6)

This equation can be rearranged as follows:

(7)

Using a 0.22-

μF capacitor results in 3.5-ms typical soft-start duration; and 0.47 μF results in 7.5-ms typical. 0.47

μF is a recommended initial value.

Once the soft-start input exceeds 0.796 V the output of the power stage will be in regulation and the 50-

μA

current is deactivated. The following conditions will reset the soft-start capacitor by discharging the SS input to

ground with an internal current sink.

•

The Enable input being pulled low

•

Thermal shutdown condition

•

8.2.2.5 Tracking Supply Divider Option

The tracking function allows the module to be connected as a slave supply to a primary voltage rail (often the

3.3-V system rail) where the slave module output voltage is lower than that of the master. Proper configuration

allows the slave rail to power up coincident with the master rail such that the voltage difference between the rails

during ramp-up is small (that is, <0.15 V typical). The values for the tracking resistive divider must be selected

such that the effect of the internal 50-µA current source is minimized. In most cases the ratio of the tracking

divider resistors is the same as the ratio of the output voltage setting divider. Proper operation in tracking mode

dictates the soft-start time of the slave rail be shorter than the master rail; a condition that is easy satisfy

master supply; once the SS/TRK rises past 0.8V the input is no longer enabled and the 50-µA internal current

source is switched off.

Figure 48. Tracking Option Input Detail

required based on the values of internal compensation in the error amplifier. Low ESR tantalum, organic

semiconductor or specialty polymer capacitor types are recommended for obtaining lowest ripple. The output

Equation 14. Beyond that, additional capacitance will reduce output ripple so long as the ESR is low enough to

permit it. Loop response verification is also valuable to confirm closed loop behavior.

20

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