LM5145 datasheet(21/62 Pages) TI

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8.3.7 Configurable Soft Start (SS/TRK)

After the EN/UVLO pin exceeds its rising threshold of 1.2 V, the LM5145-Q1 begins charging the output to the

DC level dictated by the feedback resistor network. The LM5145-Q1 features an adjustable soft start (set by a

capacitor from the SS/TRK pin to GND) that determines the charging time of the output. A 10-µA current source

charges this soft-start capacitor. Soft start limits inrush current as a result of high output capacitance to avoid

an overcurrent condition. Stress on the input supply rail is also reduced. The soft-start time, tSS, for the output

voltage to ramp to its nominal level is set by Equation 4.

REF

(4)

where

• CSS is the soft-start capacitance

• VREF is the 0.8-V reference

• ISS is the 10-µA current sourced from the SS/TRK pin

More simply, calculate CSS using Equation 5.

12.5 t

(5)

The SS/TRK pin is internally clamped to VFB + 115 mV to allow a soft start recovery from an overload event.

The clamp circuit requires a soft-start capacitance greater than 2 nF for stability and has a current limit of

approximately 2 mA.

8.3.7.1 Tracking

The SS/TRK pin also doubles as a tracking pin when master-slave power-supply tracking is required. This

tracking is achieved by simply dividing down the output voltage of the master with a simple resistor network.

Coincident, ratiometric, and offset tracking modes are possible.

If an external voltage source is connected to the SS/TRK pin, the external soft-start capability of the LM5145-Q1

is effectively disabled. The regulated output voltage level is reached when the SS/TRK pin reaches the 0.8-V

reference voltage level. It is the responsibility of the system designer to determine if an external soft-start

capacitor is required to keep the device from entering current limit during a start-up event. Likewise, the system

designer must also be aware of how fast the input supply ramps if the tracking feature is enabled.

PGOOD

2V/DIV

VOUT 1V/DIV

SS/TRK

160mV/DIV

94% VOUT

92% VOUT

10 ms/DIV

Figure 8-6. Typical Output Voltage Tracking and PGOOD Waveforms

Figure 8-6 shows a triangular voltage signal directly driving SS/TRK and the corresponding output voltage

tracking response. Nominal output voltage here is 5 V, with oscilloscope channel scaling chosen such that the

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