LM5145 datasheet(21/60 Pages) TI1

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1 Ps/DIV

SYNCIN

2 V/DIV

SW 10 V/DIV

Figure 8-5. Typical 400-kHz SYNCIN and SW Voltage Waveforms

Figure 8-5 shows a clock signal at 400 kHz and the corresponding SW node waveform (VIN = 48 V, VOUT = 5 V,

free-running frequency = 280 kHz). The SW voltage waveform is synchronized with respect to the rising edge of

SYNCIN. The rising edge of the SW voltage is phase delayed relative to SYNCIN by approximately 100 ns.

8.3.7 Configurable Soft Start (SS/TRK)

After the EN/UVLO pin exceeds its rising threshold of 1.2 V, the LV5144 begins charging the output to the

DC level dictated by the feedback resistor network. The LV5144 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 primary-secondary power-supply tracking is required. This

tracking is achieved by simply dividing down the output voltage of the primary 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 LV5144

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

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LV5144

SNVSCJ1 – AUGUST 2023

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