LM5145 datasheet(19/60 Pages) TI2

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Spread spectrum ON

frequency

randomized modulations

(a) Low-frequency

triangular modulation

(b) High-frequency

randomized modulation

Spread spectrum OFF

Frequency

Low RBW

High RBW

fs(t)

2¨Is

Figure 6-4. Dual Random Spread Spectrum Implementation

6.3.7 Soft Start

The LM25139 has an internal 3ms soft-start timer (typical). The soft-start feature allows the regulator to gradually

reach the steady-state operating point, thus reducing start-up stresses and surges.

6.3.8 Output Voltage Setpoint (FB)

The LM25139 output can be independently configured for one of two fixed output voltages without external

feedback resistors, or adjusted to the desired voltage using an external resistor divider. Set the output to 3.3V by

connecting FB directly to VCC. Alternatively, set the output to 5V by installing a 24.9kΩ resistor between FB and

VCC. See Table 6-1.

Table 6-1. Feedback Configuration Resistors

PULLUP RESISTOR TO VCC

VOUT SETPOINT

0Ω

3.3V

24.9kΩ

Not installed

External FB divider setting

Alternatively, set the output voltage with an external resistive divider from the output to AGND. The output

voltage adjustment range is between 0.8V and 36V. The regulation voltage at FB is 0.8V (VREF). Use Equation 5

to calculate the upper and lower feedback resistors, designated as RFB1 and RFB2, respectively.

RFB1= VOUT−1

VREF ×RFB2

(5)

The recommended starting value for RFB2 is between 10kΩ and 20kΩ.

If low-IQ operation is required, take care when selecting the external feedback resistors. The current

consumption of the external divider adds to the LM25139 sleep current (9.5µA typical). The divider current

reflected to VIN is scaled by the ratio of VOUT/VIN.

The FB settings are latched and cannot be changed until either the LM25139 is powered down (with the VIN

voltage decreasing below 3V) and then powered up again. FB setting can also be reconfigured if EN pin going

through a disable cycle with EN pin voltage going below 0.5V and then subsequentially rising above 0.5V.

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LM25139

SLVSJ80 – OCTOBER 2025

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