LM5145 datasheet(20/60 Pages) TI2

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6.3.9 Minimum Controllable On Time

There are two limitations to the minimum output voltage adjustment range: the LM25139 voltage reference of

0.8V and the minimum controllable switch-node pulse width, tON(min).

tON(min) effectively limits the voltage step-down conversion ratio VOUT / VIN at a given switching frequency. For

fixed-frequency PWM operation, the voltage conversion ratio must satisfy Equation 6:

VOUTVIN>tONmin×FSW

(6)

where

• tON(min) is 25ns (typical).

• FSW is the switching frequency.

If the desired voltage conversion ratio does not meet the above condition, the LM25139 transitions from fixed

switching frequency operation to a pulse-skipping mode to maintain output voltage regulation. For example, if the

desired output voltage is 5V with an input voltage of 24V and switching frequency of 2.1MHz, use Equation 7 to

verify that the conversion ratio.

5V24V>25ns×2.1MHz

(7)

0.208>0.0525

(8)

For wide-VIN applications and low output voltages, an alternative is to reduce the LM25139 switching frequency

to meet the requirement of Equation 6.

6.3.10 Error Amplifier and PWM Comparator (FB)

The LM25139 has a high-gain transconductance amplifier that generates an error current proportional to

the difference between the feedback voltage and an internal precision reference (0.8V). The output of the

transconductance amplifier connects to the COMP pin, allowing the user to provide external control loop

compensation. TI generally recommends a type-II compensation network for peak current-mode control.

In case the output voltage goes below -300mV, comp control of pwm pulses is stopped and low side drive is

turned on until the output voltage goes above -300mV. At that point , comp reasserts control of the pwm pulses.

6.3.11 Slope Compensation

The LM25139 provides internal slope compensation for stable operation with peak current-mode control and a

duty cycle greater than 50%. Use Equation 9 to calculate the buck inductance to provide a slope compensation

contribution equal to one times the inductor down slope.

LOIDEALμH = VOUTV ×RSmΩ

45×FSWMHz

(9)

• A lower inductance value generally increases the peak-to-peak inductor current, which minimizes size and

cost, and improves transient response at the cost of reduced light-load efficiency due to higher cores losses

and peak currents.

• A higher inductance value generally decreases the peak-to-peak inductor current, reducing switch peak and

RMS currents at the cost of requiring larger output capacitors to meet load-transient specifications.

6.3.12 Inductor Current Sense (ISNS, VOUT)

There are two methods to sense the inductor current of the buck power stage. The first uses a current sense

resistor (also known as a shunt) in series with the inductor, and the second avails of the DC resistance of the

inductor (DCR current sensing).

LM25139

SLVSJ80 – OCTOBER 2025

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