LM5145 datasheet(20/70 Pages) TI

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Table 8-1. Feedback Configuration Resistors

PULLUP RESISTOR TO VDDA

VOUT SETPOINT

0 Ω

3.3 V

24.9 kΩ

5 V

49.9 kΩ

12 V

Not installed

External FB divider setting

The configuration settings are latched and cannot be changed until the LM5148 is powered down (with the

VCC voltage decreasing below its falling UVLO threshold) and then powered up again (VCC rises above 3.4-V

typical). Alternatively, set the output voltage with an external resistor divider from the output to AGND. The

output voltage adjustment range is between 0.8 V and 55 V. The regulation voltage at FB is 0.8 V (VREF). Use

Equation 6 to calculate the upper and lower feedback resistors, designated as RFB1 and RFB2, respectively.

OUT

FB1

FB2

REF

(6)

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

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

consumption of the external divider adds to the LM5148 sleep current (9.5 µA typical). The divider current

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

8.3.9 Minimum Controllable On Time

There are two limitations to the minimum output voltage adjustment range: the LM5148 voltage reference of

0.8 V 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 7.

OUT

ON(min)

(7)

where

• tON(min) is 50 ns (typical).

• FSW is the switching frequency.

If the desired voltage conversion ratio does not meet the above condition, the LM5148 transitions from fixed

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

desired output voltage is 5 V with an input voltage of 24 V and switching frequency of 2.1 MHz, use Equation 8

to check the conversion ratio.

(8)

For wide VIN applications and low output voltages, an alternative is to reduce the LM5148 switching frequency to

meet the requirement of Equation 7.

8.3.10 Error Amplifier and PWM Comparator (FB, EXTCOMP)

The LM5148 has a high-gain transconductance amplifier that generates an error current proportional to

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

compensation is configured two ways. The first is using the internal compensation amplifier, which has a

LM5148

SNVSC01 – FEBRUARY 2023

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