Table 6-1. MODE Pin Configuration Table

RECOMMENDED MODE

RESISTOR, kΩ

OPERATION IN LIGHT LOAD

FUNCTION OF SS/PG PIN

FREQUENCY SPREAD

SPECTRUM, Fss

< 4kΩ, short

PFM

SS

Yes

18kΩ

PFM

PG

Yes

180kΩ

FCCM

SS

Yes

330kΩ

FCCM

PG

Yes

680kΩ

FCCM

SS

No

> 1.3MΩ, floating

FCCM

PG

No

6.3.3 Voltage Reference

The internal reference voltage, VREF, is designed at 0.6V (typical). The negative feedback system of converter

produces a precise ±1% feedback voltage, VFB, over full temperature by scaling the output of a temperature-

stable internal band-gap circuit.

6.3.4 Output Voltage Setting

A precision 0.6V reference voltage, VREF, is used to maintain a tightly regulated output voltage over the entire

operating temperature range. The output voltage is set by a resistor divider from the output voltage to the FB

pin. TI recommends using 1% tolerance resistors with a low temperature coefficient for the FB divider. Select the

bottom-side resistor, RFBB, for the desired divider current and use Equation 1 to calculate the top-side resistor,

RFBT. Lower RFBB increases the divider current and reduces efficiency at very light load. Larger RFBB makes the

FB voltage more susceptible to noise, so larger RFBB values require a more carefully designed feedback path on

the PCB. TI recommends setting RFBB = 10kΩ and RFBT in the range of 10kΩ to 300kΩ for most applications.

The tolerance and temperature variation of the resistor dividers affect the output voltage regulation.

V

OUT

FB

R

FBT

R

FBB

Figure 6-2. Output Voltage Setting

RFBT=VOUT−VREF

VREF ×RFBB

(1)

where

• VREF is the 0.6V (the internal reference voltage).

• RFBB is 10kΩ (recommended).

6.3.5 Switching Frequency Selection, Synchronization

TPSM843521 can work under RT mode and SYNC mode by different configuration of the RT/SYNC pin. In RT

mode, the switching frequency of TPSM843521 can be set with RT selection programming. Table 6-2 shows

the RT selection programming. When RT is floating or connected to GND, the condition of this input is detected

when the device is first enabled. After the converter is running, the switching frequency selection is fixed and

cannot be changed until the next power-on cycle or EN toggle. When RT is connected with resistor, the switching

frequency can be set between 200kHz and 2200kHz using the following equation.

www.ti.com

TPSM843521

SLUSG64 – APRIL 2025

Copyright © 2025 Texas Instruments Incorporated

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