TPS62933O datasheet(26/47 Pages) TI

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In addition, Adding one BST resistor R4 to reduce the spike voltage on the SW node, the resistance smaller than

10 Ω is recommended to be used between BST to the bootstrap capacitor.

9.2.2.6 Undervoltage Lockout Set Point

The undervoltage lockout (UVLO) can be adjusted using the external voltage divider network of R1 and R2.

R1 is connected between VIN and the EN pin of the TPS62933 and R2 is connected between EN and GND.

The UVLO has two thresholds: one for power up when the input voltage is rising and one for power down

or brownouts when the input voltage is falling. For the example design, the supply should turn on and start

switching when the input voltage increases above 8 V (VSTART). After the converter starts switching, it should

continue to do so until the input voltage falls below 7 V (VSTOP). Equation 3 and Equation 4 can be used

to calculate the values for the upper and lower resistor values. For the stop voltages specified, the nearest

standard resistor value for R1 is 511 kΩ and for R2 is 80.7 kΩ.

9.2.2.7 Output Inductor Selection

The most critical parameters for the inductor are the inductance, saturation current, and the RMS current. The

inductance is based on the desired peak-to-peak ripple current ΔiL, which can be calculated by Equation 15.

¿IL =

VOUT

VIN_MAX

VIN_MAX F VOUT

L × fSW

(15)

Usually, define K coefficient represents the amount of inductor ripple current relative to the maximum output

current of the device, a reasonable value of K should be 20% to 60%, experience shows that the best value of

K is 40%. Since the ripple current increases with the input voltage, the maximum input voltage is always used to

calculate the minimum inductance L. Use Equation 16 to calculate the minimum value of the output inductor.

u u

OUT

OUT _ MAX

(V -V

)

K I

(16)

where

• K = Ripple ratio of the inductor current (ΔIL / IOUT_MAX)

In general, it is preferable to choose lower inductance in switching power supplies, because it usually

corresponds to faster transient response, smaller DCR, and reduced size for more compact designs. Too low

of an inductance can generate too large of an inductor current ripple such that overcurrent protection at the full

load can be falsely triggered. It also generates more inductor core loss since the current ripple is larger. Larger

inductor current ripple also implies larger output voltage ripple with the same output capacitors.

After inductance L is determined, the maximum inductor peak current and RMS current can be calculated by

Equation 17 and Equation 18.

L_PEAK = IOUT +

¿IL

(17)

L_RMS =

¨I

OUT

2 +

¿IL

(18)

Ideally, the saturation current rating of the inductor is at least as large as the high-side switch current limit,

IHS_LIMIT (see Section 7.5). This ensures that the inductor does not saturate even during a short circuit on

the output. When the inductor core material saturates, the inductance falls to a very low value, causing the

inductor current to rise very rapidly. Although the valley current limit, ILS_LIMIT, is designed to reduce the risk of

current runaway, a saturated inductor can cause the current to rise to high values very rapidly, this can lead to

component damage, so do not allow the inductor to saturate. In any case, the inductor saturation current must

not be less than the maximum peak inductor current at full load.

For this design example, choose the following values:

TPS62933

SLUSEA4A – JUNE 2021 – REVISED DECEMBER 2021

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