SNVSCL2 Data sheet

SNVSCL2 December 2024 LM51770

PRODUCTION DATA

パッケージ・オプション

9.2.1.4 Inductor and Current Sense Resistor Selection

For boost mode, the inductor selection is based on limiting the peak-to-peak current ripple, ΔI_L, to approximately 20% of the maximum inductor current at the minimum input voltage. The target inductance for boost mode is:

Equation 19.

L_{BOOST} = \frac{V_{IN (MIN)}^{2} \times (V_{OUT} - V_{IN (MIN)})}{0 .2 \times I_{OUT (MAX)} \times f_{SW} \times V_{OUT}^{2}} = 2 .21 μH

For this application, an inductor with 1.8 μH was selected.

When selecting the current sense resistor it needs to be ensured the peak inductor current will not hit the over current limit at maximum output current. For that the peak inductor current needs to be calculated with the sum of the average and ripple current through the inductor.

The maximum peak to peak inductor current occurs at minimum input voltage and is given by:

Equation 20.

I_{L (P E A K, P E A K)} = (1 - \frac{V_{I N (M I N)}}{V_{O U T}}) \times \frac{V_{I N (M I N)}}{L \times f_{S W}} = 5.23 A

The average input current at the maximum output current with an estimated efficiency of 95% is calculated by:

Equation 21.

I_{I N, A V G (M A X)} = \frac{{V_{O U T} \times I}_{O U T (M A X)}}{95 % \times V_{I N (M I N)}} = 22.5 A

For the current sense Resistor a margin of 20% is considered to have enough headroom for the dymamic responses, e.g. load step regulation. To ensure the maximum output current can be delivered the mimium level of the peak current limit threshold is used.

Equation 22.

R_{C S} = \frac{38.5 m V}{(I_{I N, A V G (M A X)} + \frac{1}{2} I_{L (P E A K, P E A K)}) \times 1.2} = 1.28 m Ω

The standard value of R_CS = 1 mΩ with 3 times 3 mΩ is selected. With the 3 resistors in parallel it also reduces the parasitic inductance.

The maximum power dissipation in R_CS happens at V_IN(MAX):

Equation 23.

P_{R_{CS (MAX)}} = {(\frac{58.5 mV}{R_{CS}})}^{2} \times R_{CS} \times (1 - \frac{V_{OUT}}{V_{IN (MAX)}}) = 1 .90 W

Therefore, for the 3 resistors in parallel a sense resistor with 1-W power rating is sufficient for this application.

A filter network to attenuate noise in the CSA and CSB sense lines should be added. For most applications it is recommended to use a filter resistance R_DIFF1 and R_DIFF2 of 10 Ω. The capacitance C_DIFF for the filter can be calculated with Equation 15. In this configuration 180pF is used.

パッケージ・オプション

メカニカル・データ（パッケージ｜ピン）

サーマルパッド・メカニカル・データ

発注情報

9.2.1.4 Inductor and Current Sense Resistor Selection