SNVSCL2 Data sheet

SNVSCL2A December 2024 – August 2025 LM51770 , LM517701

PRODUCTION DATA

9.2.2.11 MOSFETs QH2 and QL2

The output side MOSFETs QH2 (Q3) and QL2 (Q4) see the output voltage of 16 V and additional transient spikes at SW2 during switching. Therefore, rate QH2 and QL2 for 25V or more. The gate plateau voltages of the MOSFETs need to be smaller than the minimum input voltage of the converter, otherwise, the MOSFETs may not fully enhance during start-up or overload conditions.

The power loss in QH2 in buck mode of operation is approximated by:

Equation 36.

P_{COND (QH 2)} = {I_{OUT}}^{2} \times R_{DS, On (QH 2)}

The power loss in QL2 in the boost mode of operation consists of both conduction and switching loss components given by Equation 37 and Equation 38, respectively:

Equation 37.

P_{COND (QL 2)} = (1 - \frac{V_{IN}}{V_{OUT}}) \times {(I_{OUT} \times \frac{V_{OUT}}{V_{IN}})}^{2} \times R_{DS, On (QL 2)}

Equation 38.

P_{SW (QL 2)} = \frac{1}{2} \times V_{OUT} \times (I_{OUT} \times \frac{V_{OUT}}{V_{IN}}) \times (t_{r} + t_{f}) \times f_{SW}

The rise (t_r) and the fall (t_f) times are based on the MOSFET data sheet information or measured in the lab. Typically, a MOSFET with smaller R_DSON (lower conduction loss) has longer rise and fall times (larger switching loss).

The power loss in QH2 in the boost mode of operation is shown in Equation 39:

Equation 39.

P_{C O N D (Q H 2)} = \frac{V_{I N}}{V_{O U T}} \times {(I_{O U T} \times \frac{V_{O U T}}{V_{I N}})}^{2} \times R_{D S, O n (Q H 2)}