SLVSJL6 Data sheet

SLVSJL6 June 2026 LM851772-Q1

ADVANCE INFORMATION

10.2.2.12 MOSFETs QH2 and QL2

The output side MOSFETs QH2 (Q4) and QL2 (Q3) see the output voltage of 48V and additional transient spikes at SW2 during switching. Therefore, QH2 and QL2 need to be rated for 58V or more. The gate plateau voltages of the MOSFETs need to be smaller than the minimum input voltage of the converter, otherwise, the MOSFETs do not always fully enhance during start-up or overload conditions.

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

Equation 48.

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 49.

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)}

and, respectively:

Equation 50.

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 is obtained by 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 below:

Equation 51.

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)}