SNVSCT9 Data sheet

SNVSCT9 October 2024 LM251772

PRODUCTION DATA

Package Options

Mechanical Data (Package|Pins)

RHA|40
- MPQF135D

Thermal pad, mechanical data (Package|Pins)

RHA|40
- QFND660

Orderable Information

10.2.2.10 MOSFETs QH1 and QL1

The input side MOSFETs QH1 (Q1) and QL1 (Q2) need to withstand the maximum input voltage of 48V. In addition, they must withstand the transient spikes at SW1 during switching. Therefore, QH1 and QL1 must be rated for 58V or higher. The gate plateau voltages of the MOSFETs must 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 QH1 in boost mode is approximated by:

Equation 49.

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

The power loss in QH1 in buck mode consists of both conduction and switching loss components given by Equation 50 and Equation 51, respectively:

Equation 50.

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

Equation 51.

P_{SW (QH 1)} = \frac{1}{2} \times V_{IN} \times I_{OUT} \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 (smaller conduction loss) has longer rise and fall times (larger switching loss).

The power loss in QL1 in the buck mode of operation is shown in Equation 52:

Equation 52.

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