LM5177DCPR datasheet(42/60 Pages) TI2

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8.2.2.9 Soft-Start Capacitor

The soft-start time is programmed using the soft-start capacitor. The relationship between CSS and the soft-start

time is given by:

CSS= ISS×tSSVRef= 18 nF

(30)

CSS = 18 nF gives a soft-start time of 1.8 ms.

8.2.2.10 MOSFETs QH1 and QL1

The input side MOSFETs QH1 (Q1) and QL1 (Q2) need to withstand the maximum input voltage of 36 V. In

addition, they must withstand the transient spikes at SW1 during switching. Therefore, QH1 and QL1 must be

rated for 50 V 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:

PCONDQH1 = IOUT×VOUTVIN 2× RDS,OnQH1

(31)

The power loss in QH1 in buck mode consists of both conduction and switching loss components given by

Equation 32 and Equation 33, respectively:

PCONDQH1 = IOUT×VOUTVIN 2×RDS,OnQH1

(32)

PSWQH1 =12×VIN×IOUT× tr+tf ×fSW

(33)

The rise (tr) and the fall (tf) times are based on the MOSFET data sheet information or measured in the

lab. Typically, a MOSFET with smaller RDSON (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 34:

PCONDQL1 = 1−VOUTVIN ×IOUT2× RDS,OnQL1

(34)

8.2.2.11 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, QH2 and QL2 must be rated for 25 V or more. 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 QH2 in buck mode of operation is approximated by:

PCONDQH2 =IOUT2× RDS,OnQH2

(35)

The power loss in QL2 in the boost mode of operation consists of both conduction and switching loss

components given by Equation 36 and Equation 37, respectively:

PCONDQL2 = 1− VINVOUT × IOUT×VOUTVIN 2× RDS,OnQL2

(36)

PSWQL2 =12×VOUT× IOUT×VOUTVIN × tr+tf ×fSW

(37)

LM5177

SNVSBU4F – JUNE 2022 – REVISED JANUARY 2025

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