BQ25713 datasheet(91/111 Pages) TI2

91 / 111 page

FOMtop = RDS(on) · QGD; FOMbottom = RDS(on) · QG

(5)

The lower the FOM value, the lower the total power loss. Usually lower RDS(ON) has higher cost with the same

package size.

The top-side MOSFET loss includes conduction loss and switching loss. Taking buck mode operation as

an example the power loss is a function of duty cycle (D=VOUT/VIN), charging current (ICHG), MOSFET's on-

resistance (RDS(ON)_top), input voltage (VIN), switching frequency (fS), turn-on time (ton) and turn-off time (toff):

Ptop =Pcon_top+Psw_top

(6)

Pcon_top =D · IL_RMS 2 · RDS(on)_top;

(7)

IL_RMS 2=IL_DC 2+Iripple 2/12

(8)

• IL_DC is the average inductor DC current under buck mode;

• Iripple is the inductor current ripple peak-to-peak value;

Psw_top =PIV_top+PQoss_top+PGate_top;

(9)

The first item Pcon_top represents the conduction loss which is straight forward. The second term Psw_top

represents the multiple switching loss items in top MOSFET including voltage and current overlap losses

(PIV_top), MOSFET parasitic output capacitance loss (PQoss_top) and gate drive loss (PGate_top). To calculate

voltage and current overlap losses (PIV_top):

PIV_top =0.5x VIN · Ivalley · ton· fS+0.5x VIN · Ipeak · toff · fS

(10)

Ivalley =IL_DC- 0.5 · Iripple (inductor current valley value);

(11)

Ipeak =IL_DC+ 0.5 · Iripple (inductor current peak value);

(12)

• ton is the MOSFET turn-on time that VDS falling time from VIN to almost zero (MOSFET turn on conduction

voltage);

• toff is the MOSFET turn-off time that IDS falling time from Ipeak to zero;

The MOSFET turn-on and turn-off times are given by:

off

, t

(13)

where Qsw is the switching charge, Ion is the turn-on gate driving current, and Ioff is the turn-off gate driving

current. If the switching charge is not given in MOSFET datasheet, it can be estimated by gate-to-drain charge

(QGD) and gate-to-source charge (QGS):

Qsw =QGD+QGS

(14)

Gate driving current can be estimated by REGN voltage (VREGN), MOSFET plateau voltage (Vplt), total turn-on

gate resistance (Ron), and turn-off gate resistance (Roff) of the gate driver:

REGN

plt

off

- V

, I

(15)

To calculate top MOSFET parasitic output capacitance loss (PQoss_top):

PQoss_top =0.5 · VIN· Qoss · fS

(16)

• Qoss is the MOSFET parasitic output charge which can be found in MOSFET datasheet;

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BQ25730

SLUSE65A – FEBRUARY 2021 – REVISED JANUARY 2024

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