BQ25708 datasheet(63/77 Pages) TI2

63 / 77 page

REGN

plt

off

- V

, I

= Q

off

, t

top

CHG

DS(on)

CHG

off

= D

+ t

)

bq25708

www.ti.com

SLUSCU2 – NOVEMBER 2017

Product Folder Links: bq25708

Submit Documentation Feedback

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. It is a function of duty cycle

(D=VOUT/VIN), charging current (ICHG), MOSFET's on-resistance (RDS(ON)), input voltage (VIN), switching frequency

(fS), turn on time (ton) and turn off time (toff):

(7)

The first item represents the conduction loss. Usually MOSFET RDS(ON) increases by 50% with 100°C junction

temperature rise. The second term represents the switching loss. The MOSFET turn-on and turn-off times are

given by:

(8)

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):

(9)

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:

(10)

The conduction loss of the bottom-side MOSFET is calculated with the following equation when it operates in

synchronous continuous conduction mode:

Pbottom = (1 - D) x ICHG

2 x R

DS(on)

(11)

When charger operates in non-synchronous mode, the bottom-side MOSFET is off. As a result all the

freewheeling current goes through the body-diode of the bottom-side MOSFET. The body diode power loss

depends on its forward voltage drop (VF), non-synchronous mode charging current (INONSYNC), and duty cycle (D).

PD = VF x INONSYNC x (1 - D)

(12)

The maximum charging current in non-synchronous mode can be up to 0.25 A for a 10 mΩ charging current

sensing resistor or 0.5 A if battery voltage is below 2.5 V. The minimum duty cycle happens at lowest battery

voltage. Choose the bottom-side MOSFET with either an internal Schottky or body diode capable of carrying the

maximum non-synchronous mode charging current.

BQ25708 Folha de dados(PDF) 63 Page - Texas Instruments

BQ25708 Folha de dados(HTML) 63 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

Folha de dados Download

Ligação URL