SAT

CHG

RIPPLE

I

I

+ (1/2) I

³

IN

RIPPLE

S

V

D

(1

D)

I

=

L

f

´

´

-

´

bq24725

SLUS702 – JULY 2010

www.ti.com

Table 8. Component List for Typical System Circuit of Figure 1 (continued)

PART DESIGNATOR

QTY

DESCRIPTION

Q6

1

N-channel MOSFET, 50V, 0.2A, SOT-323, Diodes, BSS138W

L1

1

Inductor, SMT, 4.7µH, 5.5A, Vishay Dale, IHLP2525CZER4R7M01

R1

1

Resistor, Chip, 430k

Ω, 1/10W, 1%, 0603

R2

1

Resistor, Chip, 66.5k

Ω, 1/10W, 1%, 0603

R3, R4, R5

3

Resistor, Chip, 10k

Ω, 1/10W, 1%, 0603

R6, R10, R11

3

Resistor, Chip, 4.02k

Ω, 1/10W, 1%, 0603

R7

1

Resistor, Chip, 316k

Ω, 1/10W, 1%, 0603

R8

1

Resistor, Chip, 100k

Ω, 1/10W, 1%, 0603

R9

1

Resistor, Chip, 10

Ω, 1/4W, 1%, 1206

R12

1

Resistor, Chip, 1.00M

Ω, 1/10W, 1%, 0603

R13

1

Resistor, Chip, 3.01M

Ω, 1/10W, 1%, 0603

RAC, RSR

2

Resistor, Chip, 0.01

Ω, 1/2W, 1%, 1206

Ri

1

Resistor, Chip, 2

Ω, 1/2W, 1%, 1210

U1

1

Charger controller, 20 pin VQFN, TI, bq24725RGR

U2

1

Dual digital transistor, 40V, 30mA, SC-74, Rohm, IMD2A

APPLICATION INFORMATION

Reverse Input Voltage Protection

Q6, R12 and R13 in Figure 1 gives system and IC protection from reversed adapter voltage. In normal operation,

Q6 is turned off by negative Vgs. When adapter voltage is reversed, Q6 Vgs is positive. As a result, Q6 turns on

to short gate and source of Q2 so that Q2 is off. Q2 body diode blocks negative voltage to system. However,

CMSRC and ACDRV pins need R10 and R11 to limit the current due to the ESD diode of these pins when turned

on. Q6 must has low Vgs threshold voltage and low Qgs gate charge so it turns on before Q2 turns on. R10 and

R11 must have enough power rating for the power dissipation when the ESD diode is on. In Figure 2, the

Schottky diode D3 gives the reverse adapter voltage protection, no extra small MOSFET and resistors are

needed.

Reduce Battery Quiescent Current

When the adapter is not present, if VCC is powered with voltage higher than UVLO directly or indirectly (such as

through a LDO or switching converter) from battery, the internal BATFET charge pump gives the BATFET pin 6V

higher voltage than the SRN pin to drive the n-channel BATFET. As a result, the battery has higher quiescent

current. This is only necessary when the battery powers the system due to a high system current that goes

through the MOSFET channel instead of the body diode to reduce conduction loss and extend the battery

working life. When the system is totally shutdown, it is not necessary to let the internal BATFET charge pump

work. The host controller can use a digital signal EN to disconnect the battery power path to the VCC pin by U2

in Figure 1. As a result, battery quiescent current can be minimized. The host controller still can get power from

BATFET body diode because the total system current is the lowest when the system is shutdown, so there is no

high conduction loss of the body diode.

Inductor Selection

The bq24725 has three selectable fixed switching frequency. Higher switching frequency allows the use of

smaller inductor and capacitor values. Inductor saturation current should be higher than the charging current

(4)

inductance (L):

(5)

26

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