REV. 0

ADP3408

–15–

Appropriate sense resistors are available from the following

vendors:

Vishay Dale

IRC

Panasonic

Charger FET Selection

The type and size of the pass transistor is determined by the

threshold voltage, input-output voltage differential, and the

charge current. The selected PMOS must satisfy the physical,

electrical and thermal design requirements.

can provide must be enough to turn on the FET. The available

gate drive voltage can be estimated using the following:

VV

V

V

GS

ADAPTER MIN

GATEDR

SENSE

=−

−

()

(8)

where:

high current limit threshold voltage.

the blocking diode, the sense resistor, and the ON resistance of

the FET at maximum charge current, where:

VV

V

V

V

DS

ADAPTER MIN

DIODE

SENSE

BAT

=−

−

−

()

(9)

R

V

I

DS ON

DS

CHR MAX

()

()

=

(10)

The thermal characteristics of the FET must be considered

next. The worst-case dissipation can be determined using:

P

V

V

V

UVLO

I

DISS

ADAPTER MAX

DIODE

SENSE

CHR

=−

−

−

()

(11)

It should be noted that the adapter voltage can be either

preregulated or nonregulated. In the preregulated case the

difference between the maximum and minimum adapter voltage

is probably not significant. In the unregulated case, the adapter

voltage can have a wide range specified. However, the maximum

voltage specified is usually with no load applied. So, the worst-case

power dissipation calculation will often lead to an over-specified

pass device. In either case, it is best to determine the load

characteristics of the adapter to optimize the charger design.

For example:

Therefore, choose a low threshold voltage FET.

VV

V

V

V

VV

V

V

mV

R

V

I

mV

mA

m

P

V

V

V

UVLO

I

PV

V

V

A

DS

ADAPT MIN

DIODE

SENSE

BAT

DS ON

DS

CHR MAX

DISS

ADAPT MAX

DIODE

SENSE

CHR

DISS

==

=

==

=

=

=

()

()

()

()

––

–

– .

– .

– .

––

–

.

– .

– .

– ..

.

5

0 5

0 160

4 2

140

140

800

175

65

05

0160

32

08

211

Ω

W

W

Appropriate PMOS FETs are available from the following

vendors:

Siliconix

IR

Fairchild

Charger Diode Selection

The diode, D1, shown in Figure 2, is used to prevent the battery from

discharging through the PMOS’ body diode into the charger’s

internal bias circuits. Choose a diode with a current rating high

enough to handle the battery charging current and a voltage

rating greater than VBAT. The blocking diode is required for

both lithium and nickel battery types.

Printed Circuit Board Layout Considerations

Use the following general guidelines when designing printed

circuit boards:

1. Connect the battery to the VBAT, VBAT2, and VRTCIN

pins of the ADP3408. Locate the input capacitor as close to

the pins as possible.

2. VAN and VTCXO capacitors should be returned to AGND.

3. VCORE, VMEM and VSIM capacitors should be returned

to DGND.

4. Split the ground connections. Use separate traces or planes

for the analog, digital, and power grounds and tie them together

at a single point, preferably close to the battery return.

5. Run a separate trace from the BATSNS pin to the battery to

prevent voltage drop error in the MVBAT measurement.

6. Kelvin-connect the charger’s sense resistor by running sepa-

rate traces to the CHRIN and ISENSE pins. Make sure that the

traces are terminated as close to the resistor’s body as possible.

7. Use the best industry practice for thermal considerations

during the layout of the ADP3408 and charger components.

Careful use of copper area, weight, and multilayer construc-

tion all contribute to improved thermal performance.