SMB139 datasheet(13/27 Pages) SUMMIT

13 / 27 page

SMB139

Summit Microelectronics, Inc

2121 3.0 6/19/2008

EXTERNAL COMPONENTS

Input and Output Capacitors

The SMB139 allows for the use of low-cost ceramic

capacitors on both the input and the output. The

minimum input capacitance value is 2.2

µF. The

minimum output capacitance of 2.2

µF is desired in

parallel with the battery installed on the BATT pin. A

µF ceramic capacitor is recommended on the

VDDCAP pin to bypass the internal band-gap voltage.

Taking account of the temperature and DC bias

degrading characteristics of ceramic capacitors, one is

encouraged to select X5R or X7R rated ceramic

capacitors.

BOARD LAYOUT RECOMMENDATIONS

The most critical components for the reliable operations

of the SMB139 are the output capacitor, the input

capacitor, and the bypass capacitor for VDDCAP. Place

those as close as possible to the SMB139. Pour

sufficient copper along the power delivery path, namely,

from the power source to the IN pin and from the OUT

pin to the battery. This minimizes the distribution loss,

therefore buys an additional margin for the IN-to-OUT

drop-out voltage. Route the TRICKLE pin, the SENSE

pin, and the BATT pin to the positive terminal of the

battery by traces wider than 10mils.

To increase ease of layout and future manufacturing,

GND from C2 and D2 can be routed through NC, D1

and a GND via placed just outside the balls, connecting

the GND balls to the GND plane. A via under the CSP

part can cause solder to wick up and push up on the

CSP, preventing a good solder connection to the board.

Additionally VDDCAP (B2) may be run through B3

CHGSET to prevent the need for pad shaving, if

minimum trace widths will not fit between a 0.4mm

pitch.

POWER DISSIPATION

The SMB139 incorporates a thermal regulation circuit

that reduces charge current when die temperature rises

to high levels (greater than 110

oC). The conditions

under which this charge current reduction finds place

can be determined by calculating device power

dissipation. Most of the SMB139 power dissipation is

generated in the internal power MOSFET. The worst-

case scenario occurs when the input voltage is at its

highest level and the device has transitioned from the

pre-charge to the fast-charge phase. In this case, both

the input-to-output differential and the charge current

level are large, resulting in high thermal dissipation.

Actual power dissipation can be calculated by using the

following formula:

PDACTUAL = (VIN – VBATT) x IOUT

Where:

VIN = input (adapter or USB port) voltage

VBATT = battery voltage

IOUT = charge current

Assuming the SMB139 operates from a 5V

±10% (worst

case: 5.5V) supply and is configured to deliver a charge

current of 120mA to a discharged Li-Ion battery with a

voltage of 3.6V, the power dissipation can be calculated

as follows:

PDACTUAL = (5.5V – 3.6V) x 0.12A = 228mW

The maximum allowable power dissipation for a specific

package and board layout can be calculate by using the

following formula:

PDMAXIMUM = (TJ – TA) / ThetaJA

Where:

TJ = maximum allowable junction (silicon) temperature

TA = maximum ambient temperature

ThetaJA = package thermal resistance (depends highly

on board layout)

Combining the two formulas (actual and maximum

allowable power dissipation) allows the user to

calculate the ambient temperature at which the

SMB139 will start reducing charge current for safe

operation.

By using our example above and an

estimated ThetaJA of 60

oC/W, the ambient temperature

can be calculated as follows:

TA = TJ – (PDMAXIMUM x ThetaJA)

= TJ – (VIN – VBATT) x IOUT x ThetaJA

= 110

oC – (5.5V – 3.6V) x 0.12A x 60oC/W

= 96.32

APPLICATIONS INFORMATION (CONTINUED)

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