SMB239 datasheet(14/30 Pages) SUMMIT

14 / 30 page

SMB239

Summit Microelectronics, Inc

2129 3.0 2/23/2009

EXTERNAL COMPONENTS

Input and Output Capacitors

The SMB239 allows for the use of low-cost ceramic

capacitors on both the input and the output. The

minimum input capacitance value is 4.7

µF. The

minimum output capacitance of 4.7

µF is desired in

parallel with the battery installed on the BATT pin.

Taking

account

the

temperature

degrading

characteristics

ceramic

capacitors,

one

encouraged to select X5R or X7R rated ceramic

capacitors.

BOARD LAYOUT RECOMMENDATIONS

The most critical components for the reliable operations

of the SMB239 are the output capacitor and the input

capacitor. Place those as close as possible to the

SMB239. 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

BATT pin to the positive terminal of the battery by

traces wider than 10mils.

POWER DISSIPATION

The SMB239 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 SMB239 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 SMB239 operates from a 5V

±10% (worst

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

current of 200mA to a discharged Li-Ion battery with a

voltage of 3.6V, the power dissipation can be calculated

as follows:

PDACTUAL = (5.5V – 3.5V) x 0.3A = 600mW

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

SMB239 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 – 3.5V) x 0.30A x 60oC/W

= 74

APPLICATIONS INFORMATION (CONTINUED)

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