SMB239
APPLICATIONS INFORMATION (CONTINUED)
EXTERNAL COMPONENTS
Input and Output Capacitors
Actual power dissipation can be calculated by using the
following formula:
The SMB239 allows for the use of low-cost ceramic
capacitors on both the input and the output. The
minimum input capacitance value is 4.7µF. The
minimum output capacitance of 4.7µF is desired in
parallel with the battery installed on the BATT pin.
Taking account of the temperature degrading
characteristics of ceramic capacitors, one is
encouraged to select X5R or X7R rated ceramic
capacitors.
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:
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.
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
POWER DISSIPATION
Where:
The SMB239 incorporates a thermal regulation circuit
that reduces charge current when die temperature rises
to high levels (greater than 110oC). 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.
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 60oC/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 oC
Summit Microelectronics, Inc
2129 3.0 2/23/2009
14
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
