SMB139 Datasheet, PDF (13/27 Pages) Summit Microelectronics, Inc. – Programmable Linear Battery Charger in 1.3 x 2.1 uCSP

SMB139
APPLICATIONS INFORMATION (CONTINUED)
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 2.2µF. The
minimum output capacitance of 2.2µF is desired in
parallel with the battery installed on the BATT pin. A
1µF 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:
PD_ACTUAL = (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:
PD_ACTUAL = (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:
PD_MAXIMUM = (TJ – TA) / Theta_JA
Where:
TJ = maximum allowable junction (silicon) temperature
TA = maximum ambient temperature
Theta_JA = 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 Theta_JA of 60^oC/W, the ambient temperature
can be calculated as follows:
TA = TJ – (PD_MAXIMUM x Theta_JA)
= TJ – (VIN – VBATT) x IOUT x Theta_JA
= 110 ^oC – (5.5V – 3.6V) x 0.12A x 60^oC/W
= 96.32 ^oC
Summit Microelectronics, Inc
2121 3.0 6/19/2008
13