MIC79050 Datasheet, PDF (13/20 Pages) Micrel Semiconductor – Simple Lithium-Ion Battery Charger Preliminary Information

MIC79050
been used by MOSFET manufacturers for years, proving
very reliable and cost effective for the user.
Thermal resistance consists of two main elements, θ_JC, or
thermal resistance junction to case and θ_CA, thermal resis-
tance case to ambient (Figure 8). θ_JC is the resistance from
the die to the leads of the package. θ_CA is the resistance from
the leads to the ambient air and it includes θ_CS, thermal
resistance case to sink, and θ_SA, thermal resistance sink to
ambient. Using the power SOP-8 reduces the θ_JC dramati-
cally and allows the user to reduce θ_CA. The total thermal
resistance, θ _JA, junction to ambient thermal resistance, is the
limiting factor in calculating the maximum power dissipation
capability of the device. Typically, the power SOP-8 has a θ_JC
of 20°C/W, this is significantly lower than the standard SOP-
8 which is typically 75°C/W. θ_CA is reduced because pins 5-
8 can now be soldered directly to a ground plane, which
significantly reduces the case to sink thermal resistance and
sink to ambient thermal resistance.
SOP-8
θ_JA
θ_JC
θ_CA
ground plane
heat sink area
AMBIENT
printed circuit board
Figure 8. Thermal Resistance
The MIC79050 is rated to a maximum junction temperature
of 125°C. It is important not to exceed this maximum junction
temperature during operation of the device. To prevent this
maximum junction temperature from being exceeded, the
appropriate ground plane heat sink must be used.
Figure 9 shows curves of copper area versus power dissipa-
tion, each trace corresponding to different temperature rises
above ambient. From these curves, the minimum area of
copper necessary for the part to operate safely can be
determined. The maximum allowable temperature rise must
be calculated to determine operation along which curve.
Micrel
900
800
700 ^{∆TJA =}
600
500
400
300
200
100
0
0 0.25 0.50 0.75 1.00 1.25 1.50
POWER DISSIPATION (W)
Figure 9. Copper Area vs. Power-SOP
Power Dissipation (∆T_JA)
Where ∆T = T_j(max) – T_a(max)
T_j(max) = 125°C
T_a(max) = maximum ambient operating
temperature
For example, the maximum ambient temperature is 40°C, the
∆T is determined as follows:
∆T = +125°C – 40°C
∆T = +85°C
Using Figure 9, the minimum amount of required copper can
be determined based on the required power dissipation.
Power dissipation in a linear regulator is calculated as fol-
lows:
P_D = (Vin-Vout)*Iout + Vin*Ignd
For example, using the charging circuit in Figure 7, assume
the input is a fixed 5V and the output is pulled down to 4.2V
at a charge current of 500mA. The power dissipation in the
MIC79050 is calculated as follows:
P_D = (5V – 4.2V)*0.5A + 5V*0.012A
P_D = 0.460W
From Figure 9, the minimum amount of copper required to
operate this application at a ∆T of 85C is less than 50mm².
Quick Method
Determine the power dissipation requirements for the design
along with the maximum ambient temperature at which the
device will be operated. Refer to Figure 10 , which shows safe
operating curves for 3 different ambient temperatures: +25°C,
+50°C and +85°C. From these curves, the minimum amount
of copper can be determined by knowing the maximum power
dissipation required. If the maximum ambient temperature is
+40°C and the power dissipation is as above, 0.46W, the
curve in Figure 10 shows that the required area of copper is
50mm².
The θ_JA of this package is ideally 63°C/W, but it will vary
depending upon the availability of copper ground plane to
which it is attached.
June 2000
13
MIC79050