MIC5236_05 Datasheet, PDF (10/12 Pages) Micrel Semiconductor – Low Quiescent Current μCap LDO Regulator

MIC5236
V_IN
5V _200k
SHUTDOWN
ENABLE
1N4148
200k
MIC5236
IN OUT
EN ERR
GND
4.7µF
V_ERR
V_OUT
C_OUT
Figure 4. Remote Enable with Short-Circuit
Current Foldback
Thermal Characteristics
The MIC5236 is a high input voltage device, intended to
provide 150mA of continuous output current in two very small
profile packages. The power SOIC-8 and power MSOP-8 al-
low the device to dissipate about 50% more power than their
standard equivalents.
Power SOIC-8 Thermal Characteristics
One of the secrets of the MIC5236’s performance is its power
SO-8 package featuring half the thermal resistance of a
standard SO-8 package. Lower thermal resistance means
more output current or higher input voltage for a given pack-
age size.
Lower thermal resistance is achieved by joining the four
ground leads with the die attach paddle to create a single-
piece electrical and thermal conductor. This concept has
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 (junc-
tion-to-case thermal resistance) and θ_CA (case-to-ambient
thermal resistance). See Figure 5. 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 (case-
to-sink thermal resistance) and θ_SA (sink-to-ambient thermal
resistance).
SOP-8
q_JA
q_JC
q_CA
ground plane
heat sink area
AMBIENT
printed circuit board
Figure 5. Thermal Resistance
Using the power SOIC-8 reduces the θ_JC dramatically 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 capabil-
ity of the device. Typically, the power SOIC-8 has a θ_JC of
20°C/W, this is significantly lower than the standard SOIC-8
Micrel, Inc.
which is typically 75°C/W. θ_CA is reduced because pins 5
through 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.
Low-dropout linear regulators from Micrel are 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.
900
800
700
600
500
400
300
200
100
00 _{0.25 0.50 0.75 1.00 1.25 1.50}
POWER DISSIPATION (W)
Figure 6. Copper Area vs. Power-SOIC
Power Dissipation (∆T_JA)
Figure 6 shows copper area versus power dissipation with
each trace corresponding to a different temperature rise
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.
ΔT = 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 50°C,
the ΔT is determined as follows:
ΔT = 125°C – 50°C
ΔT = 75°C
Using Figure 6, the minimum amount of required copper can
be determined based on the required power dissipation. Power
dissipation in a linear regulator is calculated as follows:
P_D = (V_IN – V_OUT) I_OUT + V_IN · I_GND
If we use a 3V output device and a 28V input at moderate
output current of 25mA, then our power dissipation is as
follows:
P_D = (28V – 3V) × 25mA + 28V × 250µA
P_D = 625mW + 7mW
P_D = 632mW
From Figure 6, the minimum amount of copper required to
operate this application at a ΔT of 75°C is 25mm².
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 7, which shows safe
operating curves for three different ambient temperatures:
MIC5236
10
July 2005