MIC5234_11 Datasheet, PDF (10/16 Pages) Micrel Semiconductor – Low-Quiescent Current 150mA LDO Regulator

Micrel, Inc.
resistance from the case (ePad) to the ambient air and it
includes θ_CS case (ePad)-to-sink thermal resistance and
θ_SA sink-to-ambient thermal resistance.
Figure 3. Thermal Resistance
Using the ePad 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 capability of the device. Typically, the ePad
SOIC-8 has a θ_JC of 14.7°C/W, this is significantly lower
than the standard SOIC-8 which is typically 48.8°C/W.
θ_CA is reduced because ePad 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.
The maximum allowable temperature rise must be
calculated to determine the required heat sink:
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
MIC5234
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 8V input with an
output current of 150mA, then our power dissipation is
as follows:
P_D = (8V – 3V) × 150mA + (8V × 2.6mA)
P_D = 750mW + 20.8mW
P_D = 770.8mW
A copper plane should be provided to dissipate the heat
to keep junction temperature 125°C.
Adjustable Regulator Application
The MIC5234 can be adjusted from 1.24V to 20V by
using two external resistors (see Figure 4). The resistors
set the output voltage based on Equation 1:
V_OUT
=
V_REF
× ⎜⎛1+ ^R1 ⎟⎞
⎝ ^R2 ⎠
Eq. 1
Where V_REF = 1.23V.
Figure 4. Adjustable Voltage Application
Example:
If output voltage to be set is 3.3V and R2 is selected as
1.2kΩ.
Then
R1
=
R2
×
⎜⎛
⎜⎝
V_OUT
V_REF
− 1_{⎟⎟⎠⎞}
R1 = 1.2kΩ × ⎜⎛ ^3.3V − 1⎟⎞ = 2.019 kΩ
⎝ ^1.23V ⎠
September 2011
10
M9999-092711-B