MIC3775_06 Datasheet, PDF (11/13 Pages) Micrel Semiconductor – 750mA μCap Low-Voltage Low-Dropout Regulator

Micrel, Inc.
MIC3775
with an input as low as 3.0V or 2.5V respectively. This
gives the PNP-based regulators a distinct advantage
over older, NPN-based linear regulators.
Minimum Load Current
The MIC3775 regulator is specified between finite loads.
If the output current is too small, leakage currents
dominate and the output voltage rises. A 10mA minimum
load current is necessary for proper regulation.
Adjustable Regulator Design
V_OUT
= 1.240V⎜⎛1 +
⎝
R1 _⎟⎞
R2 _⎠
Figure 2. Adjustable Regulator with Resistors
The MIC3775 allows programming the output voltage
anywhere between 1.24V and the 6V maximum
operating rating of the family. Two resistors are used.
Resistors can be quite large, up to 1MΩ, because of the
very high input impedance and low bias current of the
sense comparator. The resistor values are calculated by:
R1
=
R2_{⎜⎜⎝⎛}
V_OUT
1.240
−
1_{⎟⎟⎠⎞}
Where V_O is the desired output voltage. Figure 2 shows
component definition. Applications with widely varying
load currents may scale the resistors to draw the
minimum load current required for proper operation (see
above).
Power MSOP-8 Thermal Characteristics
One of the secrets of the MIC3775’s performance is its
power MSOP-8 package featuring half the thermal
resistance of a standard MSOP-8 package. Lower
thermal resistance means more output current or higher
input voltage for a given package 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
(junction-to-case thermal resistance) and θ_CA (case-to-
ambient thermal resistance). See Figure3. 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).
December 2006
Using the power MSOP-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 power
MSOP-8 has a θ_JA of 80°C/W, this is significantly lower
than the standard MSOP-8 which is typically 160°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 heatsink must be used.
Figure 3. Thermal Resistance
Figure 4 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 deter-
mined. The maximum allowable temperature rise must
be calculated to determine operation along which curve.
Figure 4. Copper Area vs. Power-MSOP
Power Dissipation (∆T_JA)
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