MIC37100_07 Datasheet, PDF (12/15 Pages) Micrel Semiconductor – 1A Low-Voltage μCap LDO

Micrel, Inc.
without operating in dropout, NPN-based regulators
require an input voltage of 3.7V at the very least. The
MIC37100 regulator will provide excellent performance
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 MIC37100/01/02 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_IN
ENABLE
SHUTDOWN
MIC37102
IN OUT
R1
EN ADJ
GND
R2
V_OUT
C_OUT
V_OUT
= 1.240V⎜⎛1 +
⎝
R1 _⎟⎞
R2 _⎠
Figure 2. Adjustable Regulator with Resistors
The MIC37102 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 SOIC-8 Thermal Characteristics
One of the secrets of the MIC37101/02’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 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.
MIC37100/37101/37102
Thermal resistance consists of two main elements, θ_JC
(junction-to-case thermal resistance) and θ_CA (case-to-
ambient thermal resistance). See Figure 3. 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).
SOIC-8
JA
JC
CA
ground plane
heat sink area
AMBIENT
printed circuit board
Figure 3. 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 capability 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 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.
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 4. Copper Area vs. Power SO-8
Power Dissipation
September 2007
12
M9999-090607