MIC47050 Datasheet, PDF (10/14 Pages) Micrel Semiconductor – 500mA ULDO with Low Input and Low Output Voltage

Micrel, Inc.
Power Good
The PGOOD pin is an open drain output that goes low
when the output voltage (fixed version) drops below the
PGOOD threshold voltage.
The pull-up resistor value should be large enough to
guarantee a proper “low” voltage when the PGOOD pin
pulls low. The PGOOD low voltage is typically 0.1V at
250uA current. A 10k resistor or greater is
recommended when pulling up to 3.3V bias.
If the Power Good function is not required, the PGOOD
pin may be left unconnected.
Thermal Shutdown
The MIC47050 has an internal over-temperature
protection feature. This feature is for protection only.
The device should never be intentionally operated near
this temperature as this may reduce long term reliability.
The device will turn off when the over-temperature
threshold is exceeded. A 20°C hysteresis is built in to
allow the device to cool before turning back on.
Thermal Considerations
The MIC47050 is designed to provide 0.5A of continuous
current in a very small package. Maximum ambient
operating temperature can be calculated based on the
output current and the voltage drop across the part.
Given that the input voltage is 1.8V, the output voltage is
1.2V and the output current is 0.5A. The actual power
dissipation of the regulator circuit can be determined
using the equation:
( ) PD = VIN − VOUT × IOUT + VIN × IGND + VBIAS × IBIAS
Because this device is CMOS, the ground current is
insignificant for power dissipation and can be ignored for
this calculation.
( ) PD = 1.8V − 1.2V × 0.5A = 0.3W
To determine the maximum ambient operating
temperature of the package, use the junction-to-ambient
thermal resistance of the device and the following basic
equation:
PD(MAX)
=
⎡
⎢
TJ(MAX)
−
TA
⎢⎣ ^Θ_JA
⎤
⎥
⎥⎦
MIC47050
T_J(MAX) = 125°C, the maximum junction temperature of
the die.
θ_JA thermal resistance = 90°C/W.
Table 1 shows junction-to-ambient thermal resistance for
the MIC47050 in the MLF^® package.
Table 1. Thermal Resistance
Package
θ_JA Recommended
Min. Footprint
6-pin 2mm x 2mm MLF^®
90°C/W
θ_JC
45°C/W
Substituting P_D for P_D(max) and solving for the ambient
operating temperature will give the maximum operating
conditions for the regulator circuit. The junction-to-
ambient thermal resistance for the minimum footprint is
90°C/W. The maximum power dissipation must not be
exceeded for proper operation. For example, when
operating the MIC47050-1.2YML at an input voltage of
1.8V and a 0.5A load with a minimum footprint layout,
the maximum ambient operating temperature T_A can be
determined as follows:
TA = TJ(MAX) − Θ_JA × PD(MAX)
TA = 125°C − 90°C/W × 0.3W
TA = 98°C
Therefore, a 1.2V application with 0.5A of output current
can accept an ambient operating temperature of 98°C in
a 2mm x 2mm MLF^® package.
Thermal Measurements
Measuring the IC’s case temperature is recommended to
insure it is within its operating limits. Although this might
seem like a very elementary task, it is easy to get
erroneous results. The most common mistake is to use
the standard thermal couple that comes with a thermal
meter. This thermal couple wire gauge is large, typically
22 gauge, and behaves like a heatsink, resulting in a
lower case measurement.
Two methods of temperature measurement are using a
smaller thermal couple wire or an infrared thermometer.
If a thermal couple wire is used, it must be constructed
of 36 gauge wire or higher (smaller wire size) to
minimize the wire heat-sinking effect. In addition, the
thermal couple tip must be covered in either thermal
grease or thermal glue to make sure that the thermal
couple junction is making good contact with the case of
the IC. Omega brand thermal couple (5SC-TT-K-36-36)
is adequate for most applications.
March 2010
10
M9999-032310-A