MIC4555_11 Datasheet, PDF (13/16 Pages) Micrel Semiconductor – SIM Card Level Shifter with 50mA LDO

Micrel Inc.
Application Information
The MIC4555 is a digital level shifter with a 50mA LDO
for SIM card interfaces. There are three high-speed level
shifters that can convert input voltages from the
controller and then level shift it to either 1.8V or 3.0V for
the SIM card. A voltage select pin (V_SEL) selects the
output voltage. The MIC4555 also has a 50mA LDO that
can be used to power the SIM card.
Input Capacitor
An input capacitor of 1µF is required from the V_BAT to
ground to provide stability. Low-ESR ceramic capacitors
provide optimal performance with minimum board area.
Additional high-frequency capacitors, such as small
valued NPO dielectric type capacitors, help filter out
high-frequency noise and are good practice in any RF-
based circuit. X5R or X7R dielectrics are recommended
for the input capacitor. Y5V dielectrics lose most of their
capacitance over temperature and are therefore, not
recommended.
Output Capacitor
The MIC4555 requires an output capacitor of 1µF or
greater for V_CC to maintain stability. The design is
optimized for use with low-ESR ceramic-chip capacitors.
High-ESR capacitors are not recommended because
they may cause high-frequency oscillation. The output
capacitor can be increased, but performance has been
optimized for a 1µF ceramic output capacitor and does
not improve significantly with larger capacitance.
X7R/X5R dielectric type ceramic capacitors are
recommended because of their temperature
performance. X7R-type capacitors change capacitance
by 15% over their operating temperature range and are
the most stable type of ceramic capacitors. Z5U and
Y5V dielectric capacitors change value by as much as
50% and 60%, respectively, over their operating
temperature ranges. To use a ceramic-chip capacitor
with Y5V dielectric, the value must be much higher than
an X7R ceramic capacitor to ensure the same minimum
capacitance over the equivalent operating temperature
range.
Thermal Considerations
The MIC4555 is designed to provide output current up to
50mA. The maximum ambient operating temperature
can be calculated based upon the output current and the
voltage drop across the part. For example if the input
voltage (V_BAT) is 3.6V and the output voltage (V_CC) is
1.8V at 50mA, the power dissipation of the regulator
circuit can be determined using the equation:
P_D = (V_BAT – V_CC) I _OUT + V_BAT I_GND
MIC4555
As the MIC4555 is a CMOS device, the ground current is
typically <100µA over the load range, the power
dissipation contributed by the ground current is < 1% and
may be ignored for this particular calculation.
Example:
V_BAT = 3.6V
V_CC = 1.8V
I_OUT = 50mA
P_D = (3.6V – 1.8V)50mA
P_D = 0.09W
To determine the maximum operating ambient
temperature of the package, use the junction to ambient
thermal resistance of the device and the following basic
equation:
P_D(max)
=
⎜⎜⎝⎛
T_J(max) −
θ _JA
T_A
⎟⎟⎠⎞
T_J(max) = 125°C, the maximum junction temperature of the
die, and θ_JA thermal resistance = 59°C/W for the Thin
MLF^® package.
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 maximum power dissipation must not be exceeded
for proper operation.
For example, when operating the MIC4555YMT at an
input voltage of 3.6V and 50mA load with a minimum
footprint layout, the maximum ambient operating
temperature T_A can be determined as follows:
0.09W = (125°C – T_A)/(59°C/W)
T_A = 119.69°C
Therefore, the maximum ambient operating temperature
of 119.69°C is allowed in a 3mm x 3mm thin MLF^®
package. For a full discussion of heat sinking and
thermal effects on voltage regulators, refer to the
“Regulator Thermals” section of Micrel’s Designing with
Low Dropout Voltage Regulators handbook. This
information can be found on Micrel's website at:
http://www.micrel.com/_PDF/other/LDOBk_ds.pdf
September 2011
13
M9999-092311-B