MIC5238 Datasheet, PDF (8/10 Pages) Micrel Semiconductor – Ultra-Low Quiescent Current, 150mA uCap LDO Regulator

MIC5238
Applications Information
Enable/Shutdown
The MIC5238 comes with an active-high enable pin that
allows the regulator to be disabled. Forcing the enable pin low
disables the regulator and sends it into a “zero” off-mode-
current state. In this state, current consumed by the regulator
goes nearly to zero. Forcing the enable pin high enables the
output voltage.
Input Bias Capacitor
The input capacitor must be rated to sustain voltages that
may be used on the input. An input capacitor may be required
when the device is not near the source power supply or when
supplied by a battery. Small, surface mount, ceramic capaci-
tors can be used for bypassing. Larger values may be
required if the source supply has high ripple.
Output Capacitor
The MIC5238 requires an output capacitor for stability. The
design requires 2.2µF or greater on the output to maintain
stability. The design is optimized for use with low-ESR
ceramic chip capacitors. High ESR capacitors may cause
high frequency oscillation. The maximum recommended
ESR is 3Ω. The output capacitor can be increased without
limit. Larger valued capacitors help to improve transient
response.
X7R/X5R dielectric-type ceramic capacitors are recom-
mended because of their temperature performance. X7R-
type capacitors change capacitance by 15% over their oper-
ating 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 capaci-
tor with Y5V dielectric, the value must be much higher than a
X7R ceramic capacitor to ensure the same minimum capaci-
tance over the equivalent operating temperature range.
No-Load Stability
The MIC5238 will remain stable and in regulation with no load
unlike many other voltage regulators. This is especially
important in CMOS RAM keep-alive applications.
Thermal Considerations
The MIC5238 is designed to provide 150mA of continuous
current in a very small package. Maximum power dissipation
can be calculated based on the output current and the voltage
drop across the part. To determine the maximum power
dissipation of the package, use the junction-to-ambient ther-
mal resistance of the device and the following basic equation:
P_D(MAX)
=


T_J(MAX) −
θ_JA
T_A


T_J(MAX) is the maximum junction temperature of the die,
125°C, and T_A is the ambient operating temperature. θ_JA is
layout dependent; Table 1 shows the junction-to-ambient
thermal resistance for the MIC5238.
Micrel
Package
θ_JA Recommended
Minimum Footprint
SOT-23-5
235°C/W
Table 1. SOT-23-5 Thermal Resistance
The actual power dissipation of the regulator circuit can be
determined using the equation:
P_D = (V_IN – V_OUT)I_OUT + V_INI_GND
Substituting P_D(MAX) for P_D and solving for the operating
conditions that are critical to the application will give the
maximum operating conditions for the regulator circuit. For
example, when operating the MIC5238-1.0BM5 at 50°C with
a minimum footprint layout, the maximum input voltage for a
set output current can be determined as follows:
P_D(MAX)
=


125°C − 50°C
235°C/W 
P_D(MAX) = 319mW
The junction-to-ambient (θ_JA) thermal resistance for the
minimum footprint is 235°C/W, from Table 1. It is important
that the maximum power dissipation not be exceeded to
ensure proper operation. With very high input-to-output volt-
age differentials, the output current is limited by the total
power dissipation. Total power dissipation is calculated using
the following equation:
P_D = (V_IN – V_OUT)I_OUT + V_IN x I_GND + V_BIAS x I_BIAS
Since the bias supply draws only 18µA, that contribution can
be ignored for this calculation.
If we know the maximum load current, we can solve for the
maximum input voltage using the maximum power dissipa-
tion calculated for a 50°C ambient, 319mV.
P_DMAX = (V_IN – V_OUT)I_OUT + V_IN x I_GND
319mW = (V_IN – 1V)150mA + V_IN x 2.8mA
Ground pin current is estimated using the typical character-
istics of the device.
469mW = V_IN (152.8mA)
V_IN = 3.07V
For higher current outputs only a lower input voltage will work
for higher ambient temperatures.
Assuming a lower output current of 20mA, the maximum input
voltage can be recalculated:
319mW = (V_IN – 1V)20mA + V_IN x 0.2mA
339mW = V_IN x 20.2mA
V_IN = 16.8V
Maximum input voltage for a 20mA load current at 50°C
ambient temperature is 16.8V. Since the device has a 6V
rating, it will operate over the whole input range.
Dual Suppy Mode Efficiency
By utilizing a bias supply the conversion efficiency can be
greatly enhanced. This can be realized as the higher bias
supply will only consume a few µA’s while the input supply will
require a few mA’s! This equates to higher efficiency saving
valuable power in the system. As an example, consider an
output voltage of 1V with an input supply of 2.5V at a load
MIC5238
8
August 2003