January 2002 7 MIC5235
MIC5235 Micrel
Applications Information
Enable/Shutdown
The MIC5235 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 Capacitor
The MIC5235 has high input voltage capability up to 24V. 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 MIC5235 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 an
X7R ceramic capacitor to ensure the same minimum capaci-
tance over the equivalent operating temperature range.
No-Load Stability
The MIC5235 will remain stable and in regulation with no load
unlike many other voltage regulators. This is especially
important in CMOS RAM keep-alive application.
Thermal Consideration
The MIC5235 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
TT
D(MAX)
J(MAX) A
JA
=
−
θ
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 examples of the junction-
to-ambient thermal resistance for the MIC5235.
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
IN
I
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 MIC5235-3.0BM5 at 50°C with
a minimum footprint layout, the maximum input voltage for a
set output current can be determined as follows:
P
125 C 50 C
235 C/W
D(MAX)
=
°− °
°
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. Since the MIC5235 was designed
to operate with high input voltages, careful consideration
must be given so as not to overheat the device. With Very high
input-to-output voltage 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
Due to the potential for input voltages up to 24V, ground
current must be taken into consideration.
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
– 3V)150mA + V
IN
x 2.8mA
Ground pin current is estimated using the typical character-
istics of the device.
769mW = V
IN
(152.8mA)
V
IN
= 5.03V
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
– 3V)20mA + V
IN
x 0.2mA
379mW = V
IN
x 20.2mA
V
IN
= 18.8V
Maximum input voltage for a 20mA load current at 50°C
ambient temperature is 18.8V, utilizing virtually the entire
operating voltage range of the device.
