Micrel, Inc. MIC5235
May 2008 7
M9999-051508
Application 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 capacitors 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
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
a X7R ceramic capacitor to ensure the same minimum
capacitance 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
applications.
Thermal Considerations
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 thermal resistance
of the device and the following basic equation:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
=
JA
AJ(MAX)
D(MAX)
θ
TT
P
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
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:
⎟
⎠
⎞
⎜
⎝
⎛
°
°−°
=
C/W235
C50C125
P
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
× 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 dissipation
calculated for a 50°C ambient, 319mV.
P
D(MAX)
= (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
characteristics 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:
