Micrel, Inc. MIC5305
June 2007 9
M9999-062507
Application Information
Enable/Shutdown
The MIC5305 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. The active-high
enable pin uses CMOS technology and the enable pin
cannot be left floating; a floating enable pin may cause
an indeterminate state on the output.
Input Capacitor
The MIC5305 is a high-performance, high bandwidth
device. Therefore, it requires a well-bypassed input
supply for optimal performance. A 1µF capacitor is
required from the input to ground to provide stability.
Low-ESR ceramic capacitors provide optimal perform-
ance at a minimum of space. 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.
Output Capacitor
The MIC5305 requires an output capacitor of 1µF or
greater to maintain stability. The design is optimized for
use with low-ESR ceramic chip capacitors. High ESR
capacitors 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 recom-
mended 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.
Bypass Capacitor
A capacitor can be placed from the noise bypass pin to
ground to reduce output voltage noise. The capacitor
bypasses the internal reference. A 0.1µF capacitor is
recommended for applications that require low-noise
outputs. The bypass capacitor can be increased, further
reducing noise and improving PSRR. Turn-on time
increases slightly with respect to bypass capacitance. A
unique, quick-start circuit allows the MIC5305 to drive a
large capacitor on the bypass pin without significantly
slowing turn-on time. Refer to the Typical Characteristics
section for performance with different bypass capacitors.
No-Load Stability
Unlike many other voltage regulators, the MIC5305 will
remain stable and in regulation with no load. This is
especially import in CMOS RAM keep-alive applications.
Adjustable Regulator Application
Adjustable regulators use the ratio of two resistors to
multiply the reference voltage to produce the desired
output voltage. The MIC5305 can be adjusted from
1.25V to 5.5V by using two external resistors (Figure 1).
The resistors set the output voltage based on the
following equation:
⎟
⎠
⎞
⎜
⎝
⎛
+=
R2
R1
1VV
REFOUT
V
REF
= 1.25V
MIC5305BML
VOUTVIN
ADJEN
GND
IN
V
OUT
R1
1µF
R2
1µF
Figure 1. Adjustable Voltage Application
Thermal Considerations
The MIC5305 is designed to provide 150mA of conti-
nuous 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 5.0V, the output
voltage is 2.9V and the output current = 150mA.
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
Because this device is CMOS and the ground current is
typically <100µA over the load range, the power
dissipation contributed by the ground current is < 1%
and can be ignored for this calculation.
P
D
= (5.0V – 2.9V) × 150mA P
D
= 0.32W
To determine the maximum ambient operating temp-
erature 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)
= 125°C, the max. junction temperature of
the die.
θ
JA
thermal resistance = 93°C/W
