MIC5318 Datasheet, PDF (8/11 Pages) Micrel Semiconductor – High Performance 300mA uCap ULDO TM

Micrel, Inc.
Application Information
Enable/Shutdown
The MIC5318 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 MIC5318 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 MIC5318 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
recommended because of their temperature perform-
ance. 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
January 2007
MIC5318
bypass capacitance. A unique, quick-start circuit
allows the MIC5318 to drive a large capacitor on the
bypass pin without significantly slowing turn-on time.
Refer to the Typical Characteristics subsection for
performance with different bypass capacitors.
No-Load Stability
Unlike many other voltage regulators, the MIC5318
will remain stable and in regulation with no load. This
is especially crucial for 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 MIC5318 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:
V_OUT
=
V_REF
⎜⎛1
⎝
+
R1 _⎟⎞
R2 _⎠
VREF = 1.25V
V_IN
1µF
MIC5318YMT
VIN VOUT
EN ADJ
GND
V_OUT
R1
1µF
R2
Figure 1. Adjustable Voltage Output
Thermal Considerations
The MIC5318 is designed to provide 300mA of
continuous current. 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 3.3V, the output voltage is
2.8V and the output current = 300mA.
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 = (3.3V – 2.8V) × 300mA
P_D = 0.15W
To determine the maximum ambient operating
temperature of the package, use the junction-to-
ambient thermal resistance of the device and the
following basic equation:
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