Micrel, Inc.

MIC5318

January 2007

8

M9999-011207

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

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:

⎟

⎠

⎞

⎜

⎝

⎛ +

=

R2

R1

1

V

V

REF

OUT

VREF = 1.25V

MIC5318YMT

VOUT

VIN

ADJ

EN

GND

R1

1µF

R2

1µF

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:

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.

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: