MIC5259

Micrel, Inc.

M9999-051305

8

May 2005

Applications Information

Enable/Shutdown

The MIC5259 comes with an active-high enable pin that al-

lows the regulator to be disabled. Forcing the enable pin low

disables the regulator and sends it into a “zero” off-mode-cur-

rent state. In this state, current consumed by the regulator

goes nearly to zero. Forcing the enable pin high enables

the output voltage. This part is CMOS and the enable pin

cannot be left floating; a floating enable pin may cause an

indeterminate state on the output.

Input Capacitor

The MIC5259 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 performance at a minimum of space. Addi-

tional 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 MIC5259 requires an output capacitor for stability. The

designrequires1µForgreaterontheoutputtomaintainstabil-

ity.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 300mΩ.

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-

mendedbecauseoftheirtemperatureperformance.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 operat-

ing 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 is required from the noise bypass pin to ground

to reduce output voltage noise. The capacitor bypasses

the internal reference. A 0.01µF capacitor is recommended

for applications that require low-noise outputs. The bypass

capacitor can be increased, further reducing noise and im-

proving PSRR. Turn-on time increases slightly with respect

to bypass capacitance. A unique quick-start circuit allows

the MIC5259 to drive a large capacitor on the bypass pin

without significantly slowing turn-on time. Refer to the “Typi-

cal Characteristics” section for performance with different

bypass capacitors.

Active Shutdown

The MIC5259 also features an active shutdown clamp, which

is an N-Channel MOSFET that turns on when the device is

disabled. This allows the output capacitor and load to dis-

charge, de-energizing the load.

No-Load Stability

The MIC5259 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 MIC5259 is designed to provide 300mA 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 dis-

sipation of the package, use the junction-to-ambient thermal

resistance of the device and the following basic equation:









layout dependent; Table 1 shows examples of junction-to-

ambient thermal resistance for the MIC5259.

Package

Minimum Footprint

Copper Clad

SOT-23-5

235°C/W

185°C/W

145°C/W

(M5 or D5)

MLF (ML)

90°C/W

Table 1. Thermal Resistance

The actual power dissipation of the regulator circuit can be

determined using the equation:

conditions that are critical to the application will give the

maximum operating conditions for the regulator circuit. For

example, when operating the MIC5259-2.8BML at 70°C with

a minimum footprint layout, the maximum input voltage for a

set output current can be determined as follows:

P (max)

D

=









90°C/W

125°C − 70°C

The junction-to-ambient thermal resistance for the minimum

footprint is 90°C/W, from Table 1. The maximum power dis-

sipation must not be exceeded for proper operation. Using

the output voltage of 2.8V and an output current of 200mA,

the maximum input voltage can be determined. Because this

deviceisCMOSandthegroundcurrentistypically110µAover

theloadrange,thepowerdissipationcontributedbytheground

current is <1% and can be ignored for this calculation.

Therefore, a 2.8V application at 200mA of output current can

accept a maximum input voltage of 5.85V in an MLF package.

For a full discussion of heat sinking and thermal effects on

voltage regulators, refer to the “Regulator Thermals” section

of Micrel’s Designing with Low-Dropout Voltage Regulators

handbook.