August, 2004

9

M0385-080204

MIC5255

Micrel

Applications Information

Enable/Shutdown

The MIC5255 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. This part is CMOS and the enable pin cannot

be left floating; a floating enable pin may cause an indetermi-

nate state on the output.

Input Capacitor

The MIC5255 is a high performance, high bandwidth device.

Therefore, it requires a well-bypassed input supply for opti-

mal performance. A 1

µF 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 MIC5255 requires an output capacitor for stability. The

design requires 1

µF 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 300m

Ω. The output capacitor can be increased, but

performance has been optimized for a 1

µF 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 oper-

ating 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 capaci-

tor with Y5V dielectric, the value must be much higher than an

X7R ceramic capacitor to ensure the same minimum capaci-

tance 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 by-

passes the internal reference. A 0.01

µF capacitor is recom-

mended 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 MIC5255 to drive a large capacitor on the bypass

pin without significantly slowing turn-on time. Refer to the

ent bypass capacitors.

Active Shutdown

The MIC5255 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 MIC5255 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 MIC5255 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 ther-

mal resistance of the device and the following basic equation:

P (max)

T (max) T

D

JA

JA

=

−









θ

T

125

°C, and T

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

ambient thermal resistance for the MIC5255.

Package

θθθθθ

θθθθθ

θθθθθ

JC

Minimum Footprint

Copper Clad

SOT-23-5

235

°C/W

185

°C/W

145

°C/W

(M5 or D5)

Table 1. SOT-23-5 Thermal Resistance

The actual power dissipation of the regulator circuit can be

determined using the equation:

P

Substituting P

conditions that are critical to the application will give the

maximum operating conditions for the regulator circuit. For

example, when operating the MIC5255-3.0BM5 at 50

°C with

a minimum footprint layout, the maximum input voltage for a

set output current can be determined as follows:

P (max)

125 C

50 C

235 C/W

D

=

°−

°

°









P

The junction-to-ambient thermal resistance for the minimum

footprint is 235

°C/W, from Table 1. The maximum power

dissipation must not be exceeded for proper operation. Using

the output voltage of 3.0V and an output current of 150mA,

the maximum input voltage can be determined. Because this

device is CMOS and the ground current is typically 100

µA

over the load range, the power dissipation contributed by the

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

315mW = (V

315mW = V

810mW = V

V

Therefore, a 3.0V application at 150mA of output current can

accept a maximum input voltage of 5.4V in a SOT-23-5

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 Regu-

lators handbook.