MIC5256

Micrel

June 2003

9

MIC5256

Applications Information

Enable/Shutdown

The MIC5256 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 MIC5256 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 MIC5256 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.

Error Flag

The error flag output is an active-low, open-drain output that

drives low when a fault condition AND an undervoltage

detection occurs. Internal circuitry intelligently monitors

overcurrent, overtemperature and dropout conditions and

ORs these outputs together to indicate some fault condition.

The output of that OR gate is ANDed with an output voltage

monitor that detects an undervoltage condition. That output

drives the open-drain transistor to indicate a fault. This

prevents chattering or inadvertent triggering of the error flag.

The error flag must be pulled-up using a resistor from the flag

pin to either the input or the output.

The error flag circuit was designed essentially to work with a

capacitor to ground to act as a power-on reset generator,

signaling a power-good situation once the regulated voltage

was up and/or out of a fault condition. This capacitor delays

the error signal from pulling high, allowing the down stream

circuits time to stablilize. When the error flag is pulled-up to

the input without using a pull-down capacitor, then there can

be a glitch on the error flag upon start up of the device. This

is due to the response time of the error flag circuit as the

device starts up. When the device comes out of the “zero” off

mode current state, all the various nodes of the circuit power

up before the device begins supplying full current to the

output capacitor. The error flag drives low immediately and

then releases after a few microseconds. The intelligent circuit

that triggers an error detects the output going into current limit

AND the output being low while charging the output capacitor.

The error output then pulls low for the duration of the turn-on

time. A capacitor from the error flag to ground will filter out this

glitch. The glitch does not occur if the error flag pulled up to

the output.

Active Shutdown

The MIC5256 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 MIC5256 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 MIC5256 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

TT

D(max)

J(max)

A

JA

=

−









θ

T

125

°C, and T

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

ambient thermal resistance for the MIC5256.

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 MIC5256-3.0BM5 at 50

°C with

a minimum footprint layout, the maximum input voltage for a

set output current can be determined as follows:

P

125 C

5

C

235 C/W

°−

°

°









0

P