September 2003

9

MIC5254

MIC5254

Micrel

Applications Information

Enable/Shutdown

The MIC5254 comes with an active-high enable pin for each

regulator 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

indeterminate state on the output.

Input Capacitor

The MIC5254 is a high performance, high bandwidth device.

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

mal 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 MIC5254 requires an output capacitor for stability. The

design requires 1µF 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 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-

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 downstream

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

input without using a pull-down capacitor, 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 MIC5254 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 MIC5254 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 MIC5254 is a dual LDO voltage regulator designed to

provide two output voltages from one package. Both regula-

tor outputs are capable of sourcing 150mA of output current.

Proper thermal evaluation needs to be done to ensure that

the junction temperature does not exceed it’s maximum

value, 125°C. Maximum power dissipation can be calculated

based on the output current and the voltage drop across each

regulator. The sum of the power dissipation of each regulator

determines the total power dissipation. The maximum power

dissipation that this package is capable of handling can be

determined using thermal resistance, junction to ambient,

and the following basic equation:

P

TT

D(max)

J(max)

A

JA

=

−













θ

T

resistance for a minimum footprint layout for the MIC5254.

Package

θ

θθ

θ

MSOP-10

200°C/W

Table 1. Thermal Resistance

The actual power dissipation of each regulator output can be

calculated using the following simple equation:

P

Each regulator contributes power dissipation to the overall

power dissipation of the package.

P