March 2004

5

M0382-033004

MIC5249

Micrel

Applications Information

Enable/Shutdown

The MIC5249 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 MIC5249 is a high performance, high bandwidth device.

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

mal performance. A 1.0

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 to filter out high frequency

noise and are good practice in any RF-based circuit.

Output Capacitor

The MIC5249 requires an output capacitor for stability. The

design requires 2.2

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 2.2

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.

Delay Pin Input

The power-on reset function can be implemented on the

MIC5249 by adding an external capacitor from the delay pin

to ground. This external capacitor sets the delay time (T

of the RESET output. The capacitor value required can be

easily calculated using the formula:

C

TI

V

DELAY

DELAY

DELAY

DELAY

=

×









where I

capacitor is used, there will be no delay and the POR output

acts like a standard error FLAG output.

Reset Output

The RESET output of the MIC5249 provides the power-on

reset signal based on the capacitor from the DELAY pin to

ground when input power is applied to the regulator. The

reset signal stays low for a preset time period after power is

applied to the regulator, and then goes high (see “Timing

Diagram”).

The reset 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. The output

drives an open-drain transistor to indicate a fault. This pre-

vents chattering or inadvertent triggering of the reset. The

reset must be pulled up using a resistor form the RESET pin

to either the input or the output.

Bypass Pin Input

A bypass capacitor is required 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 MIC5249 to drive a large capacitor on the bypass

pin without significantly slowing the turn-on time.

Active Shutdown

The MIC5249 also features an active shutdown clamp, which

is a 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 MIC5249 will remain stable and in regulation with no load

unlike many other voltage regulators. This is especially

important in CMOS RAM keep-alive application.