MICRF505L datasheet(19/43 Pages) MICREL

19 / 43 page

Micrel

MICRF505BML/YML

August 2006

M9999-092904

+1 408-944-0800

Low Dropout Regulator

The MICRF505LBML/YML has an internal voltage

regulator powering the MICRF505 device. This LDO

is equipped with a logic-compatible enable pin and

can be put into a zero-off-mode current state,

drawing no current when disabled. The LDO is a

Cap design operating with very small ceramic

output capacitors for stability.

Enable/Shutdown

The internal LDO 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. The active-high

enable pin uses CMOS technology and the enable

pin cannot be left floating; a floating enable pin may

cause an indeterminate state on the output.

Input Capacitor

The LDO require 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. Additional 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 LDO require an output capacitor of 1µF or

greater to maintain stability. The design is optimized

for use with low-ESR ceramic chip capacitors. High

ESR

capacitors

may

cause

high

frequency

oscillation. 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 recommended

because of their temperature performance. 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 operating

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 can be placed from the noise bypass pin

to ground to reduce output voltage noise. The

capacitor bypasses the internal reference. A 0.1µF

capacitor is recommended 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 LDO to drive a large capacitor on the

bypass pin without significantly slowing turn-on time.

Refer to the Typical Characteristics section for

performance with different bypass capacitors.

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