MICRF219A_15 datasheet(18/25 Pages) MICREL

18 / 25 page

Micrel, Inc.

MICRF219A

August 12, 2015

Revision 3.0

RadioTech@micrel.com or (408) 944-0800

Crystal Selection

The crystal resonator provides a reference clock for all

the device internal circuits. Crystal tolerance needs to

be chosen such that the down-converted signal is

always inside the IF bandwidth of MICRF219A.

From

this consideration, the tolerance should be

±50ppm on

both the transmitter and the MICRF219A side. The ESR

should be less than 300Ω, and the temperature range of

the crystal should match the range required by the

application. With the Abracon crystal listed in the Bill of

Materials, a typical MICRF219A crystal oscillator still

starts up at +105ºC

with additional 400

Ω series

resistance.

The oscillator of the MICRF219A is a Pierce-type

oscillator. Good care must be taken when laying out the

printed circuit board. Avoid long traces and place the

ground plane on the top layer close to the REFOSC pins

RO1 and RO2. When care is not taken in the layout, and

the crystals used are not verified, the oscillator may not

start or takes longer to start. Time-to-good-data will be

longer as well.

Important Note

A few customers have reported that some MICRF219A

receiver do not start up correctly.

When the issue

occurs, DO either chatters or stays at low voltage level.

An unusual operating current is observed and the part

cannot receive or demodulate data even when a strong

OOK signal is present.

Micrel has confirmed that this is the symptom of

incorrect power on reset (POR) of internal register bits.

The MICRF219A is designed to start up in shutdown

mode (SHDN pin must be in logic high during Vdd ramp

up). When the SHDN pin is tied to GND, and if the

supply is ramped up slowly, a “test bus pull down” circuit

may be activated. Once the chip enters this mode, the

POR does not have the chance to set register bits (and

hence operating modes) correctly. The test bus pull

down acts on the SHDN pin, and can be illustrated in the

following diagram.

MICRF2XX

Bias

control &

POR

Test Mode

Circuits

Test Bus

(SHDN) pin

This device turns on,

preventing POR from setting

operating modes correctly

(Vdd) pin

(SHDN) pin

3.3V

MICRF2XX

10 ohm

4.7uF

100K

2.2uF

Change the SHDN

pin and Vdd pin

connections to

To prevent the erroneous startup, a simple RC network

is recommended.

The 10

Ω resistor and the 4.7µF

capacitor provide a delay of about 200µs between the

VDD and SHDN during the power up, thus ensuring the

part to enter to shutdown stage before the part is

actually turned on. The 2.2µF capacitor bootstraps the

voltage on SHDN, ensuring that SHDN voltage leads the

supply voltage on VDD during the power up. This gives

the POR circuit time to set internal register bits.

The

SHDN pin can be brought low to turn the chip on once

the initialization is completed. The 2.2µF and 100k

Ω

network form a RC delay of about 200ms before the

SHDN pin is brought to low again. The 100k

Ω resistor

discharges the SHDN pin to turn the chip on.

The suggestion provided above will generally serve

to prevent the startup issue from happening to the

MICRF219A series ASK receiver. However, exact

values of the RC network depend on the ramp rate of

the supply voltage, and should be determined on a

case-by-case basis.

VDD pin

SHDN pin

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