Micrel

MICRF010

June 2005

8

M9999-063005

(408) 955-1690

when the devices are used in duty-cycled operation.

To further enhance duty-cycled operation, the AGC push

and pull currents are boosted for approximately 10ms

immediately after the device is taken out of shutdown. This

compensates for AGC capacitor voltage drop and reduces

the time to restore the correct AGC voltage. The current is

boosted by a factor of 45.

Selecting CAGC Capacitor in Continuous Mode

A CAGC capacitor in the range of 0.47µF to 4.7µF is

typically recommended. Caution! If the capacitor is too

large, the AGC may react too slowly to incoming signals.

AGC settling time, from a completely discharged (zero-volt)

state is given approximately by Equation 6:

(6)

∆t = 1.333 × C

where:

Selecting CAGC Capacitor in Duty-Cycle Mode

Voltage droop across the CAGC capacitor during shutdown

should be replenished as quickly as possible after the IC is

enabled. As mentioned above, the MICRF010 boosts the

push-pull current by a factor of 45 immediately after start-

up. This fixed time period is based on the reference

capacitor and the duration of the shutdown time period

should be selected such that the droop can be replenished

within this 10ms period.

Polarity of the droop is unknown, meaning the AGC voltage

could droop up or down. The worst-case from a recovery

standpoint is downward droop, since the AGC pull-up

current is 1/10th magnitude of the pull-down current. The

downward droop is replenished according to the Equation

7:

I

=

∆V

∆t

(7)

where:

I = AGC pull-up current for the initial 10ms (67.5µA)

∆t = drop recovery time

∆V = drop voltage

For example, if user desires

∆t = 10ms and chooses a

4.7µF CAGC, the allowable droop is about 144mV. Using

the same equation with 200nA, the worst-case pin leakage,

and assuming 1µA of capacitor leakage in the same

direction, the maximum allowable

∆t (shutdown time) is

about 0.56s for droop recovery in 10ms.

The ratio of decay-to-attack time-constant is fixed at 1:10

(that is, the attack time constant is 10 times of the decay

time constant). Generally the design value of 1:10 is

adequate for the vast majority of applications. If adjustment

capacitor may vary the ratio. The value of the resistor must

be determined on a case-by-case basis.