December 2000

13

MICRF007

MICRF007

Micrel

(6)

∆t 1.333C

0.44

AGC

=−

where:

C

Selecting C

Generally, droop of the AGC control voltage during shutdown

should be replenished as quickly as possible after the IC is

“turned-on”. As described in the functional description, for

about 10ms after the IC is turned on, the AGC push-pull

currents are increased to 45 times their normal values.

Consideration should be given to selecting a value for C

AGC

and a shutdown time period 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. Worst-case from a recovery stand-

point is downward droop, since the AGC pullup current is

1/10th magnitude of the pulldown current. The downward

droop is replenished according to the Equation 7:

(7)

I

C

V

t

AGC

= ∆

∆

where:

I = AGC pullup current for the initial 10ms (67.5

µA)

C

∆t = droop recovery time

∆V = droop voltage

For example, if user desires

∆t = 10ms and chooses a 4.7µF

C

same equation with 200nA worst case pin leakage and

assuming 1

µA of capacitor leakage in the same direction, the

maximum allowable

∆t (shutdown time) is about 0.56s for

droop recovery in 10ms.