AD9956-VCOSLASHPCB datasheet(31/32 Pages) AD

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AD9956

Rev. B | Page 31 of 32

CFR2<15:12> PLLREF Divider Control Bits (÷N)

These 4 bits set the PLLREF divider (÷N) ratio where N is a

value equal to 1 to 16. CFR2<15:12> = 0000 means that

N = 1 and CFR2<15:12> = 1111 means that N = 16, or simply,

N = CFR2<15:12> + 1.

CFR2<15:12> =

N =

CFR2<15:12> =

N =

0000

1000

0001

1001

0010

1010

0011

1011

0100

1100

0101

1101

0110

1110

0111

1111

CFR2<11:8> PLLREF Divider Control Bits (÷M)

These 4 bits set the PLLOSC divider (÷M) ratio where

M is a value equal to 1 to 16. CFR2<11:8> = 0000 means

that M = 1 and CFR2<11:8> = 1111 means that M = 16, or

M = CFR2<11:8> + 1.

CFR2<11:8> =

M =

CFR2<11:8> =

M =

0000

1000

0001

1001

0010

1010

0011

1011

0100

1100

0101

1101

0110

1110

0111

1111

CFR2<7:6> Open

Unused locations. Write a Logic 0.

CFR2<5> CP Polarity

This bit sets the polarity of the charge pump, in response to a

ground referenced or a supply referenced VCO.

CFR2<5> = 0 (default). The charge pump is configured to

operate with a supply referenced VCO. If PLLOSC lags

PLLREF, the charge pump will attempt to drive the VCO con-

trol node voltage higher. If PLLOSC leads PLLREF, the charge

pump will attempt to drive the VCO control node voltage low-

er.

CFR2<5> = 1. The charge pump is configured to operate with a

ground referenced VCO. If PLLOSC lags PLLREF, the charge

pump will attempt to drive the VCO control node voltage low-

er. If PLLOSC leads PLLREF, the charge pump will attempt to

drive the VCO control node voltage higher.

CFR2<4> Charge Pump Full Power-Down

This bit, when set, will put the charge pump into a full power-

down mode.

CFR2<4> = 0 (default). The charge pump is powered on and

operating normally.

CFR2<4> = 1. The charge pump is completely powered down.

CFR2<3> Charge Pump Quick Power-Down

Rather than power down the charge pump, which can take a

long time to recover from, a quick power-down mode, which

powers down only the charge pump output buffer, is included.

While this doesn’t reduce the power consumption significantly,

it does shut off the output to the charge pump and allows it to

come back on in a rapidly.

CFR2<3> = 0 (default). The charge pump is powered on and

operating normally.

CFR2<3> = 1. The charge pump is on and running, but the

output buffer is powered down.

CFR2<2:0> Charge Pump Current Scale.

A base output current from the charge pump is determined by

a resistor connected from the CP_RSET pin to ground (see the

PLL Circuitry section). However, it is possible to multiply the

charge pump output current by a value from 1:8 by program-

ming these bits. The charge pump output current is scaled by

CFR2<2:0> +1.

CFR2<2:0> = 000 (default). Scale factor = 1 to CFR2<2:0> = 111 (8).

CFR2<2:0>

Scale Factor

000

001

010

011

100

101

110

111

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