AD9981SLASHPCB datasheet(19/44 Pages) AD

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AD9981

Rev. 0 | Page 19 of 44

FIELD 1

FIELD 0

SYNC SEPARATOR THRESHOLD

FIELD 1

FIELD 0

HSYNCIN

VSYNCIN

VSYNCOUT

O/E FIELD

EVEN FIELD

QUADRANT

Figure 12. Vsync Filter—Odd/Even

Power Management

To meet display requirements for low standby power, the

AD9981 includes a power-down mode. The power-down state

can be controlled manually (via Pin 17 or Register 0x1E, Bit 3),

or completely automatically by the chip. If automatic control is

selected (0x1E, Bit 4), the AD9981’s decision is based on the

status of the sync detect bits (Register 0x24, Bits 2, 3, 6, and 7). If

either an Hsync or a sync-on-green input is detected on any

input, the chip powers up, or else it powers down. For manual

control, the AD9981 allows flexibility of control through both a

dedicated pin and a register bit. The dedicated pin allows a

hardware watchdog circuit to control power-down, while the

With manual power-down control, the polarity of the power-

down pin must be set (0x1E, Bit 2) regardless of whether the pin

is used. If unused, it is recommended to set the polarity to active

high and hardwire the pin to ground with a 10 kΩ resistor.

In power-down mode, there are several circuits that continue to

operate as normal. The serial register and sync detect circuits

maintain power so that the AD9981 can be woken up from

its power-down state. The bandgap circuit maintains power

because it is needed for sync detection. The sync-on-green and

SOGOUT functions continue to operate because the SOGOUT

output is needed when sync detection is performed by a

secondary chip. All of these circuits require minimal power to

operate. Typical standby power on the AD9981 is about 50 mW.

There are two options that can be selected when in power-

down. These are controlled by Bits 0 and 1 in Register 0x1E. The

first bit controls whether the SOGOUT pin is in high impe-

dance or not. In most cases, the user will not place SOGOUT in

high impedance during normal operation. The option to put

SOGOUT in high impedance is included mainly to allow for

factory testing modes. The second option keeps the AD9981

powered up while placing only the outputs in high impedance.

This option is useful when the data outputs from two chips are

connected on a PCB and the user wants to switch

instantaneously between the two.

Table 10. Power-Down Control and Mode Descriptions

Inputs

Mode

Auto Power-Down

Control1

Power-Down2

Sync Detect3

Powered-On or Comments

Power-Up

Everything

Power-Down

Only the serial bus, sync activity detect,

SOG, bandgap reference

Power-Up

Everything

Power-Down

Only the serial bus, sync activity detect,

SOG, bandgap reference

1 Auto power-down control is set by Register 0x1E, Bit 4.

2 Power-down is controlled by OR’ing Pin 17 with Register 0x1E, Bit 3. The polarity of Pin 17 is set by Register 0x1E, Bit 2.

3 Sync detect is determined by OR’ing Register 0x24, Bits 2, 3, 6, and 7.

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