AD9520-4SLASHPCBZ datasheet(49/84 Pages) AD

49 / 84 page

AD9520-4

Rev. 0 | Page 49 of 84

200Ω

SW1B

SW1A

SW2

QN2

QN1

OUT

4.4mA

Figure 51. LVPECL Output Simplified Equivalent Circuit

CMOS Output Drivers

The user can also individually configure each LVPECL output

as a pair of CMOS outputs, which provides up to 24 CMOS

outputs. When an output is configured as CMOS, the CMOS

Output A and CMOS Output B are automatically turned on. For

a given differential pair, either the CMOS Output A or Output B

can be turned on or off independently. The user can also select

the relative polarity of the CMOS outputs for any combination of

inverting and noninverting (see Register 0x0F0 to Register 0x0FB).

The user can power down each CMOS output as needed to save

power. The CMOS output power-down is individually controlled

by the enable CMOS output register (0x0F0[6:5] to 0x0FB[6:5]).

The CMOS driver is in tristate when it is powered down.

OUT1/

OUT1

VS_DRV

Figure 52. CMOS Equivalent Output Circuit

RESET MODES

The AD9520 has a power-on reset (POR) and several other

ways to apply a reset condition to the chip.

Power-On Reset

During chip power-up, a power-on reset pulse is issued when

VS reaches ~2.6 V (<2.8 V) and restores the chip either to the

setting stored in EEPROM (with the EEPROM pin = 1) or to

the on-chip setting (with the EEPROM pin = 0). At power-on,

the AD9520 also executes a SYNC operation, which brings the

outputs into phase alignment according to the default settings.

It takes ~70 ms for the outputs to begin toggling after the

power-on reset pulse signal is internally generated.

Hardware Reset via the RESET Pin

RESET, a hard reset (an asynchronous hard reset is executed by

briefly pulling RESET low), restores the chip either to the setting

stored in EEPROM (the EEPROM pin = 1) or to the on-chip

setting (the EEPROM pin = 0). A hard reset also executes a

SYNC operation, which brings the outputs into phase

alignment according to the default settings. When EEPROM is

inactive (the EEPROM pin = 0), it takes ~2 μs for the outputs to

begin toggling after RESET is issued. When EEPROM is active

(the EEPROM pin = 1), it takes ~20 ms for the outputs to toggle

after RESET is brought high.

Soft Reset via the Serial Port

The serial port control register allows for a soft reset by setting

Bit 2 and Bit 5 in Register 0x000. When Bit 5 and Bit 2 are set,

the chip enters a soft reset mode and restores the chip either to

the setting stored in EEPROM (the EEPROM pin = 1) or to the

on-chip setting (the EEPROM pin = 0), except for Register 0x000.

These bits are self-clearing. During the internal reset, the outputs

are held static.

Soft Reset to Settings in EEPROM when EEPROM Pin = 0 via

the Serial Port

The serial port control register allows the chip to be reset to

settings in EEPROM when the EEPROM pin = 1 via 0xB02[1].

This bit is self-clearing. This bit does not have any effect when

EEPROM pin = 0. It takes ~20 ms for the outputs to begin

toggling after soft_EEPROM register is cleared.

POWER-DOWN MODES

Chip Power-Down via PD

The AD9520 can be put into a power-down condition by

pulling the PD pin low. Power-down turns off most of the

functions and currents inside the AD9520. The chip remains in

this power-down state until PD is brought back to logic high.

When taken out of power down mode, the AD9520 returns to

the settings programmed into its registers prior to the power-

down, unless the registers are changed by new programming

while the PD pin is held low.

Powering down the chip shuts down the currents on the chip,

except the bias current necessary to maintain the LVPECL

outputs in a safe shutdown mode. The LVPECL bias currents are

needed to protect the LVPECL output circuitry from damage that

can be caused by certain termination and load configurations

when tristated. Because this is not a complete power-down, it

can be called sleep mode. The AD9520 contains special circuitry to

prevent runt pulses on the outputs when the chip is entering or

exiting sleep mode.

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