LT6553

5

6553i

Power Supplies

The LT6553 is optimized for

±5V supplies but can be

operated on as little as

±2.25V or a single 4.5V supply and

as much as

±6V or a single 12V supply. Internally, each

supply is independent to improve channel isolation. Do

not leave any supply pins disconnected or the part may not

function correctly!

Enable/Shutdown

The LT6553 has a TTL compatible shutdown mode con-

trolled by the EN pin and referenced to the DGND pin. If the

amplifier will be enabled at all times, the EN pin can be

connected directly to DGND. If the enable function is

desired, either driving the pin above 2V or allowing the

internal 46k pull-up resistor to pull the EN pin to the top rail

will disable the amplifier. When disabled, the DC output

impedance will rise to approximately 700

Ω through the

internal feedback and gain resistors. Supply current into

the amplifier in the disabled state will be primarily through

It is important that the two following constraints on the

DGND pin and the EN pin are always followed:

Split supplies of

±3V to ±5.5V will satisfy these require-

ments with DGND connected to 0V.

In single supply applications above 5.5V, an additional

resistor may be needed from the EN pin to DGND if the pin

is ever allowed to float. For example, on a 12V single

supply, a 33k resistor would protect the pin if it were

allowed to float high while still allowing the internal pull-

up resistor to disable the part.

On dual

±2.25V supplies, connecting the EN and DGND

is more than 3V.

The DGND pin should not be pulled above the EN pin since

doing so will turn on an ESD protection diode. If the EN pin

voltage is forced a diode drop below the DGND pin, current

should be limited to 10mA or less.

The enable/disable times of the LT6553 are fast when

driven with a logic input. Turn on (from 50% EN input to

50% output) typically occurs in less than 50ns. Turn off is

slower, but is nonetheless below 300ns.

Input Considerations

Therefore, on split supplies the LT6553 input range is

always larger than the output swing. On a single positive

supply, however, the input range limits the output low

swing to 2V (1V multiplied by the internal gain of 2).

The inputs can be driven beyond the point at which the

output clips so long as input currents are limited to below

±10mA. Continuing to drive the input beyond the output

limit can result in increased current drive and slightly

increased swing, but will also increase supply current and

may result in delays in transient response at larger levels

of overdrive.

Layout and Grounding

It is imperative that care is taken in PCB layout in order to

utilize the very high speed and very low crosstalk of the

LT6553. Separate power and ground planes are highly

recommended and trace lengths should be kept as short

as possible. If input or output traces must be run over a

distance of several centimeters, they should use a con-

trolled impedance with matching series and shunt resis-

tances (nominally 75

Ω) to maintain signal fidelity.

Low ESL/ESR bypass capacitors should be placed as close

to the positive and negative supply pins as possible. One

0.01

trace length on each supply pin will further improve AC and

transient response as well as channel isolation. For high

current drive and large-signal transient applications, addi-

tional 1

µF to 10µF tantalums should be added. The small-

est capacitors should be placed closest to the package.

To maintain the LT6553’s channel isolation, it is beneficial

to shield parallel input and output traces using a ground

plane or power supply traces. Vias between topside and

backside metal may be required to maintain a low induc-

tance ground near the part where numerous traces con-

verge.

ESD Protection

The LT6553 has reverse-biased ESD protection diodes on

all relevant pins. If any pins are forced a diode drop above

the positive supply or a diode drop below the negative

supply, large currents may flow through these diodes. If

the current is kept below 10mA, no damage to the devices

will occur.

APPLICATIO S I FOR ATIO