LT6003 datasheet(18/30 Pages) AD

18 / 30 page

LTC2066/LTC2067/LTC2068

Rev. B

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In the worst case, there may not be enough supply cur-

rent available to take the system up to nominal voltages.

In other cases, this transient power-up current will lead

to added power loss in duty-cycled applications.

A way to quantify the transient current loss is to integrate

the supply current during power-up to examine the total

charge loss. If there were no additional transient current,

the integrated supply current would appear as a smooth,

straight line with a slope equal to the DC supply current

of the part. Any deviation from a straight line indicates

additional transient current that is drawn from the supply.

The LTC2066/LTC2067/LTC2068 have been designed to

minimize this charge loss during power-up so that power

can be conserved in duty-cycled applications. Figure 6

shows the integrated supply current (i.e. charge) of the

LTC2066 during power-up. Likewise, Figure 7 shows the

charge loss due to enabling and disabling the part via

the SHDN pin.

1V/µs V– EDGE RATE

V+ = 5V

500µs/DIV

V–

5V/DIV

VOUT

2V/DIV

QV+

10nC/DIV

2066 F06

Figure 6. LTC2066 Charge Loss During Power-Up

500µs/DIV

VSHDN

5V/DIV

VOUT

2V/DIV

QV+

10nC/DIV

2066 F07

Figure 7. LTC2066 Charge Loss Due to

Enabling and Disabling via SHDN Pin

APPLICATIONS INFORMATION

Board leakage can be minimized by encircling the input

connections with a guard ring operated at a potential very

close to that of the inputs. The ring must be tied to a low

impedance node. For inverting configurations, the guard

ring should be tied to the potential of the positive input

(+IN). For noninverting configurations, the guard ring

should be tied to the potential of the negative input (–IN).

In order for this technique to be effective, the guard ring

must not be covered by solder mask. Ringing both sides

of the printed circuit board may be required. See Figure 5

for an example of proper layout.

Shutdown Mode

The LTC2066 in the SC70 package, the LTC2067 in the

DFN package, and the LTC2068 in the QFN package fea-

ture a shutdown mode for low-power applications. In the

OFF state, each amplifier draws less than 170nA of sup-

ply current and the outputs present a high impedance to

external circuitry.

Shutdown operation is accomplished by tying SHDN

below VL. If the shutdown feature is not required, it

is recommended that SHDN be tied to V+. A current

source pulls the SHDN pin high to keep the amplifier in

the ON state when the pin is floated, however this may

not be reliable at elevated temperatures due to board

leakage (see SHDN Circuit Block Diagram, page 14).

For operation in noisy environments, a capacitor between

SHDN and V+ is recommended to prevent noise from

changing the shutdown state. When there is a danger of

SHDN being pulled beyond the supply rails, resistance

in series with the SHDN pin is recommended to limit the

resulting current.

Start-Up Characteristics

Micropower op amps are often not micropower during

start-up, which can cause problems when used on low

current supplies. Large transient currents can conduct

during power-up until the internal bias nodes settle to

their final values. A large amount of current can be drawn

from the supplies during this transient, which can sustain

for several milliseconds in the case of a micropower part.

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