LTC2636 datasheet(24/28 Pages) LINER

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LTC2668

2668fa

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operation

The amplifiers are stable driving capacitive loads of up

to 1000pF.

Thermal Overload Protection

The LTC2668 protects itself if the die temperature exceeds

160°C. All channels power down, and the open-drain

OVRTMP interrupt pin pulls low. The reference and bias

circuits stay powered on. Once triggered, the device stays

in shutdown even after the die cools.

Thetemperatureofthediemustfalltoapproximately150°C

before the channels can be returned to normal operation.

Once the part has cooled sufficiently, the shutdown can be

cleared with any valid update operation, including LDAC

or a toggle operation. A CS/LD rising edge releases the

OVRTMP pin regardless of the die temperature.

Since the total load current of the device can easily exceed

100mA, die heating potential of the system design should

be evaluated carefully. Grounded loads as low as 1k may

be used and will not result in excessive heat.

Thermal protection can be disabled by using the Config

command to set the TS bit (see Figure 9).

Board Layout

The excellent load regulation and DC crosstalk perfor-

mance of these devices is achieved in part by minimizing

common-mode resistance of signal and power grounds.

Aswithanyhighresolutionconverter,cleanboardground-

ing is important. A low impedance analog ground plane

is necessary, as are star-grounding techniques. Keep the

board layer used for star ground continuous to minimize

ground resistances; that is, use the star-ground concept

without using separate star traces. Resistance from the

REFLO pin to the star point should be as low as possible.

For best performance, stitch the ground plane with arrays

of vias on 150 to 200 mil centers connecting it with the

ground pours from the other board layers. This reduces

overallgroundresistanceandminimizesgroundlooparea.

Using LTC2668 in 5V Single-Supply Systems

LTC2668 can be used in single-supply systems simply by

connecting the V– pins to ground along with REFLO and

GND, while V+ and AVP are connected to a 5V supply. OVP

can be connected to the 5V supply or to the logic supply

voltage if lower than 5V.

With the internal reference, use the 0V to 5V output

range. As with any rail-to-rail device, the output is

limited to voltages within the supply range. Since the

outputs of the device cannot go below ground, they may

limit at the lowest codes, as shown in Figure 10b. Simi-

larly, limiting can occur near full-scale if full-scale error

(FSE = VOS + GE) is positive, or if V+ < 2 • VREF. See

Figure 10c.

The multiplexer can be used and is fully functional. It can

pull all the way to ground, but the upper headroom limita-

tion means that it is useful for output voltages of 3.6V or

below only (V+ = 5V).

More flexibility can be afforded by using an external refer-

ence. For example, by using a 1.25V reference such as the

LTC6655, we can now select between 0x to 2x and 0x to

4x ranges, which give full-scale voltages of 2.5V and 5V,

respectively. Furthermore, the part can be configured for

reset to zero- or mid-scale codes (see the Output Ranges

section).

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