6

LTC1152

S

APPLICATI

I FOR ATIO

Rail-to-Rail Operation

The LTC1152 is a rail-to-rail input common-mode range,

rail-to-rail output swing op amp. Most CMOS op amps,

including the entire LTC zero-drift amplifier line, and even

a few bipolar op amps, can and do, claim rail-to-rail output

swing. One obvious use for such a device is to provide a

unity-gain buffer for 0V to 5V signals running from a single

5V power supply. This is not possible with the vast

majority of so-called “rail-to-rail” op amps; although the

output can swing to both rails, the negative input (which

is connected to the output) will exceed the common-mode

input range of the device at some point (generally about

1.5V below the positive supply), opening the feedback

loop and causing unpredictable and sometimes bizarre

behavior.

The LTC1152 is an exception to this rule. It features both

rail-to-rail output swing and rail-to-rail input common-

mode range (CMR); the input CMR actually extends be-

yond either rail by about 0.3V. This allows unity-gain

buffer circuits to operate with any input signal within the

power supply rails; input signal swing is limited only by the

output stage swing into the load. Additionally, signals

occurring at either rail (power supply current sensing, for

example) can be amplified without any special circuitry.

Internal Charge Pump

The LTC1152 achieves its rail-to-rail input CMR by using

a charge pump to generate an internal voltage approxi-

appear to be 2V below the front end’s power supply (Figure

1). The charge pump is contained entirely within the

LTC1152; no external components are required.

About 100

will be present on the output of the LTC1152. This

feedthrough is at 4.7MHz, higher than the gain-bandwidth

of the LTC1152, and will generally not cause any prob-

lems. Very sensitive applications can reduce this

feedthrough by connecting a capacitor from the CP pin

µF capacitor will reduce charge

pump feedthrough to negligible levels. The LTC1152 in-

cludes an internal diode from pin 8 to pin 7 to prevent

external parasitic capacitance from lengthening start-up

OUT

OUTPUT

RAIL TO RAIL

INPUT

0.1

µF*

*OPTIONAL EXTERNAL

CAPACITOR TO REDUCE

CHARGE PUMP FEEDTHROUGH

(PIN 7)

CP (PIN 8)

+IN

–IN

1152 F01

INTERNAL

CHARGE

PUMP

Figure 1. LTC1152 Internal Block Diagram

time. This diode can stand short-term peak currents of

about 50mA, allowing it to quickly charge external capaci-

µF) should not

excessive diode current from flowing at start-up. The

LTC1152 can withstand continuous short circuits be-

through the diode, destroying the LTC1152. Don’t do it.

Output Drive

The LTC1152 features an enhanced output stage that can

sink and source 10mA with a single 5V supply while

maintaining rail-to-rail output swing under most loading

conditions. The output stage can be modeled as a perfect

rail-to-rail voltage source with a resistor in series with it;

this open-loop output resistance limits the output swing

by creating a resistor divider with the output load.

The output resistance drops as total power supply voltage

increases, as shown in the typical performance curves. It

is typically 140

Ω with a single 5V supply, allowing a 4.4V

output swing into a 1k resistor with a single 5V supply.

OUT (PIN 6)

≈140Ω AT 5V SUPPLY

LTC1152

OUTPUT

DRIVER

1152 F02

Figure 2. LTC1152 Output Resistance Model