MC33502

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General Information

The MC33502 dual operational amplifier is unique in its

ability to provide 1.0 V rail−to−rail performance on both the

input and output by using a SMARTMOS

t process. The

amplifier output swings within 50 mV of both rails and is

able to provide 50 mA of output drive current with a 5.0 V

supply, and 10 mA with a 1.0 V supply. A 5.0 MHz

bandwidth and a slew rate of 3.0 V/

ms is achieved with high

speed depletion mode NMOS (DNMOS) and vertical PNP

transistors. This device is characterized over a temperature

range of −40

°C to 105°C.

Circuit Information

Input Stage

One volt rail−to−rail performance is achieved in the

MC33502 at the input by using a single pair of depletion

mode NMOS devices (DNMOS) to form a differential

amplifier with a very low input current of 40 fA. The normal

input common mode range of a DNMOS device, with an ion

implanted negative threshold, includes ground and relies on

the body effect to dynamically shift the threshold to a

positive value as the gates are moved from ground towards

the positive supply. Because the device is manufactured in

a p−well process, the body effect coefficient is sufficiently

large to ensure that the input stage will remain substantially

saturated when the inputs are at the positive rail. This also

applies at very low supply voltages. The 1.0 V rail−to−rail

input stage consists of a DNMOS differential amplifier, a

folded cascode, and a low voltage balanced mirror. The low

voltage cascoded balanced mirror provides high 1st stage

gain and base current cancellation without sacrificing signal

integrity. Also, the input offset voltage is trimmed to less

than 1.0 mV because of the limited available supply voltage.

The body voltage of the input DNMOS differential pair is

internally trimmed to minimize the input offset voltage. A

common mode feedback path is also employed to enable the

offset voltage to track over the input common mode voltage.

The total operational amplifier quiescent current drop is

1.3 mA/amp.

Output Stage

An additional feature of this device is an “on demand”

base current cancellation amplifier. This feature provides

base drive to the output power devices by making use of a

buffer

amplifier

to

perform

a

voltage−to−current

conversion. This is done in direct proportion to the load

conditions. This “on demand” feature allows these

amplifiers to consume only a few micro−amps of current

when the output stage is in its quiescent mode. Yet it

provides high output current when required by the load. The

rail−to−rail output stage current boost circuit provides

50 mA of output current with a 5.0 V supply (For a 1.0 V

supply output stage will do 10 mA) enabling the operational

amplifier to drive a 600

W load. A buffer is necessary to

isolate the load current effects in the output stage from the

input stage. Because of the low voltage conditions, a

DNMOS follower is used to provide an essentially zero

voltage level shift. This buffer isolates any load current

changes on the output stage from loading the input stage. A

high speed vertical PNP transistor provides excellent

frequency performance while sourcing current. The

operational amplifier is also internally compensated to

provide a phase margin of 60 degrees. It has a unity gain of

5.0 MHz with a 5.0 V supply and 4.0 MHz with a 1.0 V

supply.

Low Voltage Operation

The MC33502 will operate at supply voltages from 0.9 to

7.0 V and ground. When using the MC33502 at supply

voltages of less than 1.2 V, input offset voltage may

increase slightly as the input signal swings within

approximately 50 mV of the positive supply rail. This effect

occurs only for supply voltages below 1.2 V, due to the input

depletion mode MOSFETs starting to transition between the

saturated to linear region, and should be considered when

designing high side dc sensing applications operating at the

positive supply rail. Since the device is rail−to−rail on both

input and output, high dynamic range single battery cell

applications are now possible.