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BG REF:Curvature CompensationComparison of various topology that can be implemented in F021 CMOS Technology

Architecture ReviewState of Art

Bodhayan

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Agenda

Second order Compensation

Metal Compensation

Exponential Compensation

PWL (piece-wise linear) Compensation

Accurate Compensation VBE based

IS based

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Second Order Compensation

Metal compensation

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Metal Compensation scheme

Principle Involved

By using ZTC resistors in the bandgap loop and a resistor with a tempco,

the curvature of bandgap can be minimized by dropping PTAT current

across it

Pros

Trimming is easier No power expenditure for improving curvature

Least area penalty

Cons

Not much ppm reduction is possible due to much variation in second order

due to two types of resistors with process

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Piece-wise Compensation

Current mode and Voltage mode

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Current mode PWL Compensation

Principle Involved

cancellation of a 2nd order term, with a piece-wise linear current by mainly

using the non-ideal properties of the transistors as current sources

Pros

Lower power and area

Disabling the curvature compensation circuitry is simple

Cons

Requires more accurate current source

Temperature range dependent

Mid-point needs to be chosen for optimum accuracy

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Voltage mode PWL Compensation

Principle Involved

cancellation of a 2nd order term, with a piece-wise linear voltage by mainly

using the non-ideal properties of the transistors as current sources

Pros

Unlike PWL current mode circuit, constant current not required

Disabling the curvature compensation circuitry is simple Lowest power and area

Cons

Temperature range dependent

Mid-point needs to be chosen for optimum accuracy*VB

G

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Accurate Compensation

VBE Compensation technique

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VBE based Compensation

Principle Involved

When 2 BJTs are biased with PTAT and Constant Ic, the V is equal to

(k*T/q)*ln(T/Tr). This error is scaled and fed back into the bandgap loop to

correct the higher order terms.

Pros

Highest accuracy Lowest power and area

Cons

Constant current source is required

Higher power and area

Due to constant current source, VBG needs to be trimmed at the time of curvature

compensation, resulting in tedious trimming procedure

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VBE based Compensation

Principle Involved

When 2 BJTs are biased with Ic of different temperature coefficients, the V

is a scaled value of(k*T/q)*ln(T/Tr). This error is scaled and fed back into

the bandgap loop to correct the higher order terms.

By dropping PTAT voltage across CTAT R, a PTATn (n>1) can be

generated

Pros

Highest accuracy

Unlike previous circuit, constant current not required, needs PTAT2 current

Non-linear current introduces 1st order, which can anyway be trimmed

Can reduce the resistance required by having a tapping point in between the VBGand the VPTAT

Cons

Multiple amplifier required

Larger area and power (less compared to previous scheme)