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JAJSOR6 September 2024 LM706A0-Q1

PRODUCTION DATA

7.1.2 Error Amplifier and Compensation

A Type-ll compensator using a transconductance error amplifier (EA) is shown in Figure 7-3. The dominant pole of the EA open-loop gain is set by the EA output resistance, R_OEA, and effective bandwidth-limiting capacitance, C_BW as shown by Equation 23.

Equation 23. LM706A0-Q1

Equation 24.

G_{E A (o p e n l o o p)} (s) = - \frac{g_{m} \times R_{O E A}}{1 + s \times R_{O E A} \times C_{B W}}

The EA high-frequency pole is neglected in the above expression. The compensator transfer function from output voltage to COMP node, including the gain contribution from the (internal or external) feedback resistor network, is calculated in Equation 25.

Equation 25.

G_{c} (s) = \frac{{\hat{v}}_{c} (s)}{{\hat{v}}_{o u t} (s)} = - \frac{V_{R E F}}{V_{O U T}} \times \frac{g_{m} \times R_{O E A} \times (1 + \frac{s}{ω_{z 1}})}{(1 + \frac{s}{ω_{p 1}}) \times (1 + \frac{s}{ω_{p 2}})}

where

V_REF is the feedback voltage reference of 0.8V
g_m is the EA gain transconductance of 1200µS
R_O-EA is the error amplifier output impedance of 64MΩ

Equation 26.

ω_{z 1} = \frac{1}{R_{C O M P} \times C_{C O M P}}

Equation 27.

ω_{p 1} = \frac{1}{R_{O E A} \times (C_{C O M P} + C_{H F} + C_{B W})} ≅ \frac{1}{R_{O E A} \times C_{C O M P}}

Equation 28.

ω_{p 2} = \frac{1}{R_{C O M P} \times (C_{C O M P} | | (C_{H F} + C_{B W}))} ≅ \frac{1}{R_{C O M P} \times C_{H F}}

The EA compensation components create a pole close to the origin, a zero, and a high-frequency pole. Typically, R_COMP << R_O-EA and C_COMP >> C_BW and C_HF, so the approximations are valid.Figure 7-3 circles the poles in red and the zero in blue.

LM706A0-Q1 Error
Amplifier and Compensation Network

Figure 7-3 Error Amplifier and Compensation Network