With a PWM controller, like the MAX668, one gets the advantage of constant-frequency operation which is good for EMI control in noise sensitive audio and RF applications. But the advantage is achieved at the price of less flexibility in inductor value and vulnerability to a right-hand plane zero that moves into the control bandwidth and contributes to instability at high boost ratios. When designing with the MAX668, the inductor value is limited by requirements for proper slope compensation to reduce risk of instability from subharmonic oscillation. And the right-hand plane zero is an issue when the inductor value allows operation in continuous conduction mode. "Continuous conduction mode" is simply geek jargon that means that the inductor current doesn't ramp to zero during each switching cycle.
With a minimum off-time PFM controller, like the MAX1771, MAX863 or MAX608, one gets the advantages of fast transient response, great flexibility in inductor selection for cost-performance trade-offs and invulnerability to the right-hand plane zero at the cost of variable frequency EMI and sensitivity to board layout.
If you're designing an audio or RF project, and not boosting too much, the MAX668 is a first option to consider. For general use or when you're doing something wild, like boo
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... like boosting to 500V, then the MAX1771 is a better option.