Design Article
Current-mode controller benefits defined
Bruce Haug, Linear Technology
1/29/2013 5:39 AM EST
Features
The LTC3866 comprises two positive sense pins, SNSD+ and SNSA+, to acquire the ramp signal and process it internally to provide a 14dB signal-to-noise ratio improvement in response to low voltage sense signals. The current limit threshold is still a function of the inductor peak current and its DCR value, and can be accurately set from 10mV to 30mV in a 5mV steps. The part-to-part current limit error is only about 1mV over the full temperature range assuring good accuracy.
In addition, since the LTC3866 uses constant frequency peak current mode control architecture, it guarantees cycle-by-cycle peak current limit and current sharing between different power supplies. It is especially well-suited to low voltage, high current supplies because of a unique architecture that enhances the signal-to-noise ratio of the current sense circuit. The improved signal to noise ratio minimizes jitter due to switching noise, which could corrupt the signal. The worst case switching jitter is reduced by 60 percent when compared to a standard current mode controller.
Low output ripple application
Because the LTC3866 only requires a ramp signal about a quarter of the sense signal of the next best current mode converters, output ripple can be drastically reduced by increasing the inductance and capacitance of the output filter. Figure 3 shows a high efficiency converter with the benefit of low output ripple voltage. The much lower output voltage ripple of less than 10 mV, as shown in figure 4 is critical for extremely noise sensitive applications such as test/measurement systems and audio devices.
Figure 3: High efficiency, 1.5V/25A step-down converter with very low output ripple
Figure 4: Low output ripple curve of LTC3866 figure 3 schematic
The LTC3866 comprises two positive sense pins, SNSD+ and SNSA+, to acquire the ramp signal and process it internally to provide a 14dB signal-to-noise ratio improvement in response to low voltage sense signals. The current limit threshold is still a function of the inductor peak current and its DCR value, and can be accurately set from 10mV to 30mV in a 5mV steps. The part-to-part current limit error is only about 1mV over the full temperature range assuring good accuracy.
In addition, since the LTC3866 uses constant frequency peak current mode control architecture, it guarantees cycle-by-cycle peak current limit and current sharing between different power supplies. It is especially well-suited to low voltage, high current supplies because of a unique architecture that enhances the signal-to-noise ratio of the current sense circuit. The improved signal to noise ratio minimizes jitter due to switching noise, which could corrupt the signal. The worst case switching jitter is reduced by 60 percent when compared to a standard current mode controller.
Low output ripple application
Because the LTC3866 only requires a ramp signal about a quarter of the sense signal of the next best current mode converters, output ripple can be drastically reduced by increasing the inductance and capacitance of the output filter. Figure 3 shows a high efficiency converter with the benefit of low output ripple voltage. The much lower output voltage ripple of less than 10 mV, as shown in figure 4 is critical for extremely noise sensitive applications such as test/measurement systems and audio devices.
Figure 3: High efficiency, 1.5V/25A step-down converter with very low output ripple
Click on image to enlarge
Figure 4: Low output ripple curve of LTC3866 figure 3 schematic
Alternatively, the LTC3866 can be used with power blocks and DrMOS devices for a more compact and very high output current. Figure 5 shows a dual-phase, high efficiency, 1.8V/80A power supply based on two LTC3866 controllers driving power blocks in parallel. The current share between phases is within +/-5 percent due to the current mode control of the LTC3866. If two voltage mode controllers were used in place of the LTC3866, accurate current sharing would not be possible due to there being no means of controlling the phase current.
Figure 5: 1.5V/80A power with two LTC3866’s paralleled, each using power blocks
In applications where a higher value DCR inductor or sense resistor is used, the LTC3866 can be configured like any typical current mode controller by disabling the SNSD+ pin by shorting it to ground. An RC filter can be used to sense the output inductor signal. If the RC filter is used, its time constant, R • C, is set equal to L/DCR of the output inductor. In these applications, the current limit is normally five times larger for the specified current sense.
Conclusion
The LTC3866 allows the use of an ultralow DCR current sensing element to increase the efficiency in high current applications. Its current mode control provides the benefits over the alternative voltage mode controller of high re-liability with fast, cycle-by-cycle current sensing, simple feedback loop compensation and the ability to use all ceramic capacitors for the smallest solution size. The LTC3866 is suitable for low voltage, high current step-down converter applications needing high efficiency and high reliability. Tracking, strong on-chip drivers, multichip operation and external sync capability fill out its menu of features. The LTC3866 applies to point-of-load computer and telecom systems, industrial and medical instruments, and DC power distribution systems. Finally, a power supply designer can have a controller that incorporates the best of both current and voltage mode control schemes.
About the author:
Bruce Haug is senior product marketing engineer at Linear Technology
See related links:
UMC copper process targets power management ICs
Distributed power management cuts wiring weight
Power management IC market set to decline
TI leads power management IC ranking
Optimizing FPGAs for power: A full-frontal attack
Figure 5: 1.5V/80A power with two LTC3866’s paralleled, each using power blocks
In applications where a higher value DCR inductor or sense resistor is used, the LTC3866 can be configured like any typical current mode controller by disabling the SNSD+ pin by shorting it to ground. An RC filter can be used to sense the output inductor signal. If the RC filter is used, its time constant, R • C, is set equal to L/DCR of the output inductor. In these applications, the current limit is normally five times larger for the specified current sense.
Conclusion
The LTC3866 allows the use of an ultralow DCR current sensing element to increase the efficiency in high current applications. Its current mode control provides the benefits over the alternative voltage mode controller of high re-liability with fast, cycle-by-cycle current sensing, simple feedback loop compensation and the ability to use all ceramic capacitors for the smallest solution size. The LTC3866 is suitable for low voltage, high current step-down converter applications needing high efficiency and high reliability. Tracking, strong on-chip drivers, multichip operation and external sync capability fill out its menu of features. The LTC3866 applies to point-of-load computer and telecom systems, industrial and medical instruments, and DC power distribution systems. Finally, a power supply designer can have a controller that incorporates the best of both current and voltage mode control schemes.
About the author:
Bruce Haug is senior product marketing engineer at Linear Technology
See related links:
UMC copper process targets power management ICs
Distributed power management cuts wiring weight
Power management IC market set to decline
TI leads power management IC ranking
Optimizing FPGAs for power: A full-frontal attack
|
|
|
|
|
Navigate to related information