Introducing the LTC3890
The LTC3890/-1 is a high voltage dual output synchronous step-down DC/DC controller that draws only 50uA when one output is active and 60uA when both outputs are enabled. With both outputs shut down, the LTC3890/-1 draws only 14uA. The 4V to 60V input supply range is designed to protect against high voltage transients, continue operation during automotive heavy equipment and truck cold cranking along with covering a broad range of input sources and battery chemistries. Each output can be set from 0.8V to 24V at output currents up to 25 amps, with efficiencies as high as 98 percent making it well suited for 12V, 24V or 28V automotive, truck, heavy equipment and industrial control applications.
The LTC3890/-1 operates with a selectable fixed frequency between 50kHz and 900kHz, and can be synchronized to an external clock from 75kHz to 850kHz with its phase-locked loop (PLL). The user can select from continuous operation, pulse skipping and low ripple Burst Mode operation during light loads. The LTC3890’s 2-phase operation reduces input filtering and capacitance requirements. Its current mode architecture provides easy loop compensation, fast transient response and excellent line regulation. Output current sensing is accomplished by measuring the voltage drop across the output inductor (DCR) for the highest efficiency or by using an optional sense resistor. Current foldback limits MOSFET heat dissipation during overload conditions. These features, combined with a minimum on-time of just 95ns, make this controller an ideal choice for high step-down ratio applications.
The device is available in two versions; the LTC3890 is the fully featured part with functions including a clock out, clock phase modulation, two separate power good outputs and adjustable current limit. The LTC3890-1 does not have these extra features and is available in a 28-pin SSOP package. The LTC3890 is available is a 32-lead 5mm x 5mm QFN package. Both packaged versions are available in H-grade and MP-grades which have an operating junction temperature from -40°C to 150°C and -55°C to 150°C, respectively.
Burst Mode operation, pulse skipping or forced continuous mode
The LTC3890/-1 can be enabled to enter high efficiency Burst Mode operation, constant frequency pulse skipping, or forced continuous conduction mode at low load currents. When configured for Burst Mode operation and during a light load condition, the converter will burst out a few pulses to maintain the charge voltage on the output capacitor. It then turns off the converter and goes into sleep mode with most of its internal circuits shut down. The output capacitor supplies the load current and when the voltage across the output capacitor drops to a programmed level, the converter starts back up delivering more current to replenish the charge voltage. The action of shutting down and turning off most of its internal circuits significantly reduces quiescent current, thereby helping to extend the battery run-time in an “always-on” system when the system is not running. Figure 2
shows the conceptual timing diagram of how this works.
Figure 2: Burst Mode operation voltage diagram for the LTC3890
Click on image to enlarge
The Burst Mode output ripple is load independent so only the length of the sleep intervals will change. In sleep mode, much of the internal circuitry is turned off except for the critical circuitry needed to respond quickly, further reducing its quiescent current. When the output voltage drops low enough, the sleep signal goes low and the controller resumes normal Burst Mode operation by turning on the top external MOSFET. Alternatively, there are instances when the user will want to operate in forced continuous or constant frequency pulse skipping mode at light load currents. Both of these modes are easily configurable but will have a higher quiescent current and a lower peak to peak output ripple.
In addition, when the controller is enabled for Burst Mode operation, the inductor current is not allowed to reverse. The reverse current comparator, IR, turns off the bottom external MOSFET just before the inductor current reaches zero, preventing it from going negative. Thus, the controller also operates in discontinuous mode when configured for Burst Mode operation.
Furthermore, in forced continuous operation or when clocked by an external clock source, the inductor current is allowed to reverse at light loads or under large transient conditions. Continuous operation has the advantage of lower output voltage ripple and results a higher quiescent current.