Datasheet LT3433 (Analog Devices) - 9

LT3433
U U W U APPLICATIO S I FOR ATIO Antislope Compensation
switches the VOUT side of the inductor. The LT3433 bridged topology merges the elements of buck and boost Most current mode switching controllers use slope com- topologies, providing switches on both sides of the induc- pensation to prevent current mode instability. The LT3433 tor. Operating both switches simultaneously achieves is no exception. A slope compensation circuit imposes an both step-up and step-down functionality. artificial ramp on the sensed current to increase the rising slope as duty cycle increases. Unfortunately, this addi-
Step-Down (VIN > VOUT)
tional ramp corrupts the sensed current value, reducing SW L the achievable current limit value by the same amount as VIN VOUT the added ramp represents. As such, current limit is CIN D COUT typically reduced as duty cycles increase. The LT3433 contains circuitry to eliminate the current limit
Step-Up (V
reduction associated with slope-compensation, or anti-
IN < VOUT)
D slope compensation. As the slope compensation ramp is V L IN VOUT added to the sensed current, a similar ramp is added to the C C current limit threshold reference. The end result is that IN SW OUT current limit is not compromised so the LT3433 can provide full power regardless of required duty cycle.
Step-Up/Step-Down (VIN > VOUT or VIN < VOUT) Mode Switching
V SW L D IN VOUT The LT3433 switches between buck and buck/boost modes CIN D SW COUT of operation automatically. While in buck mode, if the 3433 F01 converter input voltage becomes close enough to the output voltage to require a duty cycle greater than 75%, Maximum duty cycle capability (DC the LT3433 enables a second switch which pulls the MAX) gates the dropout capabilities of a buck converter. As V output side of the inductor to ground during the switch-on IN – VOUT is reduced, the required duty cycle increases until DC time. This “bridged” switching configuration allows volt- MAX is reached, beyond which the converter loses regulation. With a age conversion to continue when VIN approaches or is less second switch bridging the switched inductor between V than V IN OUT. and ground, the entire input voltage is imposed across the When the converter input voltage falls to where the duty inductor during the switch-on time, which subsequently cycle required for continuous buck operation is greater reduces the duty cycle required to maintain regulation. than 75%, the LT3433 enables its ground-referred switch, Using this topology, regulation is maintained as VIN ap- changing the converter operation to a dual-switch bridged proaches or drops below VOUT. configuration. Because the voltage available across the switched inductor is greater while bridged, operational
Inductor Selection
duty cycle will decrease. Voltage drops associated with The primary criterion for inductor value selection in LT3433 external diodes and loss terms are estimated internally so applications is the ripple current created in that inductor. that required operating duty cycle can be calculated re- Design considerations for ripple current are converter gardless of specific operating voltages. output capabilities in bridged mode, output voltage ripple In the simplest terms, a buck DC/DC converter switches and the ability of the internal slope compensation wave- the VIN side of the inductor, while a boost converter form to prevent current mode instability. 3433f 9