Flexible Voltage Regulation with LTC3440
In various applications, powering a circuit from a battery poses challenges when the required supply voltage falls within the battery’s discharge curve. A new solution is offered by Linear Technology through the LTC3440 buck/boost voltage converter, accessible at www.linear.com. Illustrated in Figure 1, this switching regulator efficiently converts input voltages ranging from +2.7 V to +4.5 V into output voltages spanning +2.5 V to +5.5 V, all accomplished with a single compact coil. The output voltage level is precisely determined by the voltage divider created by resistors R2 and R3. The device adeptly transitions between step-up (or ‘boost’) operation when Vin is less than Vout and step-down (or ‘buck’) operation when Vin surpasses Vout, ensuring consistent and reliable voltage regulation regardless of the battery’s state of charge.
Optimal Voltage Range Adaptation
A common dilemma arises when a circuit is powered by a battery, especially when the desired supply voltage lies within the battery’s discharge curve. Initially, when the battery is brand new, the circuit might receive a voltage higher than required. Conversely, as the battery approaches the end of its lifespan, the voltage supplied might fall below the desired level. Linear Technology’s innovative solution, the LTC3440 buck/boost voltage converter, effectively addresses this challenge. Operating within a broad input voltage range of +2.7 V to +4.5 V, this converter ensures seamless conversion to an output voltage spanning +2.5 V to +5.5 V, all regulated by a miniature coil. Precise voltage adjustment is achieved through the voltage divider configuration of resistors R2 and R3. Notably, the device swiftly switches between step-up (or ‘boost’) operation for Vin < Vout and step-down (or ‘buck’) operation for Vin > Vout, guaranteeing optimal voltage adaptation irrespective of the battery’s discharge state.
Efficient Voltage Conversion with LTC3440
With a maximum output current capacity of 600 mA, the LTC3440 integrates four MOSFET switches, as depicted in Figure 2. These switches offer versatile connections, allowing them to link the input side of coil L1 to either Vin or ground, and the output side of L1 to either the output voltage or ground. In step-up mode, switch A remains continuously on while switch B is consistently off. Switches C and D alternate, enabling energy storage from the input within the inductor and its subsequent release into the output, thus elevating the output voltage beyond the input level. Conversely, in step-down mode, switch D maintains a constant closed state, and switch C is perpetually open.
Here, switches A and B take turns closing, resulting in a reduced voltage at Vout in direct proportion to the switching signal’s mark-space ratio. Coil L1, in conjunction with the output capacitor, forms an effective low-pass filter. When the input and output voltages closely match, the IC enters pulse-width modulation mode employing all four switches. The IC’s switching frequency is determined by resistor R1, with its preset value yielding approximately 1.2 MHz, ensuring the feasibility of using a compact coil like the DT1608C-103 from Coilcraft (www.coilcraft.com).
Control and Efficiency Enhancement
The IC boasts a shutdown capability through the SHDN/SS input. Additionally, a ‘soft start’ feature can be introduced by applying a gradually ascending voltage to this pin, implemented via an RC network. To further enhance efficiency, the MODE pin offers the selection between fixed-frequency operation (MODE linked to ground) and burst mode operation (MODE=Vin). Burst mode operation is especially favorable for low currents, delivering an efficiency range between 70% and 80%. As the current increases to around 100 mA, efficiency surges to over 90%. To maximize efficiency, consider incorporating the two Schottky diodes illustrated with dotted lines in the circuit diagram. These diodes come into play during the brief period when both active switches are momentarily open, ensuring smooth operation.