High power 3.3V bus in Li-ion handheld devices

Generating an output voltage that is always above or below the input voltage can easily be accomplished by a conventional boost or buck regulator. However, when the output voltage is at a fixed voltage that is within a wide input voltage range, like when producing a 3.3V bus from a single cel Li-Ion battery, conventional designs fall short, suffering from low efficiency, polarity inversion and complex circuitry.

While buck (step-down) converters excel in efficiency when converting a nominal 2.7V to 4.1V Li-Ion battery voltage to a lower output voltage such as 1.8V, and boost (step-up) converters can efficiently develop higher output voltages such as 5V, neither provides an optimal solution for generating the ever present 3.3V bus. Topologies such as the Single-Ended Primary Inductance Converter (SEPIC) and cascading a boost and buck converter can utilize the full battery capacity but suffer from drawbacks such as low efficiency, high cost, increased board area, higher parts count and design complexity.

A single or dual cell Li-Ion battery is usually charged from a wall adapter in the 5V to 9V output voltage range. It is advantageous to power the DC-DC converter, normally used to deliver the 3.3V rail when battery operated, directly from the wall adaptor during the battery recharge cycle using a simple Power Path controller. This, of course, requires the converter to be able to not only operate at the minimum battery input voltage but also from elevated input voltages. Traditionally, manufacturers of battery powered handheld devices draw power directly from the battery voltage even while the battery is being charged. This type of DC/DC converter also needs a very low quiescent current to conserve battery energy during standby or idle mode.

Monolithic IC based synchronous buck-boost converters are very efficient, (up to 95%) and can be commonly used for battery-powered devices needing a regulated 3.3V bus. However, they have limitations. For example, the maximum output current is usually limited to about one amp and the maximum input voltage is typically 5.5V, which is not suitable for the higher input voltages used during the battery re-charge cycle from a wall adaptor. This presents a design challenge for Li-Ion and Li-Polymer powered systems requiring efficiencies of greater than 95% when delivering a 3.3V output at higher output currents.

A Synchronous Buck-Boost Controller is an Ideal Solution

Linear Technology’s LTC3785 synchronous buck-boost switching regulator DC/DC controller represents an ideal solution to the complex problem of providing a fixed 3.3V output rail in a single cell Li-Ion powered handheld device. Its proprietary buck-boost topology requires only a single inductor to efficiently generate a fixed output voltage from an input voltage that can be above, below or equal to the output. The LTC3785 operates with both input and output voltages between 2.7V to 10V, making it an excellent choice for one or two Li-Ion or Li-Polymer cells, or multiple cell NiMH, NiCad or Alkaline batteries. Using only a small number of tiny external components, this highly integrated controller offers many programmable features including soft start, switching frequency, and a current limit threshold voltage.

The application schematic, with corresponding efficiency curves in Figure 1, utilizes the LTC3785 controller in a synchronous, 4-switch buck-boost converter topology produce a fixed output of 3.3V at 3 amps from a 2.7V to 10V input with efficiencies as high as 96%. The LTC3785 provides all N-channel MOSFET gate drive, facilitating the use of low RDS(ON) single package multiple power switch technology. Its proprietary topology and control architecture employs MOSFET RDS(ON) sensing for forward and reverse current limiting, yielding unrivaled efficiency. An optional sense resistor may be used when increased current limit accuracy is desired. Moreover, the LTC3785 incorporates Burst Mode® operation which reduces light load quiescent current to less than 100µA, which is crucial to help extend battery operating life in portable applications that spend the majority of their time in standby mode. The LTC3785 also incorporates true output disconnect during shutdown so that the battery is not connected to the system load.

Advanced Control Maximizes Efficiency

The LTC3785 is based on a standard H-bridge buck-boost power stage as shown in Figure 2. It contains both buck and boost switching MOSFETs that are connected through a single inductor. The LTC3785 utilizes a proprietary design that switches only two MOSFETs at a time during the buck or boost mode. During the time when the input voltage is approximately equal to the output voltage the LTC3785 enters the buck-boost mode where all 4 MOSFETs are switching in a controlled manner. Many buck-boost control schemes exhibit efficiency drops, power supply jitter, or unstable output voltage at the transition points. However, the LTC3785 transitions seamlessly between the buck, buck-boost and boost regions of operation maintaining low noise performance across all operational modes. This control scheme also significantly reduces unnecessary switching and conduction losses maximizing the converter’s efficiency.

Modes of Operation

When the input voltages is well above the output, the converter operates in buck mode, switches A and B commutate the input voltage, and switch D stays on, connecting L to the output (see figure 2). As the input voltage is reduced and approaches the output, the converter approaches the maximum duty cycle for buck mode operation, and the boost section of the bridge side starts to switch, thus entering the buck-boost or 4-switch region of operation. As the input is reduced further, the converter enters the boost region. At the minimum boost duty cycle, switch A stays on, connecting the inductor to the input and switches C and D commutate the output side of the inductor between the output capacitor and ground functioning as a synchronous boost converter.

More Flexibility

The LTC3785 contains additional features that enhance its usability in portable applications which require, for example, an extremely low quiescent current to extend battery life. For these portable applications, the part can be configured to operate in Burst Mode operation in order to conserve battery life. During Burst Mode, the LTC3785 delivers packets of energy to the output until the output voltage reaches regulation. At this point the part is put into a sleep state where the drive to the external MOSFETs are turned off and only critical circuitry is kept alive with the LTC3785 drawing less than 100uA. The load current during this period is supplied by the output capacitor. When the output voltage has dropped below the lower regulation boundary, the part “wakes up” and starts switching again, thereby replenishing the output capacitor.

The LTC3785 also provides overload and short circuit protection by sensing and limiting the input current drawn from the input supply by MOSFET A. If the user-programmed current limit is reached, the soft-start capacitor attached to the RUN/SS pin is re-used as a fault timer and begins discharging when in standby mode. If the current limit condition persists for a long enough period of time the converter will be disabled and a reset timer is started to re-start the converter. If the LTC3785 is unable to restart and the overload condition persists, this mode of operation will continue limiting the overall power dissipation. The part can be commanded to latch-off instead of automatically restarting by sourcing a small current into the RUN/SS pin. MOSFET drain to source sensing is typically not very accurate due to variations in the resistance of the external MOSFETs. A current sense resistor can be added if tighter current limit accuracy is necessary. The LTC3785 can be programmed to provide full class D operation allowing the converter to source and sink current equal to the current limit set point. This is achieved by asserting a high logic level signal on the CCM pin.

An internal P-channel low dropout regulator produces 4.35V at the VCC pin from the input supply voltage. This voltage powers the drivers and internal circuitry of the LTC3785 and can supply a peak current of 100 mA and must be bypassed to ground with a minimum capacitor value of 4.7µF. The VCC regulator can be connected to VOUT through a Schottky diode to provide even higher gate drive current.

Finally, the LTC3785 incorporates overvoltage and undervoltage functions for fault protection and transient limitation. If the output voltage is sensed to be more than 9.5% above its target regulation point, switching is terminated. The output voltage will then drop to a safer level as no energy is supplied to the output. Switching will recommence once the output has sufficiently dropped. During an undervoltage condition, the IC is forced to operate in fixed frequency mode with Burst mode operation disabled.

Power N-Channel MOSFET Selection and Solution Size

The LTC3785 requires four external N-channel power MOSFETs, two for the top switches and two for the bottom switches. In a typical 3 amp output design, two SO-8 packages can be used, each of which containing two MOSFETs. Important parameters are the breakdown voltage VBR(DSS), gate threshold voltage VGS(TH), on-resistance RDS(ON) and maximum current IDS(MAX). The drive voltage is set by the 4.35V VCC supply. For most applications where the input voltage is expected go below 5V, sub-logic gate threshold MOSFETs can be used. Furthermore, a typical LTC3785 DC/DC converter utilizes all ceramic input and output capacitors. The complete circuit size of a 10 watt output takes less than 2cm x 2cm of board area with the inductor being the tallest profile part at 0.32cm.

Conclusion:

Converting a Li-Ion or Li-Polymer battery and a wall adapter input voltage to 3.3V requires careful consideration. A SEPIC or cascading a boost and buck converter utilize the full battery capacity but suffer from low efficiency, high cost, increased board area, higher parts count and design complexity. A monolithic synchronous buck-boost converter typically has limited output current and input voltage range.

However, a LTC3785 based design operates seamlessly throughout the entire input voltage range from 2.7V to 10V and features selectable Burst Mode operation to maintain high efficiency at light loads that helps preserve battery life during standby or idle mode. Protection features include over voltage, over current and short circuit in all operating modes. It also incorporates true output disconnect during shutdown and can be an excellent choice when needing up to a 35W DC/DC converter to produce a fixed output voltage when the input voltage is above, below or equal to the output voltage. Typical applications include tablet PCs, handheld instruments, portable media players, along with a wide variety of single or dual-cell Li-Ion or multi-cell alkaline/NiMH powered devices.


Captions

Figure 1. LTC3785: Buck-Boost Converter Schematic and associated Efficiency Curve

Figure 2. Block Diagram of Power Section and Operation Modes

Click here for the illustrations:

Figure 1, Figure 2