Tip of the Week: High power 3.3-volt bus for a Li-ion handheld device

Indeed, step-down converters excel in efficiency when converting a nominal 2.7 to 4.1 volt battery voltage to a lower output such as 1.8 volts, and step-up converters can efficiently develop higher output voltages such as 5 volts. But neither provides an optimal solution for generating the ever present 3.3-volt bus at higher currents. Topologies such as the single-ended primary inductance converter (SEPIC), and cascaded buck-boost stages 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. Here's how you can apply a suitable synchronous buck-boost controller to get the job done right.

Overview
A single- or dual-cell lithium-ion battery is usually charged from a wall adapter that provides an output in the 5- to 9-volt range. It's advantageous to power the DC/DC converter, which is normally used to deliver the 3.3-volt rail when battery operated, directly from the wall adapter during the battery recharge cycle using a simple power path controller. This 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 even while it 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 3.3-volt bus. But they have limitations. For example, the maximum output current is usually limited to about one amp and the maximum input voltage is typically 5.5 volts, which is not suitable for the higher input voltages used during the battery recharge cycle. This presents a design challenge for lithium-ion and lithium-polymer powered systems that require efficiencies of greater than 95 percent when delivering a 3.3-volt output at the higher output currents.

Choose the synchronous buck-boost
Regulators such as the LTC3785 synchronous buck-boost controller solve the problem. Its 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 2.7 to 10 volts. Thus it can be used with one or two lithium-ion/polymer cells, or multiple cell NiMH, NiCd, or alkaline batteries. It offers programmable features including soft start, switching frequency, and a current limit threshold voltage. Relatively few external components are required.

In this case, we apply the LTC3785 controller in a synchronous, four-switch buck-boost converter topology to produce a fixed output of 3.3 volts at 3 amps from a 2.7-to-10 volt input (Figure 1). Its efficiency is as high as 96 percent (Figure 2). The LTC3785 provides all n-channel MOSFET gate drive, facilitating the use of low R_DS(ON) devices. Its topology and control architecture employs MOSFET R_DS(ON) sensing for forward and reverse current limiting. An optional sense resistor may be used if the designer desires increased current-limit accuracy. Moreover, the chip incorporates Burst Mode operation, which reduces the (light-load) quiescent current to less than 100 microamps. The LTC3785 also incorporates true output disconnect during shutdown so that the battery is not connected to the system load.

Maximizing efficiency
The LTC3785 is based on a standard H-bridge buck-boost power stage (Figure 3). 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 four 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. In this case, the circuitry transitions seamlessly between the buck, buck-boost and boost regions of operation to maintain low-noise performance across all operational modes.

This control scheme also significantly reduces unnecessary switching and conduction losses to maximizing the converter's efficiency.

Modes of Operation
When the input voltage 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.

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 four-switch region of operation (Figure 4). 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. Thus the circuit functions as a synchronous boost converter.

This synchronous buck-boost design 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 chip delivers a controlled bursts of current until the output voltage reaches regulation. At this point the part is put into a sleep state where the drive to the external MOSFETs is turned off and only critical circuitry is kept alive, with the chip drawing less than 100 microamps. 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 chip circuitry 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 chip's 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 time, the converter will be disabled and a reset timer is started to restart 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.

The chip's internal p-channel low-dropout regulator produces 4.35 volts at the V_CC 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. The output must be bypassed to ground (minimum capacitor value is 4.7 microfarads). The V_CC regulator can be connected to V_OUT through a Schottky diode to provide even higher gate drive current.

The chip circuitry also incorporates overvoltage and undervoltage functions for fault protection and transient limitation. If the circuit senses the output voltage to be more than 9.5 percent above its target regulation point, the chip will terminate switching. 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.

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, V_BR(DSS), gate threshold voltage V_GS(TH), on-resistance, and maximum current I_DS(MAX). The drive voltage is set by the 4.35 volt V_CC. For most applications where the input voltage is expected go below 5 volts, 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 supply takes less than 2-by-2 cm of board area. The inductor is the tallest profile part (0.32 cm).

Converting a lithium-ion/polymer battery and wall adapter input voltage to 3.3 volts requires careful consideration. As mentioned previously, a SEPIC design, or using a cascaded boost and buck converter, utilize the full battery capacity but, again, suffer from low efficiency, high cost, increased board area, and higher parts count and design complexity. The typical monolithic synchronous buck-boost converter typically has limited output current and input-voltage range.