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Sponsored by IntersilA Supplement to Electronic Design

dc voltage based on the ratio of the on-time to cycle-time (the duty

cycle D) of a switch that opens and closes at a controlled rate. It

uses pulse-width modulation (PWM) to control the step-down ratio

between the input voltage (VI) and the output voltage (V

O).

Reduced to the simplest elements, the buck converter consists

of a voltage source, a load, an inductor, a capacitor, a high-side

switchgenerally a power MOSFET, but an insulated gate bipolar

transistor (IGBT) or a bipolar junction transistor (BJT) also can be

usedand a low-side switch. The low-side switch can be just adiode, as shown. But for greater efficiency, it can be replaced by

another MOSFET that closes when the high-side switch opens and

opens when the high-side switch closes.

The relationship between input voltage and output voltage can be

reduced for simplicity to:

VO= V

I/(1 + (2LI

O)/(D2V

IT))

where VOis output voltage, V

Iis input voltage, L is the inductor value,

D is the duty cycle, IOis the output current, and T is the period of one

cycle or 1/FSW

(switching frequency).

The Boost ConverterIn the ideal boost converter, when the low-side switch is closed

(on), current through the inductor increases and energy is stored

in the inductor (Fig. 1b). When the low-side switch is open, current

continues through the inductor, through the flyback diode (or high-

side switch in a synchronous implementation), then to the capacitor

and the load.

In continuous mode, L di/dt determines the voltage. The

expression for the output voltage is VO= V

I/(1 D). The closer the

duty cycle is to unity, the higher the output voltage. In discontinuous

mode, where the inductor current falls to zero during part of each

commutation period:

VO

= VI+ (V

I2D2T)/(2LI

O)

where T is again the length of one commutation cycle.

Buck-Boost-InvertingA buck-boost circuit can be as simple as the buck or boost

circuits requiring just a switch, diode, inductor, output capacitor, and

modulator or considerably more complex depending on the require-

ments.

The inverting buck-boost is the simplest of the circuits, providingan output whose polarity is opposite of the inputs (Fig. 1c). When S1

is closed, energy is stored in the inductor. When S1 is opened as it

is in the figure, the voltage across the inductor reverses and current

flows from the inductor to the output cap and load and then back to

the inductor via the diode.

With the use of a second inductor, the Cuk converter reduces the

high ripple current that is seen on the input capacitor (Fig. 1d).

Buck-Boost-NoninvertingThere are a number of solutions for non-inverting buck-boost

regulations, such as the charge pump, flyback, and single-ended

primary inductor coupled (SEPIC). However, all of them have theirlimitations. In todays integrated world, while the charge pump pro-

vides a simple, low-cost solution, it is generally limited to a couple

hundred milliamps. The flyback requires a costly coupled inductor

usually specifically wound for the application (Fig. 1e). Additionally,

the flyback is inherently noisy. The SEPIC circuit is similar to the

Cuk, but with L2 and the diode swapped, resulting in a non-inverting

output (Fig. 1f).

Though the efficiency of the SEPIC converter has improved over

the years with the development of monolithic capacitors, it still

suffers lower efficiency do to the losses in CCand L2. Slightly better

efficiency may be witnessed if the L1 and L2 are wound on the samecore. Because of the cost, solution size, and poor efficiency, the

SEPIC never really gained acceptance in handheld devices.

The H-bridge buck-boost converter is simply a buck and boost

sharing one inductor (Fig. 1f). The diode for the buck section has

been replaced with a synchronous FET, S2, for improved efficiency.

Likewise, S4 has replaced the diode in the boost section. Given that

the FETs can be integrated for mobile applications as well as for

many industrial applications, the solution can be quite small and

simple to design with, requiring just two external capacitors, CInand

COut

, and the inductor.

H-Bridge Advantages And ChallengesThe obvious benefit of the H-bridge buck-boost is the simplicity

for the system designer. But additionally, the H-bridge offers high

efficiency, ample load current capability, and good line transient re-

sponse. It also demands little quiescent current. And, a simple buck

or boost would use off-the-shelf capacitors and inductors.

Probably the greatest challenge in designing a buck-boost is in

meeting stringent transient requirements in todays microprocessor-

based equipment. This is especially difficult when VIapproaches the

desired output voltage. The H-bridge has to decide whether to oper-

ate in buck, boost, or buck-boost mode. The transition point between

these operating modes and the speed at which the device can transi-

tion can be critical to maintaining a regulated output voltage.

H-Bridge Design Considerations Ripple: As the voltages for loads such as microprocessors and

memory drop along with increasingly sensitive analog and RF

circuits, the need for lowered ripple voltage on the output is

becoming an absolute.

Input voltage transients: An autonomous buck-boost is expected

to ride through any input voltage changes. In some cases, these

changes can be quite rapid as in hot-swapping or emergency

backup applications. The capability of the buck-boost to ride

through these abrupt changes is often the most critical perfor-mance requirement.

Load transients: In boost operation, S1 is on 100% of the time

and S2 remains off. So, the device reacts just as a simple boost

converter would during an increase in load.

The difference between a simple boost and buck-boost comes into

play when the source has a high output impedance and suddenly the

load drops considerably. This may result in VIrising close to or even

above the desired output voltage. So not only does the device need

to respond to a change in load, possibly transitioning into pulse-fre-

quency modulation (PFM) mode, it may also need to rapidly change

to buck-boost or even buck operation.For buck operation, S4 is on 100% of the time and S3 remains

off. And similar to the boost operation above, if the load suddenly

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Sponsored by IntersilA Supplement to Electronic Design

increases during buck operation from a high impedance source, the

device may need to quickly begin operating as a buck-boost or in

boost mode to hold up the output. Adding to the complexity, the de-

vice may simultaneously be transitioning into PWM operation from

PFM. The response time, the max voltage delta, and the recovery

time during this transition are all parameters system designers need

to be concerned about.

The VIntransition also poses additional challenges to the control

circuit. With four switches operating in alternating pairs, it isimportant to understand the impact of dead-time between switches.

In the buck mode, when S2 is initially on, it conducts the inductor

current while S1 is off. This is followed by S2 turning off, and S1

remains off for a few nanoseconds.

This time between S2 turning off and S1 turning on is the dead

time, and it is required as a safety measure to avoid shoot-through

between VInand ground through a low impedance path. During this

time, the inductor current cannot instantly change to zero. Hence,

the S2 body diode will conduct.

In the next part of the switching cycle, S1 turns on and starts

conducting the inductor current. The S2 body diode undergoes re-

verse recovery while S2 remains off. A similar event happens in the

boost mode, involving S3 and S4. This process can have a signifi-

cant impact on the transient response of the converter, particularly

when the part is switching back and forth between buck and boost

modes and also possibly undergoing a load transient event.

Combining buck and boost operation, for stiff sources, the load

transient response will depend for the most part on the control loop

and switching frequency. For sources with high output impedance,

where the input voltage can vary in and out of the buck-boost range

of operation, the transient behavior can be quite severe. Care needs

to be taken, then, when selecting a buck-boost converter to ensure

it meets the performance needs of the circuit.

Performance TradeoffsMany buck-boosts either provide a way of setting the F

SWwith an

external component or in providing an input to sync to an external

clock. This often requires external compensation to work with the

selected FSW

. Also, even some fixed-frequency converters require

an external compensation network. Often, these components are

unique to the system, increasing the bill of materials (BOM) size.

In the cases where the FSW

is set by an external component, due

to the variability of both the converters own circuitry and that of

the external component, the converter may be limited to simple

applications. Converters that provide a sync pin can also be used in

applications where precise frequency control is necessary to pre-

vent the generation of beat frequencies and other electromagnetic

interference (EMI) that might disrupt sensitive circuits.

Buck-boost converters that offer a frequency window that the

device can be synchronized to without requiring any external com-

pensation, then, add considerable value to the solution.

Transition Points For Buck-Boost OperationThe voltage window width and the methodology of buck-boost

operation can have a significant impact on battery run time. For

instance, if the desired output is 3.3 V, operating from a Li-ion or

polymer battery and the window of buck-boost operation is 10%

of VO, the device will be in buck-boost operation for nearly 50% of

the batterys discharge cycle. Thus, the efficiency of buck-boost

mode operation is critical in these cases. There are three methods

of implementing buck-boost operation:

Pass through: Both high-side FETs (100% duty cycle) and low-

side FETs are off (0% duty cycle). This theoretically can provide

the highest efficiency, since there are no switching losses in driv-

ing the gates and no channel transitions losses. However, output

regulation can be quite poor.

Quadrature: The input high-side (S1) switch and output low-side

(S3) switch are first turned on, creating an increasing field

across the inductor. Then both switches are turned off and the

input low-side (S2) and output high-side (S4) switch are turned

on to transfer the inductors energy to the output cap and load.

The advantage of this approach is good line transient response.

Theoretically, though, it has the poorest efficiency since all four

switches are being controlled each cycle.

Bang-bang: The bang-bang methodology runs in boost and then

in buck operation, keeping the output between an upper and

lower limit (Fig. 2). The voltage deviations are far smaller than

(generally less than 0.5% of VO) the voltage regulation tolerance,

ensuring such deviations will not impact the operation of the load.

Balanced PerformanceThere are many applications for buck-boost converters in

todays mobile devices. Some require a device that can run from

two alkaline or nickel-metal-hydride (NiMH) cells. Other applica-

tions require a device that can range down to low voltages during

standby conditions.

However, the device needs to be simple to design in and takelittle space. And for mobile devices, where battery runtime is criti-

cal, they have to be efficient. The capability to ride through line and

load transients on the part of the buck-boost may be the difference

between a successful and unsuccessful product development.

As examples of the H-bridge architecuture intended for battery-

powered mobile devices, Intersils ISL9110 and ISL9112 represent

the state of the art in H-bridges. The ISL9112 also features an

I2C interface that allows programmability of the output voltage on

the fly and also controls the rate of change of the output voltage

transition. The low losses in their physical structure provide high ef-

ficiency, and a high switching frequency provides excellent transient

response and a flexible sync range that doesnt require externalcompensation. They accept a wide range of voltages and provide

voltages down to 1 V.

Upper limit

VOsetting

Lower limit

2. There are various ways of synchronizing the two extra MOSFET

switches in H-bridge buck-boost converters, including the pass-

through, quadrature, and bang-bang approaches. The first two can be

understood by inspecting the topology of the converter. This diagramof V

Ohelps explain why the bang-bang approach can achieve better

efficiency than quadrature and better regulation than pass-through.