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LTC7813

7813f

For more information www.linear.com/LTC7813

applicaTions inForMaTion

Compensation and VMID Capacitance in a Cascaded

Boost+Buck Regulator

When using the LTC7813 as a cascaded Boost+Buck

regulator, the boost and buck regulator control loops are

compensated individually. While this may seem more

complicated, this is actually advantageous, as the inher-

ently fast buck loop can be designed to handle the output

load transient, while the boost loop is less important and

can be slower.

TheamountofcapacitanceneededontheintermediateVMID

node (boost output) and the buck output VOUTdepends on

a number of factors, including the input voltage, output

voltage, load current and the nature of any transients,

and the mode of operation (Burst Mode operation, forced

continuous mode, or pulse-skipping mode).

In general, the buck regulator should be designed to

handle any output load transients and provide sufficiently

low output ripple.

The boost regulator does not need to respond as fast, as

the VMID node can tolerate relatively high ripple and/or

transient dips and therefore does not necessarily need a

lot of capacitance. The VMID node capacitance needs to

be able to handle the input ripple current from the buck

regulator. It also needs to be large enough that the boost

regulator’s voltage ripple and/or transient dips do not ap-

pear as significant input line steps to the buck regulator

and feed through to the buck regulator’s output.

The ripple on the VMIDnodeishigherinBurstModeopera-

tion and pulse-skipping mode than in forced continuous

mode, especially at light loads and/or if the input voltage

isslightlybelowtheregulatedboostoutput(VMID)voltage.

Thus, Burst Mode operation and pulse-skipping mode

generally require more VMID capacitance than in forced

continuous mode to maintain a similar amount of ripple.

The capacitance on the VMID node can be all ceramic, or

some combination of ceramic and polarized (tantalum,

electrolytic, etc.) capacitors.

Choosing the VMID Voltage in Cascaded Boost+Buck

Regulator

There are many performance trade-offs when considering

where to set the VMID (boost output) regulation voltage

(VMID_REG) relative to the input voltage (VIN) range and

output (buck) regulation voltage (VOUT_REG). These trade-

offsincludeefficiency,quiescentcurrent,switchingnoise/

EMI, and voltage ripple.

Remember that VMID will follow VIN if VIN > VMID_REG

(see the Boost Controller Operation When VIN > VOUT

section in the Operation section). If VIN < VMID_REG, VMID

is regulated to VMID_REG.

Consider as an example an automotive application that

requires a regulated 12V output voltage generated from a

vehicle battery. The battery spends most of its operating

lifetime in a normal range of 10V to 16V, but may dip to

as low as 2.5V during engine start and rise as high as 38V

during high voltage transients.

We can designate the minimum normal operating voltage

as VIN_MIN_OP = 10V, and the maximum normal operating

voltage as VIN_MAX_OP = 16V. So what voltage should we

choose for VMID_REG?

REGULATED OUTPUT VOLTAGE

Inthisexample,notethatwewantatightlyregulatedoutput

(VOUT_REG =12V), which is within our normal operating

range (VIN_MIN_OP < VOUT_REG < VIN_MAX_OP). We want

VMID_REG > VOUT_REG to provide headroom for the buck

regulator, but we have a choice of whether to set VMID_REG

above or below VIN_MAX_OP.

OPTION A: VMID_REG > VOUT_REG and VMID_REG >

VIN_MAX_OP

In this option, we set VMID_REG > VIN_MAX_OP (e.g.,

VMID_REG =18V). Both the boost regulator and the buck

regulator are switching (at full, constant frequency if in

forced continuous mode) over the full 10V to 16V nor-

mal operating range. Since the boost regulator is always

switching, the efficiency is lower and the input ripple and

EMI, while predictable and still low, are higher than other

potential options.

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