LTC7813 datasheet(20/36 Pages) LINER

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LTC7821

Rev A

For more information www.analog.com

RdsON product. M3 and M4 operate in a manner similar

to traditional buck converter, with M3 hard switching while

M4operatesinzerovoltageswitching(ZVS).ThereforeM3

and M4 should be chosen with the lowest (Qgd • RdsON)

and (Qg • RdsON) product, respectively.

MOSFET input capacitance is a combination of several

components but can be taken from the typical gate charge

curve included on most data sheets (see Figure 10). The

curve is generated by forcing a constant input current into

the gate of a common source, current source loaded stage

and then plotting the gate voltage versus time.

The initial slope is the effect of the gate-to-source and

the gate-to-drain capacitance. The flat portion of the

curve is the result of the Miller multiplication effect of the

voltage, but can be adjusted for different VDS voltage by

multiplying the ratio of the application VDS to the curve

specified VDS value. A way to estimate the CMILLER term is

to take the change in gate charge from point a-and-b on a

manufacturer’s data sheet and divide by the specified VDS

voltage. CMILLER is the most important selection criteria

for determining the transition loss term in the MOSFET

M3 but is not directly specified on MOSFET data sheets.

CRSS and COss are specified sometimes but definitions of

these parameters are not included.

In a traditional synchronous buck converter, the current

flowing through the upper and lower MOSFET is the same

as the inductor current (see Figure 11). In the hybrid topol-

ogy of the LTC7821, the MOSFET and inductor currents

do not match, because the capacitors play a role in energy

transfer to the output.

In the first phase (see Figure 12a), M1 and M3 are ON

and the capacitor CMID provides part of the inductor cur-

rent via M3. The rest of the inductor’s current is provided

through CFLY via M1. If the capacitance of CFLY is the same

as CMID, then the inductor’s current is equally supplied

by both capacitors as shown in Figure 12b. Therefore

compared to the traditional buck converter with the same

amount of inductor current, less current flowing through

M3meanslowerswitchinglossandconductionloss.Since

M3 switches hard, this reduction in current reduces the

switching loss significantly.

In the 2nd phase, M2 and M4 are ON (see Figure 13a).

In this phase, M4 not only has to supply the full inductor

APPLICATIONS INFORMATION

M3 CURRENT FLOW

M4 CURRENT FLOW

TIME

7821 F11

M3 CURRENT

M4 CURRENT

Figure 11. MOSFET’s Current of Traditional Synchronous Buck Converter

Figure 10. Gate Change Characteristic

gate-to-drain capacitance as the drain drops the voltage

across the current source load. The upper sloping line is

due to the drain-to-gate accumulation capacitance and

the gate-to-source capacitance. The Miller charge (the

increase in coulombs on the horizontal axis from a-to-b

while the curve is flat) is specified for a given VDS drain

MILLER

EFFECT

QIN

CMILLER = (QB – QA)/VDS

VIN

7821 F10

VDS

VGS

+–

–

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