FDMF2011 datasheet(27/31 Pages) ONSEMI

27 / 31 page

www.fairchildsemi.com

FDMF2011 Rev.1.0

However, similar operation can arise when a

switching converter (such as a synchronous

buck) is pulling energy from the output filter

capacitors and delivering the energy back to

the input filter capacitors.

Figure 67. Synchronous Boost Converter

operating in CCM

From

module

perspective,

the

main

difference here versus the previous (buck)

operating mode is that this situation will cause

the LS FET (Q2) to act as the control

MOSFET and hard switch while the HS FET

(Q1) acts as a synchronous rectifier and

undergoes soft switching with body diode

recovery. This type of operation can drastically

change power losses dissipated in Q1 and Q2

versus buck operating mode.

dVDS/dt control using external

gate resistors

The FDMF2011 also provides module pins for

placing external gate resistors. The module

provides pins for the HO and LO signals

(driver output signals) and the HG and LG

(Power MOSFET gate pins). Resistors can be

placed in series with the MOSFET gate to

control the SW node edge rates.

Independently controlling MOSFET (slower)

turn-on and (faster) turn-off slew rates can

also be accomplished by using the resistor

and diode circuit shown in Figure 68..

Figure 68. Gate drive resistor-diode

circuit

CGD x dVDS/dt turn-on

CGD x dVDS/dt turn-on is a false (unwanted)

turn-on event that often creates a brief and

uncontrolled shoot through current between

the HS (Q1) and LS (Q2) MOSFETs.

Typically, a CGD x dVDS/dt “shoot-through”

condition

arises from

capacitive

feedback

current flowing through CGD into CGS inducing

a gate-bounce-induced channel turn-on of the

MOSFET. Holding the gate below threshold

can become challenging because the high-

frequency

capacitive

displacement

current

from CGD (due to dVDS/dt) couples back to

circuit ground through the gate electrode.

Figure 69. CGD x dVDS/dt current flow

Ld_LS

Ls_LS

LG_brd

LG_pack

RDRIVE_LS

CGD

CGS

ZGate_Drv ~ R + ωL

ZMOS_Gate ~ 1/ωC

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