LTC3890
applicaTions inForMaTion
Thepeak-to-peakdrivelevelsaresetbytheINTV voltage.
where δ is the temperature dependency of R
DR
and
CC
DS(ON)
This voltage is typically 5.1V during start-up (see EXTV
R
(approximately 2Ω) is the effective driver resistance
CC
Pin Connection). Consequently, logic-level threshold
at the MOSFET’s Miller threshold voltage. V
is the
THMIN
MOSFETs must be used in most applications. Pay close
typical MOSFET minimum threshold voltage.
attentiontotheBV specificationfortheMOSFETsaswell.
DSS
2
BothMOSFETshaveI RlosseswhilethetopsideN-channel
equation includes an additional term for transition losses,
Selection criteria for the power MOSFETs include the
on-resistance, R
, Miller capacitance, C
DS(ON)
, input
MILLER
which are highest at high input voltages. For V < 20V
IN
voltage and maximum output current. Miller capacitance,
, can be approximated from the gate charge curve
the high current efficiency generally improves with larger
C
MOSFETs, while for V > 20V the transition losses rapidly
MILLER
IN
usually provided on the MOSFET manufacturers’ data
increasetothepointthattheuseofahigherR
device
DS(ON)
sheet. C
is equal to the increase in gate charge
withlowerC
actuallyprovideshigherefficiency.The
MILLER
MILLER
along the horizontal axis while the curve is approximately
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period.
flat divided by the specified change in V . This result is
DS
then multiplied by the ratio of the application applied V
DS
to the gate charge curve specified V . When the IC is
DS
operating in continuous mode the duty cycles for the top
The term (1+ δ) is generally given for a MOSFET in the
and bottom MOSFETs are given by:
form of a normalized R
vs Temperature curve, but
DS(ON)
VOUT
δ = 0.005/°C can be used as an approximation for low
Main Switch Duty Cycle =
voltage MOSFETs.
V
IN
The optional Schottky diodes D3 and D4 shown in
Figure 11 conduct during the dead-time between the
conduction of the two power MOSFETs. This prevents
the body diode of the bottom MOSFET from turning on,
storing charge during the dead-time and requiring a
reverse recovery period that could cost as much as 3%
V − V
IN
OUT
Synchronous Switch Duty Cycle =
V
IN
The MOSFET power dissipations at maximum output
current are given by:
VOUT
2
in efficiency at high V . A 1A to 3A Schottky is generally
IN
PMAIN
=
=
I
1+ δ R
+
)
(
)
(
)
MAX
DS(ON)
V
a good compromise for both regions of operation due
to the relatively small average current. Larger diodes
result in additional transition losses due to their larger
junction capacitance.
IN
2
IMAX
2
V
R
C
MILLER
•
(
)
(
)
(
IN
DR
1
1
+
f
( )
C and C
IN
Selection
OUT
VINTVCC – VTHMIN
V
THMIN
The selection of C is simplified by the 2-phase architec-
IN
V – V
2
IN
OUT
ture and its impact on the worst-case RMS current drawn
throughtheinputnetwork(battery/fuse/capacitor).Itcanbe
shown that the worst-case capacitor RMS current occurs
when only one controller is operating. The controller with
P
I
(
1+ δ R
) ( )
MAX
SYNC
DS(ON)
V
IN
the highest (V )(I ) product needs to be used in the
OUT OUT
formula shown in Equation 1 to determine the maximum
3890fb
18
Linear Systems [ Linear Systems ]
