LTC3890EGN-1 LINER [Linear Technology], LTC3890EGN-1 Datasheet - Page 18

• Manufacturer Part Number: LTC3890EGN-1
• Description: 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3890-1
The peak-to-peak drive levels are set by the INTV
voltage. This voltage is typically 5.1V during start-up
(see EXTV
threshold MOSFETs must be used in most applications.
Pay close attention to the BV
MOSFETs as well.
Selection criteria for the power MOSFETs include the
on-resistance, R
voltage and maximum output current. Miller capacitance,
C
usually provided on the MOSFET manufacturers’ data
sheet. C
along the horizontal axis while the curve is approximately
fl at divided by the specifi ed change in V
then multiplied by the ratio of the application applied V
to the Gate charge curve specifi ed V
operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
The MOSFET power dissipations at maximum output
current are given by:
APPLICATIONS INFORMATION
18
MILLER
Main Switch Duty Cycle
P
P
Synchronous S
MAIN
SYNC
, can be approximated from the gate charge curve
MILLER
=
=
CC
( )
V
V
V
V
V
OUT
IN
Pin Connection). Consequently, logic-level
IN
INTVCC
IN
– V
V
is equal to the increase in gate charge
DS(ON)
2
IN
(
I
MAX
OUT
I
w w itch Duty Cycle
MAX
2
1
– V
, Miller capacitance, C
)
(
2
I
THMIN
MAX
(
( )
1+
R
DR
=
)
2
)
DSS
V
+
(
(
V
R
OUT
1+
C
V
IN
DS(ON)
MILLER
THMIN
specifi cation for the
1
)
DS
=
R
DS(ON)
DS
. When the IC is
V
+
IN
)
( )
. This result is
•
f
MILLER
−
V
IN
V
OUT
, input
DS
CC
where δ is the temperature dependency of R
R
at the MOSFET’s Miller threshold voltage. V
typical MOSFET minimum threshold voltage.
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
the high current effi ciency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
with lower C
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.
The term (1+ δ) is generally given for a MOSFET in the
form of a normalized R
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
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%
in effi ciency at high 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.
C
The selection of C
ture and its impact on the worst-case RMS current drawn
through the input network (battery/fuse/capacitor). It can
be shown that the worst-case capacitor RMS current occurs
when only one controller is operating. The controller with
the highest (V
formula shown in Equation 1 to determine the maximum
IN
DR
and C
(approximately 2Ω) is the effective driver resistance
OUT
MILLER
Selection
OUT
IN
)(I
actually provides higher effi ciency. The
is simplifi ed by the 2-phase architec-
OUT
2
IN
R losses while the topside N-channel
IN
DS(ON)
> 20V the transition losses rapidly
. A 1A to 3A Schottky is generally
) product needs to be used in the
vs Temperature curve, but
DS(ON)
THMIN
DS(ON)
IN
device
< 20V
is the
38901f
and