LTC3868IUH#TRPBF Linear Technology, LTC3868IUH#TRPBF Datasheet - Page 19

• Manufacturer Part Number: LTC3868IUH#TRPBF
• Description: IC CTRLR STP-DN SYNC DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3868IUH#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 14 V
• Frequency - Switching: 50kHz ~ 900kHz
• Voltage - Input: 4 ~ 24 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

APPLICATIONS INFORMATION
Pin Connection). Consequently, logic-level threshold
MOSFETs must be used in most applications. The only
exception is if low input voltage is expected (V
then, sub-logic level threshold MOSFETs (V
should be used. Pay close attention to the BV
fi cation for the MOSFETs as well; many of the logic-level
MOSFETs are limited to 30V or less.
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:
where δ is the temperature dependency of R
R
at the MOSFET’s Miller threshold voltage. V
typical MOSFET minimum threshold voltage.
MILLER
DR
Main Switch Duty Cycle =
Synchronous Switch Duty Cycle =
P
P
MAIN
SYNC
(approximately 2Ω) is the effective driver resistance
, can be approximated from the gate charge curve
MILLER
=
=
⎡
⎢
⎣
( )
V
V
V
V
V
OUT
IN
IN
INTVCC
IN
– V
is equal to the increase in gate charge
V
DS(ON)
2
IN
(
⎛
⎜
⎝
I
MAX
OUT
I
MAX
1
2
– V
, Miller capacitance, C
)
(
2
I
⎞
⎟ R
⎠
THMIN
MAX
(
( )
1+ δ
DR
)
2
)
+
(
V
(
R
1+ δ
V
OUT
C
V
DS(ON)
IN
MILLER
THMIN
1
)
DS
R
DS(ON)
. When the IC is
DS
+
V
⎤
⎥ f
⎦
)
IN
( )
. This result is
•
MILLER
GS(TH)
THMIN
− V
V
DS(ON)
DSS
IN
IN
OUT
< 4V);
speci-
, input
< 3V)
is the
and
DS
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 D1 and D2 shown in Figure 10
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
to 3A Schottky is generally 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
RMS capacitor current requirement. Increasing the out-
put current drawn from the other controller will actually
decrease the input RMS ripple current from its maximum
value. The out-of-phase technique typically reduces the
input capacitor’s RMS ripple current by a factor of 30%
to 70% when compared to a single phase power supply
solution.
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V
IN
and C
OUT
MILLER
Selection
OUT
)(I
IN
actually provides higher effi ciency. The
is simplifi ed by the 2-phase architec-
OUT
2
IN
R losses while the topside N-channel
DS(ON)
> 20V the transition losses rapidly
) product needs to be used in the
vs Temperature curve, but
OUT
)/(V
LTC3868
IN
). To prevent
DS(ON)
IN
IN
19
device
< 20V
. A 1A
3868fd