LTC3857IUH#PBF Linear Technology, LTC3857IUH#PBF Datasheet - Page 18

• Manufacturer Part Number: LTC3857IUH#PBF
• Description: IC CTRLR STP-DN SYNC DUAL 32QFN
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3857IUH#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.8 ~ 24 V
• Frequency - Switching: 50kHz ~ 900kHz
• Voltage - Input: 4 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 32-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

LTC3857
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.
The only exception is if low input voltage is expected
(V
(V
BV
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
flat divided by the specified change in V
then multiplied by the ratio of the application applied V
to the gate charge curve specified 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
IN
GS(TH)
DSS
Main Switch Duty Cycle=
P
P
Synchronous Switch Duty Cycle=
MAIN
SYNC
< 4V); then, sub-logic level threshold MOSFETs
specification for the MOSFETs as well; many of the
, can be approximated from the gate charge curve
MILLER
< 3V) should be used. Pay close attention to the
=
=
CC
V
V
( )
⎡
⎢
⎣
V
OUT
IN
V
V
IN
Pin Connection). Consequently, logic-level
DS(ON)
IN
INTVCC
–V
V
IN
is equal to the increase in gate charge
(
2
OUT
I
MAX
⎛
⎝ ⎜
I
, Miller capacitance, C
MAX
1
–V
2
(
I
)
MAX
2
THMIN
(
⎞
⎠ ⎟
1+δ
(
R
)
2
DR
(
)
+
1+δ
V
R
V
OUT
)
DS(ON)
V
IN
(
C
THMIN
)
MILLER
1
R
DS
DS(ON)
+
. When the IC is
V
DS
⎤
⎥ f
⎦
IN
( )
. This result is
)
V
-V
MILLER
•
IN
OUT
, input
CC
DS
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 efficiency 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 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 efficiency 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 oc-
curs when only one controller is operating. The controller
with the highest (V
in the formula shown in Equation (1) to determine the
IN
DR
and C
(approximately 2Ω) is the effective driver resistance
OUT
MILLER
Selection
IN
actually provides higher efficiency. The
OUT
is simplified by the 2-phase architec-
2
IN
R losses while the topside N-channel
IN
)(I
DS(ON)
> 20V the transition losses rapidly
. A 1A to 3A Schottky is generally
OUT
) product needs to be used
vs Temperature curve, but
DS(ON)
THMIN
DS(ON)
IN
device
< 20V
is the
3857fc
and