LTC3855IFE#PBF Linear Technology, LTC3855IFE#PBF Datasheet - Page 21

• Manufacturer Part Number: LTC3855IFE#PBF
• Description: IC CTLR DC/DC MULTIPHASE 38SSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3855IFE#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 3.3 V, 0.6 ~ 12.5 V
• Current - Output: 25A
• Frequency - Switching: 250kHz ~ 770kHz
• Voltage - Input: 4.5 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 38-TSSOP Exposed Pad, 38-eTSSOP, 38-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

applicaTions inForMaTion
A reasonable starting point is to choose a ripple current
that is about 40% of I
40%. Note that the largest ripple current occurs at the
highest input voltage. To guarantee that ripple current does
not exceed a specified maximum, the inductor should be
chosen according to:
For duty cycles greater than 40%, the 10mV current
sense ripple voltage requirement is relaxed because the
slope compensation signal aids the signal-to-noise ratio
and because a lower limit is placed on the inductor value
to avoid subharmonic oscillations. To ensure stability for
duty cycles up to the maximum of 95%, use the following
equation to find the minimum inductance.
where
Inductor Core Selection
Once the inductance value is determined, the type of in-
ductor must be selected. Core loss is independent of core
size for a fixed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
L
f
L ≥
L
SW
MIN
MIN
f
is in units of MHz
OSC
V
is in units of µH
>
IN
f
SW
•I
– V
RIPPLE
•
OUT
I
V
LOAD MAX
OUT
•
(
OUT(MAX)
V
V
OUT
IN
)
• .
1 4
for a duty cycle less than
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Power MOSFET and Schottky Diode
(Optional) Selection
Two external power MOSFETs must be selected for each
controller in the LTC3855: one N-channel MOSFET for the
top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
The peak-to-peak drive levels are set by the INTV
voltage. This voltage is typically 5V during start-up
(see EXTV
threshold MOSFETs must be used in most applications.
The only exception is if low input voltage is expected (V
< 5V); then, sub-logic level threshold MOSFETs (V
< 3V) should be used. Pay close attention to the BV
specification for the MOSFETs as well; most 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
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:
MILLER
Main Switch Duty Cycle =
Synchronous Switch Duty Cycle =
, can be approximated from the gate charge curve
MILLER
CC
Pin Connection). Consequently, logic-level
DS(ON)
is equal to the increase in gate charge
, Miller capacitance C
V
V
OUT
IN
DS
V
LTC3855
IN
. When the IC is
DS
– V
V
. This result is
IN
MILLER
OUT

, input
GS(TH)
DSS
3855f
DS
CC
IN