LTC3865EFE#PBF Linear Technology, LTC3865EFE#PBF Datasheet - Page 15

• Manufacturer Part Number: LTC3865EFE#PBF
• Description: IC BUCK SYNC ADJ DUAL 38TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3865EFE#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.6 ~ 5 V
• Frequency - Switching: 250kHz ~ 770kHz
• Voltage - Input: 4.5 ~ 38 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 38-TSSOP Exposed Pad, 38-eTSSOP, 38-HTSSOP
• Primary Input Voltage: 15V
• No. Of Outputs: 2
• Output Voltage: 5V
• Output Current: 25A
• No. Of Pins: 38
• Operating Temperature Range: -40°C To +85°C
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

APPLICATIONS INFORMATION
Filter components mutual to the sense lines should be
placed close to the LTC3865/LTC3865-1, and the sense
lines should run close together to a Kelvin connection
underneath the current sense element (shown in Figure 1).
Sensing current elsewhere can effectively add parasitic
inductance and capacitance to the current sense element,
degrading the information at the sense terminals and mak-
ing the programmed current limit unpredictable. If DCR
sensing is used (Figure 2b), sense resistor R1 should be
placed close to the switching node, to prevent noise from
coupling into sensitive small-signal nodes. The capacitor
C1 should be placed close to the IC pins.
Figure 1. Sense Lines Placement with Inductor or Sense Resistor
Low Value Resistors Current Sensing
A typical sensing circuit using a discrete resistor is shown
in Figure 2a. R
output current.
The current comparator has a maximum threshold
V
common mode range of the current comparator is 0V
to 5V. The current comparator threshold sets the peak of
the inductor current, yielding a maximum average output
current I
peak ripple current, ΔI
value, use the equation:
Because of possible PCB noise in the current sensing loop,
the AC current sensing ripple of ΔV
also needs to be checked in the design to get a good
signal-to-noise ratio. In general, for a reasonably good
PCB layout, a 15mV ΔV
as a conservative number to start with, either for R
or DCR sensing applications.
SENSE(MAX)
R
SENSE
MAX
=
equal to the peak value less half the peak-to-
determined by the I
I
V
(
MAX
SENSE MAX
SENSE
)
INDUCTOR OR R
+
(
Δ
is chosen based on the required
2
L
I
. To calculate the sense resistor
L
SENSE
)
TO SENSE FILTER,
NEXT TO THE CONTROLLER
SENSE
voltage is recommended
3865
LIM
F01
SENSE
C
OUT
setting. The input
= ΔI
L
• R
SENSE
SENSE
*PLACE C1 NEAR SENSE
For previous generation current mode controllers, the
maximum sense voltage was high enough (e.g., 75mV for
the LTC1628 / LTC3728 family) that the voltage drop across
the parasitic inductance of the sense resistor represented
a relatively small error. For today’s highest current density
solutions, however, the value of the sense resistor can be
less than 1mΩ and the peak sense voltage can be as low
as 20mV. In addition, inductor ripple currents greater than
50% with operation up to 1MHz are becoming more com-
mon. Under these conditions the voltage drop across the
sense resistor’s parasitic inductance is no longer negligible.
A typical sensing circuit using a discrete resistor is shown
in Figure 2a. In previous generations of controllers, a small
RC fi lter placed near the IC was commonly used to reduce
the effects of capacitive and inductive noise coupled in
SENSE
LTC3865
LTC3865
–
SGND
SGND
PINS
Figure 2. Two Different Methods of Sensing Current
SENSE
SENSE
(2b) Using the Inductor DCR to Sense Current
SENSE
SENSE
INTV
BOOST
INTV
BOOST
PGND
PGND
PLACED NEAR SENSE PINS
V
SW
SW
(2a) Using a Resistor to Sense Current
V
BG
BG
TG
TG
CC
CC
IN
FILTER COMPONENTS
IN
+
–
+
–
+
,
LTC3865/LTC3865-1
R1 || R2
C
C1*
F
R
R
F
F
•
C1 =
R2
R1
DCR
L
R
SENSE(EQ)
V
SENSE RESISTOR
C
PLUS PARASITIC
INDUCTOR
CANCELLATION
IN
F
L
INDUCTANCE
R
• 2
POLE-ZERO
S
V
= DCR
RF
IN
≤ ESL/R
DCR
ESL
R1 + R2
R2
S
3865
F02a
15
3865
3865fb
F02b
V
V
OUT
OUT