LTC3855EFE#PBF Linear Technology, LTC3855EFE#PBF Datasheet - Page 17

• Manufacturer Part Number: LTC3855EFE#PBF
• Description: IC BUCK SYNC ADJ 25A DL 38TSSOP
• Manufacturer: Linear Technology
• Series: PolyPhase®r
• Type: Step-Down (Buck)r

Specifications of LTC3855EFE#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 ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 38-TSSOP Exposed Pad, 38-eTSSOP, 38-HTSSOP
• Primary Input Voltage: 38V
• No. Of Outputs: 2
• Output Voltage: 12.5V
• No. Of Pins: 38
• Operating Temperature Range: -40°C To +85°C
• Msl: MSL 1 - Unlimited
• Switching Frequency Max: 770kHz
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

applicaTions inForMaTion
the resulting waveform looks resistive again, as shown
in Figure 4. For applications using low maximum sense
voltages, check the sense resistor manufacturer’s data
sheet for information about parasitic inductance. In the
absence of data, measure the voltage drop directly across
the sense resistor to extract the magnitude of the ESL
step and use the equation above to determine the ESL.
However, do not over-filter. Keep the RC time constant less
than or equal to the inductor time constant to maintain a
high enough ripple voltage on V
3855
F02a
Figure 4. Voltage Waveform Measured After the
Sense Resistor Filter. C
LTC3855
SGND
20mV/DIV
Figure 3. Voltage Waveform Measured
Directly Across the Sense Resistor.
20mV/DIV
(2a) Using a Resistor to Sense Current
V
V
SENSE
SENSE
SENSE
SENSE
INTV
BOOST
PGND
SW
V
BG
TG
CC
PLACED NEAR SENSE PINS
IN
+
–
FILTER COMPONENTS
C
F
R
R
500ns/DIV
F
F
500ns/DIV
F
= 1000pF , R
RSENSE
Figure 2. Two Different Methods of Sensing Current
V
SENSE RESISTOR
C
PLUS PARASITIC
CANCELLATION
IN
F
3855
3855
INDUCTANCE
R
• 2
POLE-ZERO
S
F04
F03
.
RF
F
≤ ESL/R
= 100Ω.
ESL
V
ESL(STEP)
S
V
OUT
**PLACE R1 NEXT TO
The above generally applies to high density/high current
applications where I
tors are used. For applications where I
to 10 Ohms and C
starting point.
The filter components need to be placed close to the IC.
The positive and negative sense traces need to be routed
as a differential pair and Kelvin connected to the sense
resistor.
Inductor DCR Sensing
For applications requiring the highest possible efficiency at
high load currents, the LTC3855 is capable of sensing the
voltage drop across the inductor DCR, as shown in Figure
2b. The DCR of the inductor represents the small amount
of DC winding resistance of the copper, which can be less
than 1mΩ for today’s low value, high current inductors.
In a high current application requiring such an inductor,
conduction loss through a sense resistor would cost sev-
eral points of efficiency compared to DCR sensing.
If the external R1|| R2 • C1 time constant is chosen to be
exactly equal to the L/DCR time constant, the voltage drop
across the external capacitor is equal to the drop across
the inductor DCR multiplied by R2/(R1 + R2). R2 scales the
voltage across the sense terminals for applications where
the DCR is greater than the target sense resistor value.
To properly dimension the external filter components, the
DCR of the inductor must be known. It can be measured
using a good RLC meter, but the DCR tolerance is not
R
INDUCTOR
NTC
TEMP COMP
OPTIONAL
NETWORK
R
S
(2b) Using the Inductor DCR to Sense Current
R
P
*PLACE C1 NEAR SENSE
SENSE
ITEMP
SGND
LTC3855
–
PINS
SENSE
SENSE
INTV
F
BOOST
PGND
(MAX)
to 1000pF . This will provide a good
V
SW
TG
BG
CC
IN
+
–
+
,
>10A and low values of induc-
R1 || R2
C1*
×
C1 =
R2
R1**
DCR
L
R
(MAX)
SENSE(EQ)
LTC3855
INDUCTOR
L
= DCR
<10A, set R
V
IN
DCR
R1 + R2
R2

V
OUT
3855f
3855
F02b
F