LTC3856EFE#PBF Linear Technology, LTC3856EFE#PBF Datasheet - Page 17

• Manufacturer Part Number: LTC3856EFE#PBF
• Description: IC DCDC SWITCH
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3856EFE#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 5 V
• 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
• Current - Output: -
• Power - Output: -

applicaTions inForMaTion
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 Equation 1 to determine the ESL. However, do not
overfilter. Keep the RC time constant, less than or equal
to the inductor time constant to maintain a high enough
ripple voltage of ∆V
to high density/high current applications where I
10A and low values of inductors are used. For applications
where I
will provide a good starting point. The filter components
need to be placed close to the IC. The positive and nega-
tive 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 LTC3856 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 several 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
always the same and varies with temperature; consult the
manufacturers’ data sheets for detailed information.
MAX
< 10A, set R
SENSE
F
. The equation generally applies
to 10Ω and C
F
to 1000pF . This
MAX
>
Using the inductor ripple current value from the Inductor
Value Calculation and Output Ripple Current section, the
target sense resistor value is:
To ensure that the application will deliver full load current
over the full operating temperature range, choose the
minimum value for the maximum current sense threshold
(V
45mV or 68mV, depending on the state of the I
Next, determine the DCR of the inductor. Where provided,
use the manufacturer’s maximum value, usually given at
20°C. Increase this value to account for the temperature
coefficient of resistance, which is approximately 0.4%/°C.
A conservative value for T
maximum inductor DCR to the desired sense resistor
value, use the divider ratio:
C1 is usually selected to be in the range of 0.047µF to
0.47µF . This forces R1|| R2 to around 2k, reducing error
that might have been caused by the SENSE
current. T
The equivalent resistance R1|| R2 is scaled to the room
temperature inductance and maximum DCR:
The sense resistor values are:
SENSE(MAX)
R
R
R R
R
D
1
1
SENSE EQUIV
||
=
=
R
DCR
2
1
L(MAX)
(
=
R
R
||
D
(
SENSE EQUIV
R
) in the Electrical Characteristics table (25mV,
MAX
DCR at
2
)
;
is the maximum inductor temperature.
=
at T
(
R
I
V
(
MAX
2
SENSE MAX
L MAX
L
20
(
=
°
)
R
)
1
C
+
1
(
L(MAX)
−
) •
)
•
∆
R
R
2
I
C
D
L
D
)
1
is 100°C. To scale the
LTC3856
+
pins’ ±1µA
LIM

pin).
3856f