LTC3856EFE#PBF Linear Technology, LTC3856EFE#PBF Datasheet - Page 19

• 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
NTC Compensated DCR Sensing
For DCR sensing applications where a more accurate
current limit is required, a network consisting of an NTC
thermistor placed from the ITEMP pin to ground will
provide correction of the current limit over temperature.
Figure 2b shows this network. Resistors R
linearize the impedance the ITEMP pin sees. To implement
NTC compensated DCR sensing, design the DCR sense
filter network per the same procedure mentioned in the
previous selection, except calculate the divider compo-
nents using the room temperature value of the DCR. For
a typical application:
1. Set the ITEMP pin resistance to 50k at 25°C. With
2. Calculate the ITEMP pin resistance and the maximum
Calculate the values for R
graph the following R
the y-axis and R
Next, find the value of R
which will be the point where the curves intersect. Once
R
P
10µA flowing out of the ITEMP pin, the voltage on the
ITEMP pin will be 0.5V at room temperature. Current
limit correction will occur for inductor temperatures
greater than 25°C.
inductor temperature, which is typically 100°C. Use the
following equations:
R
R
R
I
V
is known, solve for R
MAX
S
S
ITEMP
ITEMP
= R
= R
•
ITEMP25C
ITEMP100C
100
DCR
100
C
C
MAX
=
=
P
0 5
on the x-axis.
V
– R
.
ITEMP
– R
•
10
V
R
S
NTC25C
V
NTC100C
1
−
S S ENSE MAX
versus R
µA
R
S
+
P
P
100
1
.
2
R
and R
.3 3
that satisfies both equations,
2
C
||R
(
•
•
||R
P
(
S
100
P
. A simple method is to
P
)
equations with R
° −
C
25
S
° •
and R
C
)
100
0 4
P
S
.
will
on
The resistance of the NTC thermistor can be obtained
from the vendor’s data sheet either in the form of graphs,
tabulated data or formulas. The approximate value for the
NTC thermistor for a given temperature can be calculated
from the following equation:
Where
R = resistance at temperature T, in degrees C
R
B = B-constant of the thermistor.
Figure 5 shows a typical resistance curve for a 100k therm-
istor and the ITEMP pin network over temperature.
Starting values for the NTC compensation network are:
• NTC R
• R
• R
But, the final values should be calculated using the previ-
ous equations and checked at 25°C and 100°C.
O
R R
= resistance at temperature T
S
P
=
= 20k
= 50k
10000
Figure 5. Resistance vs Temperature for the
ITEMP Pin Network and the 100k NTC
1000
O
O
100
10
•
= 100k
0
–40
exp
–20
R
R
R
100k NTC
ITMP
S
P



= 20k
= 43.2k
B
INDUCTOR TEMPERATURE (°C)
:
•
0

 
THERMISTOR RESISTANCE:
R
T
B = 4334 for 25°C/100°C
O
O
T
20
= 25°C
= 100k
+
1
273
40
60
−
O
T
80
, typically 25°C
O
+
100
1
273
LTC3856
3856 F05
120

 




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