LTC3855IFE#PBF Linear Technology, LTC3855IFE#PBF Datasheet - Page 19

• 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
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
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
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 above
equations and checked at 25°C and 100°C.
P
R
R
R = Resistance at temperature T, which is in degrees C
R
B = B-constant of the thermistor
R
R R
V
I
is known, solve for R
MAX
ITEMP
S
S
O
S
P
ITEMP
=
= R
= R
= 20k
= 50k
= Resistance at temperature T
•
O
O
ITEMP25C
ITEMP100C
100
100
DCR MAX R
•
R
= 100k
exp
1
C
C
+
(
=
=
R

 
P
0 5
B
2
V
.
on the x-axis.
ITEMP
•
– R
– R
V
10
V

 
)
SENSE MAX
•
T
−
S
NTC25C
µA
NTC100C
+
100
1 3
versus R
2
S
1
.
P
P
273
.
•
(
and R
C
that satisfies both equations
•
(1 1 00
|| R
−
|| R
)
P
T
S
° −
P
O
C
. A simple method is to
P
equations with R
+
100
1
O
273
25
, typically 25°C
° •
C

 
)

 
0 4
.
S
on
After determining the components for the temperature
compensation network, check the results by plotting
I
equations:
where
Use typical values for V
A will provide a minimum value for V
values are summarized in Table 2.
Table 2.
I
V
A
The resulting current limit should be greater than or
equal to I
and 100°C.
Typical values for the NTC compensation network are:
• NTC R
• R
• R
Generating the I
first using the above values as a starting point and then
adjusting the R
approach. Figure 6 shows a typical curve of I
inductor temperature. For PolyPhase applications, tie the
ITEMP pins together and calculate for an ITEMP pin cur-
rent of 10µA • #phases.
The same thermistor network can be used to correct for
temperatures less than 25°C. But make sure V
LIM
MAX
SENSE(MAX)
V
I
DCR MAX
V
MAX
ITEMP
S
P
SENSEMAX ADJ
versus inductor temperature using the following
≈ 20k
≈ 50k
(
=
O
TYP
MAX
= 10µA • (R
= 100k, B-constant = 3000 to 4000
)
(
for inductor temperatures between 25°C
V
at 25 C C
MAX
S
SENSEMAX ADJ
and R
)
versus inductor temperature curve plot
=
°
30mV
V
S
GND
5mV
•
SENSE MAX
P
SENSE(MAX)
+ R

 
values as necessary is another
1
(
+
P
2
|| R
(
(
T
)
L MAX
−
NTC
(
∆
)
. Subtracting constant
FLOAT
•
50mV
V
)
5mV
SENSE
1 8
)
.
−
SENSE(MAX)
LTC3855
V V
25
−
1 3
°
.
C
ITEMP
)
MAX
•
INTV
75mV
100
7mV
0 4
ITEMP

. These
.
versus
CC
−

 
A
3855f
is