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LTC3875
APPLICATIONS INFORMATION
V
= V
SENSEMAX(ADJ)
SENSE(MAX)
V
= 30µA • (R
+ R
ITEMP
S
I
= Maximum average inductor current.
DC(MAX)
TC is the inductor temperature.
The resulting current limit should be greater than or equal
to I
for inductor temperatures between 25°C and 100°C.
MAX
Typical values for the NTC compensation network are:
• NTC R
= 100k, B-constant = 3000 to 4000
O
• R
≈ 3.92k
S
• R
≈ 24.3k
P
Generating the I
versus inductor temperature curve plot
MAX
first using the above values as a starting point, and then
adjusting the R
and R
S
P
approach. Figure 6 shows a curve of I
temperature. For PolyPhase
ITEMP pins together and calculate for an TCOMP/ITEMP
pin current of 30µA • #phases.
For the most accurate temperature detection, place the
thermistors next to the inductors as shown in Figure 7.
Take care to keep the TCOMP/ITEMP pins away from the
switch nodes.
Slope Compensation and Inductor Peak Current
70
60
NOMINAL
50
I
MAX
R
= 3.92k
S
R
= 24.3k
P
40
NTC THERMISTOR:
R
= 100k
O
T
= 25°C
O
B = 4334
30
–40
–20
0
20
INDUCTOR TEMPERATURE (°C)
Figure 6. Worst-Case I
vs Inductor Temperature Curve with
MAX
and without NTC Temperature Compensation
22
2.2 – V
ITEMP
•
1.5
||R
)
P
NTC
values as necessary, is another
versus inductor
MAX
applications, tie the TCOMP/
®
CORRECTED I
MAX
UNCORRECTED
I
MAX
40
60
80
100
120
3875 F06
For more information
CONNECT TO
ITEMP1
NETWORK
V
OUT1
R
NTC1
L1
GND
SW1
(7a) Dual Output Dual Phase DCR Sensing Application
V
OUT
L1
SW1
(7b) Single Output Dual Phase DCR Sensing Application
Figure 7. Thermistor Locations. Place Thermistor Next to
Inductor(s) for Accurate Sensing of the Inductor Temperature,
but Keep the ITEMP Pins away from the Switch Nodes and
Gate Traces
Slope compensation provides stability in constant fre-
quency architectures by preventing sub-harmonic oscil-
lations at high duty cycles. It is accomplished internally
by adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. Normally, this re-
sults in a reduction of maximum inductor peak current for
duty cycles > 40%. However, the LTC3875 uses a scheme
that counteracts this compensating ramp, which allows
the maximum inductor peak current to remain unaffected
throughout all duty cycles.
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency, f
the inductor's peak-to-peak ripple current:
V
V
OUT
IN
I
=
RIPPLE
V
f
IN
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest efficiency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
www.linear.com/LTC3875
CONNECT TO
ITEMP2
V
NETWORK
OUT2
R
NTC2
L2
GND
SW2
3875 F07a
R
NTC
L2
SW2
3875 F07b
, directly determine
OSC
– V
OUT
•L
OSC
3875fb
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