LTC3856 LINER [Linear Technology], LTC3856 Datasheet - Page 20

• Manufacturer Part Number: LTC3856
• Description: 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp
• Manufacturer: LINER [Linear Technology]

LTC3856
applicaTions inForMaTion
After determining the components for the temperature
compensation network, check the results by plotting
I
equations:
where
Use typical values for V
will provide a minimum value for V
are summarized in Table 2.
Table 2. Values for V
The resulting current limit should be greater than or
equal to I
and 100°C.
0
I
V
A
MAX
LIM
SENSE(MAX)
Figure 6. Worst Case I
with and without NTC Temperature Compensation
V
I
DCR
V
MAX
ITMP
SENSEMAX ADJ
versus inductor temperature using the following
MAX
=
25
20
15
10
= 10µA • (R
Typ
0
MAX
5
–40
R
R
NTC THERMISTOR:
R
T
B = 4334
O
S
P
O
AT
–20
= 25°C
= 20k
= 43.2k
V
= 100k
for inductor temperatures between 25°C
(
SENSEMAX ADJ
INDUCTOR TEMPERATURE (°C)
25
SENSE(MAX)
0
30mV
)
GND
5mV
° •
=
S
MAX
C
20
SENSE(MAX)
V
+ R
SENSE MAX
vs Inductor Temperature Curve



40
P
(
1 ( (
UNCORRECTED
||R
+
CORRECTED
60
(
NTC
I
T
I
)
MAX
MAX
L MAX
−
SENSE(MAX)
(
FLOAT
80
. Subtracting constant A
50mV
)
5mV
∆
)
•
V
100
SENSE
)
1 8
2
.
−
3856 F06
120
25
V V
1 3
NOMINAL
I
−
MAX
. These values
° •
.
C
ITMP
)
INTV
75mV
7mV
100
0 4
CC
.
−
A



Typical values for the NTC compensation network are:
• NTC R
• R
• R
Another approach for generating the I
temperature curve plot is to first use the aforementioned
values as a starting point and then adjusting the R
R
of I
The same thermistor network can be used to correct for
temperatures less than 25°C. But, ensure that V
greater than 0.2V for duty cycles of 25% or more, oth-
erwise temperature correction may not occur at elevated
ambients. For the most accurate temperature detection,
place the thermistor next to the inductors, as shown in
Figure 7. Take care to keep the ITEMP pins away from the
switch nodes.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant-frequen-
cy, current mode architectures by preventing subharmonic
oscillation 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
results in a reduction of maximum inductor peak current
for duty cycles greater than 40%. However, the LTC3856
uses a scheme that counteracts this compensating ramp,
which allows the maximum inductor peak current to remain
unaffected throughout all duty cycles.
P
Figure 7. Thermistor Location. Place Thermistor Next to
Inductor(s) for Accurate Sensing of the Inductor Temperature,
But Keep the ITEMP Pin Away from the Switch Nodes and
Gate Drive Traces.
MAX
values as necessary. Figure 6 shows a typical curve
S
P
≈ 20k
≈ 50k
versus inductor temperature.
O
= 100k, B-constant = 3000 to 4000
V
SW1
OUT
L1
RNTC
SW2
L2
3856 F07
MAX
versus inductor
ITEMP
S
and
3856f
is