Datasheet

LTC3856
20
3856f
applicaTions inForMaTion
After determining the components for the temperature
compensation network, check the results by plotting
I
MAX
versus inductor temperature using the following
equations:
I
V
V
DCR AT C
MAX
SENSEMA X ADJ
SENSE
MAX
=
−
° • +
( )
∆
2
25 1 (( )
.
( )
T C
L MAX
− ° •






25
0 4
100
where
V V
V V
A
SENSEMA X ADJ SENSE MAX
ITMP
( ) ( )
.
.
= •
−
−
1 8
1 3
V
ITMP
= 10µA • (R
S
+ R
P
||R
NTC
)
Use typical values for V
SENSE(MAX)
. Subtracting constant A
will provide a minimum value for V
SENSE(MAX)
. These values
are summarized in Table 2.
Table 2. Values for V
SENSE(MAX)
I
LIM
GND FLOAT INTV
CC
V
SENSE(MAX)
Typ 30mV 50mV 75mV
A 5mV 5mV 7mV
The resulting current limit should be greater than or
equal to I
MAX
for inductor temperatures between 25°C
and 100°C.
Typical values for the NTC compensation network are:
• NTC R
O
= 100k, B-constant = 3000 to 4000
• R
S
≈ 20k
• R
P
≈ 50k
Another approach for generating the I
MAX
versus inductor
temperature curve plot is to first use the aforementioned
values as a starting point and then adjusting the R
S
and
R
P
values as necessary. Figure 6 shows a typical curve
of I
MAX
versus inductor temperature.
The same thermistor network can be used to correct for
temperatures less than 25°C. But, ensure that V
ITEMP
is
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.
Figure 6. Worst Case I
MAX
vs Inductor Temperature Curve
with and without NTC Temperature Compensation
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.
3856 F07
V
OUT
L1
SW1
L2
SW2
RNTC
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.
INDUCTOR TEMPERATURE (°C)
–40
I
MAX
(A)
20
100
15
10
0
40
–20
20
60 80 120
5
0
25
3856 F06
NOMINAL
I
MAX
R
S
= 20k
R
P
= 43.2k
NTC THERMISTOR:
R
O
= 100k
T
O
= 25°C
B = 4334
UNCORRECTED
I
MAX
CORRECTED
I
MAX