Datasheet

LTC3867
19
3867f
After determining the components for the temperature
compensation network, check the results by plotting I
MAX
versus inductor temperature using the following equations:
I
MAX
=
V
SENSEMAX(ADJ)
V
SENSE
/ 2
DCR(MAX) at 25°C 1+ T
L(MAX)
25°C
( )
0.4 / 100
( )
where:
V
SENSEMAX(ADJ)
= V
SENSE(MAX)
1.8V
V
ITEMP
2.8
1.3
A
V
ITEMP
= 30µA 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 1.
Table 1
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.
These are typical values for the NTC compensation network:
• NTC R
O
= 100k, B-constant = 3000 to 4000
• R
S
≈ 20k
• R
P
≈ 50k
Generating the I
MAX
versus inductor temperature curve plot
first using the above values as a starting point and then
adjusting the R
S
and R
P
values as necessary is another
approach. Figure 7 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 make sure V
ITEMP
is
greater than 0.6V 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 thermistors next to the inductor as shown in
Figure 8. Take care to keep the ITEMP pin away from the
switch nodes.
APPLICATIONS INFORMATION
Figure 6. Resistance Versus Temperature for the ITEMP Pin
Network and the 100k NTC
Figure 7. Worst-Case I
MAX
Versus Inductor Temperature Curve
with and without NTC Temperature Compensation
Figure 8. Thermistor Location. Place Thermistor Next to
Inductor for Accurate Sensing of the Inductor Temperature,
But Keep the ITEMP Pin Away from the Switch Nodes and Gate
Drive Traces
INDUCTOR TEMPERATURE (°C)
10
RESISTANCE (kΩ)
100
1000
10000
–40 20 40 60 10080 120
1
–20 0
3867 F06
THERMISTOR RESISTANCE
R
O
= 100k
T
O
= 25°C
B = 4334 FOR 25°C/100°C
R
ITMP
R
S
= 20k
R
P
= 43.2k
100k NTC
INDUCTOR TEMPERATURE (°C)
–40
I
MAX
(A)
15
20
25
20 60 120
3867 F07
10
5
0
–20 0
40
80 100
CORRECTED
I
MAX
NOMINAL
I
MAX
UNCORRECTED
I
MAX
R
S
= 20k
R
P
= 43.2k
NTC THERMISTOR:
R
O
= 100k
T
O
= 25°C
B = 4334
V
OUT
R
NTC
L1
SW1
3867 F08