Datasheet

LTC3816
23
3816f
applicaTions inForMaTion
the NTC compensation network. To determine the compo-
nent values, first, select the NTC with room temperature
resistance approximately equal to R
VDCR
that has the
smallest temperature coefficient (β constant in the NTC
data sheet. Using an NTC with a higher β constant gen-
erates a less optimal temperature compensation). Next,
calculate the resistances R
PAR
and R
SER
from the following
equations where the NTC resistances at different tempera-
tures is obtained from the manufacturer’s data sheet.
R R at C
R R R at C
PAR NTC
SER PAR NTC
= °
≈ °
( )
( )
|| –
25
10
3
0 RR R at C
R R at C
PAR NTC
PAR NTC
||
– ||
75
25
°
( )
( )




°
( ))
( )
Note that the above equations optimize temperature
compensation at hot. At extreme cold temperature, the
temperature compensation is less effective.
With the NTC resistor network, the temperature compen-
sated AVP transfer function becomes:
V
OUT
= V
DAC
– A
AVP(DCRN)
• I
L
= V
DAC
– A
G(DCRN)
• I
L
• R
DCR
where A
AVP(DCRN)
and A
G(DCRN)
are the AVP and DCR gain
using the inductor DCR current sense with NTC temperature
compensation configuration.
A A R and A
R
R
AVP DCRN G DCRN DCR G DCRN
NTCNET
A
( ) ( ) ( )
•= =
VVPDCRN
VDCRN
NTCNET DCR
NTCNET SER PAR
C
L
R R
R R R R
=
= +
•
||
NNTC
( )




Figure 11a shows the room temperature AVP DC transfer
curves obtained using inductor DCR current sense with and
without NTC temperature compensation. There is only a
slight difference in the transfer curve at heavy load. Figure
11b shows the AVP transfer curve obtained at 125°C, it
shows the improvement in AVP accuracy with the NTC
resistor network.
Figure 12 shows another easy way to compensate for the
inductor DCR temperature coefficient. In this configura-
tion, a linear PTC resistor is connected from the SW node
to the I
TCFB
pin. The PTC thermistor ’s temperature coef-
ficient of 0.411%/°C compensates for the change in DCR
I
LOAD
(A)
0
V
OUT
(V)
0.97
0.98
0.99
15
25
3816 F11a
0.96
0.95
0.94
5 10 20
1.00
1.01
1.02
30
IDEAL + 1.5%
IDEAL
IDEAL – 1.5%
WITH NTC
WITHOUT NTC
T
A
= 25°C
Figure 11a. AVP Transfer Curve Using Vishay
IHLP-5050CE-01 0.33µH (DCR = 1.3mΩ) Inductor
DCR Current Sense with A
G(DCRN)
= 1 at T
A
= 25°C
I
LOAD
(A)
0
V
OUT
(V)
0.97
0.98
0.99
15
25
3816 F11b
0.96
0.95
0.94
5 10 20
1.00
1.01
1.02
30
IDEAL + 1.5%
IDEAL
IDEAL – 1.5%
WITH NTC
WITHOUT NTC
T
A
= 125°C
Figure 11b. Same Setup as Figure 11a. Improvement
in AVP Accuracy with NTC Temperature Compensation
Network at T
A
= 125°C
+
Q
B
D
Q
T
V
IN
L
C
OUT
3916 F12
V
OUT
I
L
INDUCTOR
DCR
LPTC
C
VDCRP
LPTC: VISHAY TPFT1206 SERIES, 4110ppm/°C
TG
BG
BSOURCE
I
TCFB
I
SENN
I
TC
LTC3816
SW
R
VDCRP
+
–
AITC
Figure 12. AVP Using Inductor DCR Current Sense
with Linear PTC Temperature Compensation