FAN53168MTC FAIRCHILD [Fairchild Semiconductor], FAN53168MTC Datasheet - Page 18

• Manufacturer Part Number: FAN53168MTC
• Description: 6-Bit VID Controlled 2-4 Phase DC-DC Controller
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

FAN53168
• Sumida Electric Company
• Vishay Intertechnology
Output Droop Resistance
The design requires that the regulator output voltage
measured at the CPU pins drops when the output current
increases. The specified voltage drop corresponds to a DC
output resistance (R
The output current is measured by summing together the
voltage across each inductor and then passing the signal
through a low-pass filter. This summer-filter is the CS
amplifier configured with resistors R
R
set by the following equations, where R
output inductors:
Next, use Equation 7 to solve for C
Inductor DCR Temperature Correction
With the inductor’s DCR being used as the sense element,
and copper wire being the source of the DCR, one needs to
compensate for temperature changes of the inductor’s wind-
ing. Fortunately, copper has a well-known temperature
coefficient (TC) of 0.39%/°C.
If R
change in resistance to that of the wire, it will cancel the
temperature variation of the inductor's DCR. Due to the
18
R
R
R
C
C
CS
PH X
PH X
O
CS
CS
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CS
=
( )
( )
and C
Ω
=
=
----------------- -
R
is designed to have an opposite and equal percentage
------------------------
R
------------------------------------------ -
1.6mΩ 100kΩ
R
=
=
PH X
L
CS
CS
------- -
R
1.6mΩ
----------------- -
1.3mΩ
R
( )
×
L
O
L
650nH
R
(filter). The output resistance of the regulator is
×
×
CS
×
R
R
CS
L
×
O
100kΩ
).
=
4.06nF
=
123kΩ
Ω
CS
PH(X)
:
L
is the DCR of the
(summers), and
nonlinear nature of NTC thermistors, resistors R
R
produce the desired temperature tracking.
The following procedure and expressions will yield values to
use for R
for a given R
1.
2.
3.
TC = 0.0039
T
T
4.
r
CS2
1
2
To V
CS2
sense
= 50°C
= 90°C
OUT
Select an NTC to be used based on type and value. Since
we do not have a value yet, start with a thermistor with a
value close to R
tolerance of better than 5%.
Based on the type of NTC, find its relative resistance
value at two temperatures. The temperatures to use that
work well are 50°C and 90°C. We will call these resis-
tance values A (A is R
R
is always 1 at 25°C.
Next, find the relative value of R
these temperatures. This is based on the percentage
change needed, which we will initially make 0.39%/°C.
We will call these r
Compute the relative values for R
using:
r
r
r
R
r
are needed (see Figure 5) to linearize the NTC and
1
2
=
TH(90°C)
TH
CS1
PH3
and well away from
Figure 5. Temperature Compensation Circuit
Switch Node lines
Keep this path as
short as possible
=
=
(
----------------------------------------------------------------------------------------------------------------------------------- -
CS1
A B
To Switch Nodes
=
---------------------------------------------------- -
(
---------------------------------------------------- -
(
R
=
1
1
–
PH2
---------------------------------------
-------------------- -
1 r
, R
A
+
+
CS
------------------------------------------------- -
-------------------- -
1 r
–
/R
×
TC
TC
) r
–
1
CS2
×
value.
R
TH(25°C)
(
CS2
1
PH1
1 B
CS2
×
×
1
1
, and R
–
1
1
(
×
(
(
1 A
CS
–
T
T
r
–
–
----------- -
r
R
)
1
2
2
. The NTC should also have an initial
CS1
CS2
1
×
----------------------
r
1
–
–
–
). Note that the NTC's relative value
1
and r
r
)
25
25
TH
A
–
1
TH(50˚C)
A
r
×
–
)
)
R
CS2
R
1.8nF
(the thermistor value at 25˚C)
)
)
CS1
B
(
TH
2
C
1 B
CS
×
where:
–
(
1 A
inductor or low-side
possible to nearest
PRODUCT SPECIFICATION
/R
Place as close as
)
–
TH(25˚C)
CS
×
MOSFET
CS1
r
)
2
18
17
16
required for each of
×
+
, R
CSCOMP
CSSUM
r
CSREF
REV. 1.0.0 6/9/03
B
2
CS2
) and B (B is
–
×
(
(
A B
, and R
1 A
CS1
–
–
and
)
) r
CSA
×
TH
1