Datasheet

19
LTC3778
3778f
APPLICATIO S I FOR ATIO
WUUU
Figure 8. CPU Core Voltage Regulator with Active Voltage Positioning 1.25V/20A at 300kHz
∆=






∆
=
()
Ω
()()
=
I
V
V
RI
AV
TH
RNG
SENSE OUT
12
24 0 002 20 0 96..
The corresponding change in the output voltage is deter-
mined by the gain of the error amplifier and feedback
divider. The LTC3778 error amplifier has a transcon-
ductance g
m
that is constant over both temperature and a
wide ± 40mV input range. Thus, by connecting a load
resistance R
VP
to the I
TH
pin, the error amplifier gain can
be precisely set for accurate voltage positioning.
∆=






∆IgR
V
V
V
TH m VP
OUT
OUT
06.
Solving for this resistance value:
R
VI
Vg V
VV
VmSmV
k
VP
OUT TH
m OUT
=
∆
∆
==
(. )
(. )(. )
(. )(. )( )
.
06
125 096
06 17 75
15 7
The gain setting resistance R
VP
is implemented with two
resistors, R
VP1
connected from I
TH
to ground and R
VP2
connected from I
TH
to INTV
CC
. The parallel combination of
these resistors must equal R
VP
and their ratio determines
nominal value of the I
TH
pin voltage when the error
amplifier input is zero. To set the beginning of the load line
at the regulation point, the I
TH
pin voltage must be set to
correspond to zero output current. The relation between
voltage and the output current is:
I
V
V
RI I V
V
V
AA V
V
TH NOM
RNG
SENSE OUT L()
–.
.
.–..
.
=






∆






+
=






Ω
()






+
=
12 1
2
075
12
05
0 002 0
1
2
58 075
061
Solving for the required values of the resistors:
R
V
VI
R
V
VV
k
k
R
V
I
R
V
V
kk
VP
TH NOM
VP
VP
TH NOM
VP
1
2
5
5
5
5061
15 7
18
55
061
15 7 129
==
=
===
––.
.
.
.
()
()
The active voltage positioned circuit is shown in Figure 8.
Refer to Linear Technology Design Solutions 10 for addi-
tional information about output voltage positioning.
C
C1
100pF
R
VP1
18k
R
VP2
129k
C
C1
2.2nF
C
SS
0.1µF
C
FB
100pF
C
ION
0.01µF
R
ON
330k
R
C
20k
R
RNG2
45.3k
R
RNG1
4.99k
3778 F08
R
PG
100k
R
F
1Ω
C
B
0.33µF
M1
IRF7811
×2
D1
UPS840
L1
0.68µH
M2
IRF7811
×3
D
B
CMDSH-3
C
VCC
4.7µF
C
F
0.1µF
5V
C
IN
22µF
50V
×3
V
IN
7V TO 24V
V
OUT
1.25V
20A
C
OUT
270µF
2V
×3
C
IN
: UNITED CHEMICON
THCR70EIH226ZT
C
OUT
: PANASONIC EEFUE0D271
L1: SUMIDA CEP125-4712-T007
R
SENSE
0.002Ω
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
RUN/SS
V
ON
PGOOD
V
RNG
I
TH
FCB
SGND
I
ON
V
FB
EXTV
CC
BOOST
TG
SW
SENSE
+
SENSE
–
PGND
BG
DRV
CC
INTV
CC
V
IN
LTC3778
12.7k
11.7k