LTC3778 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC3778' PDF : 24 Pages View PDF
LTC3778
APPLICATIO S I FOR ATIO
∆ITH
=
12V
VRNG
RSENSE
∆IOUT
( )( )( ) = 24 0.002Ω 20A = 0.96V
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 gm that is constant over both temperature and a
wide ± 40mV input range. Thus, by connecting a load
resistance RVP to the ITH pin, the error amplifier gain can
be precisely set for accurate voltage positioning.
∆ITH
=
gm RVP
0.6V
VOUT
∆VOUT
Solving for this resistance value:
RVP
=
VOUT ∆ITH
(0.6V)gm ∆VOUT
= (1.25V)(0.96V) = 15.7k
(0.6V)(1.7mS)(75mV)
these resistors must equal RVP and their ratio determines
nominal value of the ITH pin voltage when the error
amplifier input is zero. To set the beginning of the load line
at the regulation point, the ITH pin voltage must be set to
correspond to zero output current. The relation between
voltage and the output current is:
ITH(NOM)
=
12V
VRNG
RSENSEIOUT
–
1
2
∆IL
+
0.75V
( ) =
12V
0.5V
0.002Ω
0A
–
1
2
5.8A
+ 0.75V
= 0.61V
Solving for the required values of the resistors:
RVP1
=
5V
5V
– ITH(NOM)
RVP
=
5V
5V 15.7k
– 0.61V
= 18k
RVP2
=
5V
ITH(NOM)
RVP
=
5V 15.7k
0.61V
=
129k
The gain setting resistance RVP is implemented with two
resistors, RVP1 connected from ITH to ground and RVP2
connected from ITH to INTVCC. The parallel combination of
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.
CSS
0.1µF
RRNG1 RRNG2
4.99k 45.3k
1
20
RUN/SS BOOST
2
LTC3778
19
RPG
VON
TG
100k
3
18
PGOOD
SW
CC1
2.2nF RC
20k
RVP2
129k
RVP1
18k
CION 0.01µF
4
VRNG
5
CC1
ITH
100pF 6
FCB
7
SGND
8
ION
SENSE+ 17
SENSE– 16
15
PGND
14
BG
13
DRVCC
11.7k
RON
330k
CFB
100pF
9
VFB
12.7k
10
5V
EXTVCC
12
INTVCC
11
VIN
DB
CMDSH-3
CB
0.33µF
M1
IRF7811 L1
×2
0.68µH
M2
IRF7811
×3
RSENSE
0.002Ω
D1
UPS840
VIN
CIN 7V TO 24V
22µF
50V
×3 VOUT
1.25V
20A
COUT
270µF
2V
×3
CVCC
4.7µF
RF
1Ω
CF
0.1µF
CIN: UNITED CHEMICON
THCR70EIH226ZT
COUT: PANASONIC EEFUE0D271
L1: SUMIDA CEP125-4712-T007
3778 F08
Figure 8. CPU Core Voltage Regulator with Active Voltage Positioning 1.25V/20A at 300kHz
3778f
19
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