ltc3711 Linear Technology Corporation, ltc3711 Datasheet - Page 20

• Manufacturer Part Number: ltc3711
• Description: 5-bit Adjustable, Wide Operating Range, No Rsense? Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3711
The corresponding change in the output voltage is deter-
mined by the gain of the error amplifier and feedback
divider. The LTC3711 error amplifier has a transconduc-
tance g
tance R
precisely set for accurate active voltage positioning.
Solving for this resistance value:
The gain setting resistance R
resistors, R
connected from I
these resistors must equal R
nominal value of the I
amplifier input is zero. To center the load line around the
regulation point, the I
spond to half the output current. The relation between I
voltage and the output current is:
Solving for the required values of the resistors:
20
40mV input range. Thus, by connecting a load resis-
R
I
R
R
TH NOM
VP
VP
VP
I
TH
(
1
m
2
VP
that is constant over both temperature and a wide
( .
( .
12 44
g R
)
to the I
0 8
5
0 8
I
TH NOM
VP1
m VP
V I
.
V
(
1 17
5
V
–
V g
OUT
( .
.
V
1 5
0 5
)( .
V
)
connected from I
12
12
5
k
TH
TH NOM
RNG
.
1 7
V
TH
m
V
)
V
V
(
V
V
U
to INTV
0 8 .
V
R
I
)( .
mS
OUT
pin, the error amplifier gain can be
TH
TH
V
VP
1 08
R
0 003
V
OUT
)
SENSE OUT
TH
pin voltage must be set to corre-
.
)(
R
U
125
CC
V
VP
1 17
pin voltage when the error
V
VP
)
. The parallel combination of
.
VP
OUT
5
mV
V
and their ratio determines
I
is implemented with two
V
5
7 5
TH
)
V
9 53
.
W
.
– .
–
to ground and R
A
5
9 53
1 17
2
1
V
.
–
k
2
1
k
I
L
4 7
V
40 73
.
9 53
.
A
.
U
0 8
.
k
k
V
0 8
.
VP2
TH
V
The modified circuit is shown in Figure 10. Figures 11
and 12 show the transient response without and with
active voltage positioning. Both circuits easily stay within
active voltage positioning accomplishes this with only
three output capacitors rather than five. Refer to Design
Solutions 10 for additional information about active volt-
age positioning.
PC Board Layout Checklist
When laying out the printed circuit board, use the follow-
ing checklist to ensure proper operation of the controller.
These items are also illustrated in Figure 11.
• Segregate the signal and power grounds. All small
• Place M2 as close to the controller as possible, keeping
• Connect the input capacitor(s) C
• Keep the high dV/dT SW, BOOST and TG nodes away
• Connect the INTV
• Connect the top driver boost capacitor C
• Connect the V
• VID0-VID4 interface circuitry must return to SGND.
100mV of the 1.5V output. However, the circuit with
signal components should return to the SGND pin at
one point which is then tied to the PGND pin close to the
source of M2.
the PGND, BG and SENSE
MOSFETs. This capacitor carries the MOSFET AC
current.
from sensitive small-signal nodes.
to the INTV
BOOST and SW pins.
the V
IN
and PGND pins.
CC
IN
and PGND pins.
pin decoupling capacitor C
CC
decoupling capacitor C
+
traces short.
IN
close to the power
B
closely to the
F
VCC
closely to
closely
3711f