LTC3714 Linear, LTC3714 Datasheet - Page 22

• Manufacturer Part Number: LTC3714
• Description: Wide Operating Range / Step-Down Controller with Internal Op Amp
• Manufacturer: Linear

APPLICATIO S I FOR ATIO
LTC3714
Because the top MOSFET is on for such a short time, a
single IRF7811 will be sufficient. Checking its power
dissipation at current limit with
The junction temperatures will be significantly less at
nominal current, but this analysis shows that careful
attention to heat sinking will be necessary in this circuit.
C
temperature. The output capacitors are chosen for a low
ESR of 0.005 to minimize output voltage changes due to
inductor ripple current and load steps. The ripple voltage
will be only:
However, a 0A to 15A load step will cause an output
change of up to:
The complete circuit is shown in Figure 8.
Active Voltage Positioning
Active voltage positioning (also termed load “deregula-
tion” or droop) describes a technique where the output
voltage varies with load in a controlled manner. It is useful
in applications where rapid load steps are the main cause
of error in the output voltage. By positioning the output
voltage at or above the regulation point at zero load, and
below the regulation point at full load, one can use more
of the error budget for the load step. This allows one to
reduce the number of output capacitors by relaxing the
ESR requirement.
In the design example, Figure 8, five 0.025 capacitors
are required in parallel to keep the output voltage within
tolerance. Using active voltage positioning, the same
specification can be met with only three capacitors. In this
case, the load step will cause an output voltage change of:
22
IN
T
= 27mV
P
J
TOP
is chosen for an RMS current rating of about 6A at
V
V
= 50 C + (0.652W)(50 C/W) = 82.6 C
OUT(RIPPLE)
OUT(STEP)
1 7 24
1 15
.
0 299
24
.
.
V
= I
V
W
V
= I
20
LOAD
U
2
A
L(MAX)
0 353
20
.
2
(ESR) = (15A) (0.005 ) = 75mV
A
1 2 0 013
U
.
60
W
(ESR) = (5.4A) (0.005 )
pF
.
0 652
80 C
.
W
300
= 1.2:
kHz
W
U
By positioning the output voltage 60mV above the regula-
tion point at no load, it will drop 65mV below the regulation
point after the load step. However, when the load disap-
pears or the output is stepped from 15A to 0A, the 65mV
is recovered. This way, a total of 65mV change is observed
on V
150mV is seen on V
Implementing active voltage positioning requires setting a
precise gain between the sensed current and the output
voltage. Because of the variability of MOSFET on-resis-
tance, it is prudent to use a sense resistor with active
voltage positioning. In order to minimize power lost in this
resistor, a low value of 0.003 is chosen. The nominal
sense voltage will now be:
To maintain a reasonable current limit, the voltage on the
V
INTV
voltage.
Next, the gain of the LTC3714 error amplifier must be
determined. The change in I
change in the output current is:
The corresponding change in the output voltage is deter-
mined by the gain of the error amplifier and feedback
divider. The LTC3714 error amplifier has a
transconductance g
ture and a wide 40mV input range. Thus, by connecting
a load resistance R
can be precisely set for accurate voltage positioning.
RNG
V
SNS(NOM)
V
I
I
OUT
CC
TH
TH
OUT STEP
pin is reduced to 0.5V by connecting it between
and GND, corresponding to a 50mV nominal sense
in all conditions, whereas a total of 75mV or
(
g R
24 0 003
m VP
V
12
RNG
= (0.003 )(15A) = 45mV
)
V
.
VP
R
15
m
V
0 6 .
OUT
SENSE
to the I
OUT
that is constant over both tempera-
A
V
without voltage positioning.
15
3
1
TH
TH
A
V
I
OUT
0 025
voltage for a corresponding
OUT
pin, the error amplifier gain
.
1 08
.
V
125
mV
3714f