ltc3711 Linear Technology Corporation, ltc3711 Datasheet - Page 18

• 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
Figure 9 will provide adequate compensation for most
applications. For a detailed explanation of switching
control loop theory see Application Note 76.
Design Example
As a design example, take a supply with the following
specifications: V
timing resistor with V
and choose the inductor for about 40% ripple current at
the maximum V
Selecting a standard value of 1 H results in a maximum
ripple current of:
Next, choose the synchronous MOSFET switch. Because
of the narrow duty cycle and large current, a single SO-8
MOSFET will have difficulty dissipating the power lost in
the switch. Choosing two IRF7811A (R
C
of:
Tying V
range for a nominal value of 140mV with current limit
occurring at 186mV. To check if the current limit is
acceptable, assume a junction temperature of about 100 C
above a 50 C ambient with
and double check the assumed T
18
RSS
100mV, I
V
R
L
I
LIMIT
SNS(NOM)
ON
I
= 60pF,
L
RNG
300
300
OUT(MAX)
300
kHz
to INTV
0 5 1 6 0 012
= (15A)(0.5)(1.3)(0.012 ) = 117mV
.
JA
kHz
1 5
kHz
1 5
IN
.
IN
.
= 50 C/W) yields a nominal sense voltage
186
:
0 4 15
1
V
= 7V to 24V (15V nominal), V
V
.
.
U
= 15A, f = 300kHz. First, calculate the
1
CC
10
mV
ON
H
will set the current sense voltage
pF
.
= V
A
U
1
OUT
–
150 C
1
330
1 5
24
.
:
J
1 5
V
24
k
V
2
.
1
in the MOSFET:
W
= 1.6:
V
V
4 7
.
4 7
DS(ON)
A
.
0 8
A
.
18
= 0.013 ,
U
OUT
H
A
= 1.5V
Because the top MOSFET is on for such a short time, a
single IRF7811A 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 9.
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 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
T
T
P
P
J
BOT
J
is chosen for an RMS current rating of about 6A at
BOT
V
V
= 50 C + (2.12W)(50 C/W) = 156 C
= 50 C + (0.84W)(50 C/W) = 92 C
OUT(RIPPLE)
OUT(STEP)
1 7 24
1 5
24
.
0 46
24
.
.
V
V
V
24
W
– .
= I
21 7
V
V
1 5
= I
= (4.7A) (0.005 ) = 24mV
.
2
LOAD
0 38
V
.
A
21 7
L(MAX)
21 7
2
.
W
(ESR) = (15A) (0.005 ) = 75mV
1 3 0 012
A
.
2
.
A
(ESR)
60
0 84
.
2
.
pF
1 6 0 012
90 C
W
.
300
= 1.3:
.
kHz
2 12
.
3711f
W