LTC1771 LINER [Linear Technology], LTC1771 Datasheet - Page 11

• Manufacturer Part Number: LTC1771
• Description: Low Quiescent Current High Efficiency Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Design Example
As a design example, assume V
15V
tion, we can easily calculate all the important components.
To optimize low current efficiency, MODE pin is tied to V
to enable Burst Mode operation, thus the minimum induc-
tance necessary is:
15 H is chosen for the application.
For the feedback resistors, choose R1 = 1M to minimize
supply current. R2 can then be calculated to be:
Assume that the MOSFET dissipation is to be limited to
P
If T
83 C/W, then the junction temperatures will be 91 C and
be calculated:
P
P
R
L
R2 = (V
= 0.33. The required R
= 0.25W.
A
(MAX)
MIN
SENSE
I
= 70 C and the thermal resistance of the MOSFET is
L
= 70 H(3.3V + 0.5)(0.05 ) = 13.3 H
, V
3 5
OUT
= 100mV/2A = 0.05
.
OUT
/1.23 – 1) • R1 = 1.68M
s
= 3.3V, and I
3 3
.
U
15
V
H
0 5
DS(ON)
U
.
V
MAX
IN
for the MOSFET can now
R1
= 2A. With this informa-
0 89
= 10V (nominal), V
R2
W
.
C
ITH
BOLD LINES INDICATE HIGH CURRENT PATHS
A
R
ITH
C
SS
C
Figure 3. LTC1771 Layout Diagram
FF
1
2
3
4
U
RUN/SS
I
V
GND
TH
FB
LTC1771
0.1 F
IN
IN
=
SENSE
PGATE
MODE
V
IN
Since the gate of the MOSFET will see the full input voltage,
a MOSFET must be selected whose V
P-channel MOSFET that meets both the V
R
The most stringent requirement for the Schottky diode
occurs when V
V
With a 0.05
increasing the 0.5V Schottky diode dissipation to 1W.
C
temperature. C
output ripple. The output voltage ripple due to ESR is
approximately:
8
7
6
5
IN
IN
DS(ON)
V
P
P
. In this case the worst-case dissipation rises to:
MODE
D
is chosen for a RMS current rating of at least 1A at
ORIPPLE
- Channel R
I
requirement is the Si6447DQ.
SC AVG
+
+
(
C
C
IN
OUT
(R
OUT
OUT
sense resistor I
)
ESR
DS(ON)
D1
V
is chosen with an ESR of 0.05 for low
D
= 0V (i.e., short circuit) at maximum
)( I
V
Q1
L
IN
L
1771 F03
) = 0.05 (0.89A
V
V
0 130
OUT
IN
.
3 3
10
V
.
D
V
V
SC(AVG)
0 5
0 5
.
0 25
.
.
V
V
GS(MAX)
= 2A will result,
W
P-P
LTC1771
2
A
GS(MAX)
) = 45mV
2
> 15V. A
1 33
11
.
and
P-P