LTC1779 LINER [Linear Technology], LTC1779 Datasheet - Page 10

• Manufacturer Part Number: LTC1779
• Description: 250mA Current Mode Step-Down DC/DC Converter in ThinSOT
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1779
Foldback Current Limiting
As described in the Output Diode Selection, the worst-
case dissipation occurs with a short-circuited output
when the diode conducts the current limit value almost
continuously. To prevent excessive heating in the diode,
foldback current limiting can be added to reduce the
current in proportion to the severity of the fault.
Foldback current limiting is implemented by adding di-
odes D
pin as shown in Figure 6. In a hard short (V
current will be reduced to approximately 50% of the
maximum output current.
10
FB1
and D
Figure 6. Foldback Current Limiting
FB2
I
TH
LTC1779
U
/RUN
between the output and the I
C
R
ITH
ITH
V
FB
U
Figure 7. LTC1779 Layout Diagram (See PC Board Layout Checklist)
R2
R1
BOLD LINES INDICATE HIGH CURRENT PATHS
+
W
1
2
3
I
GND
V
TH
FB
/RUN
1779 F06
LTC1779
D
D
OUT
FB1
FB2
SENSE
V
U
OUT
= 0V), the
SW
V
TH
IN
–
/RUN
6
5
4
R
S
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1779. These items are illustrated graphically in the
layout diagram in Figure 7. Check the following in your
layout:
1. Large switch currents flow into the input capacitor C
2. Is the input decoupling capacitor (0.1 F) connected
3. Keep the switching node SW away from sensitive small
4. Does the V
0.1 F
the power switch and the Schottky diode D1. The loop
formed by these components should be as small as
possible.
closely between V
signal nodes.
resistors? The resistive divider R1 and R2 must be
connected between the (+) plate of C
ground. Locate R1 and R2 close to the V
+
C
SW
IN
D1
FB
L1
pin connect directly to the feedback
IN
R1
+
1779 F07
(Pin 5) and ground (Pin 2)?
C
OUT
R2
V
V
IN
OUT
OUT
FB
and signal
pin.
IN
,