ltc1772b Linear Technology Corporation, ltc1772b Datasheet - Page 10

• Manufacturer Part Number: ltc1772b
• Description: Constant Frequency Current Mode Step-down Dc/dc Controller In Sot-23
• Manufacturer: Linear Technology Corporation

APPLICATIONS
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 continu-
ously. 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 diodes
D
shown in Figure 6. In a hard short (V
will be reduced to approximately 50% of the maximum
output current.
LTC1772B
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
10
FB1
and D
FB2
Figure 6. Foldback Current Limiting
between the output and the I
I
TH
LTC1772B
U
/RUN
C
R
ITH
ITH
V
FB
INFORMATION
U
Figure 7. LTC1772B Layout Diagram (See PC Board Layout Checklist)
R2
R1
BOLD LINES INDICATE HIGH CURRENT PATHS
+
W
OUT
1
2
3
= 0V), the current
I
GND
V
TH
FB
/RUN
LTC1772B
1772 F05
D
D
TH
FB1
FB2
/RUN pin as
SENSE
V
PGATE
OUT
U
V
IN
–
6
5
4
R
0.1 F
SENSE
LTC1772B. These items are illustrated graphically in the
layout diagram in Figure 7. Check the following in your
layout:
1. Is the Schottky diode closely connected between ground
2. Does the (+) plate of C
3. Is the input decoupling capacitor (0.1 F) connected
4. Connect the end of R
5. Is the trace from SENSE
6. Keep the switching node PGATE away from sensitive
7. Does the V
(Pin 2) and drain of the external MOSFET?
as closely as possible? This capacitor provides AC
current to the MOSFET.
closely between V
possible. The V
comparator.
kept short? Does the trace connect close to R
small signal nodes.
resistors? The resistive divider R1 and R2 must be
connected between the (+) plate of C
ground.
M1
+
C
IN
D1
FB
L1
pin connect directly to the feedback
IN
R1
IN
+
pin is the SENSE
1772 F06
(Pin 5) and ground (Pin 2)?
SENSE
C
IN
OUT
–
connect to the sense resistor
R2
(Pin 4) to the Sense resistor
as close to V
V
V
IN
OUT
+
OUT
of the current
IN
and signal
(Pin 5) as
SENSE
?