LTC1772 Linear Technology, LTC1772 Datasheet - Page 10

• Manufacturer Part Number: LTC1772
• Description: Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23
• Manufacturer: Linear Technology

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 5. In a hard short (V
will be reduced to approximately 50% of the maximum
output current.
LTC1772
Design Example
Assume the LTC1772 is used in a single Lithium-Ion
battery-powered cellular phone application. The V
operating from a maximum of 4.2V down to a minimum of
2.7V. Load current requirement is a maximum of 1.5A but
most of the time it will be on standby mode, requiring only
2mA. Efficiency at both low and high load current is
important. Output voltage is 2.5V.
From Figure 2, SF = 57%.
10
FB1
R
Maximum
SENSE
and D
FB2
Figure 5. Foldback Current Limiting
12
between the output and the I
Duty Cycle
I
TH
LTC1772
I
U
/RUN
OUT
SF
V
FB
INFORMATION
100
U
V
IN MIN
V
R2
R1
OUT
(
12 1 5
+
0 57
W
OUT
)
.
V
.
V
= 0V), the current
D
D
1772 F05
D
D
TH
FB1
FB2
/RUN pin as
0 0317
93
V
OUT
.
U
%
IN
will be
In the application, a 0.03
inductor, the required value is:
In the application, a 5.6 H inductor is used to reduce ripple
current.
For the selection of the external MOSFET, the R
must be guaranteed at 2.5V since the LTC1772 has to work
down to 2.7V. Let’s assume that the MOSFET dissipation
is to be limited to P
is 50 C/W. Hence the junction temperature at T
will be 37.5 C and p = 0.005 (37.5 – 25) = 0.0625. The
required R
The P-channel MOSFET requirement can be met by an
Si6433DQ.
The requirement for the Schottky diode is the most strin-
gent when V
R
Schottky is 0.1/0.03 = 3.3A. An MBRS340T3 Schottky
diode is chosen. With 3.3A flowing through, the diode is
rated with a forward voltage of 0.4V. Therefore, the worst-
case power dissipated by the diode is 1.32W. The addition
of D
tion to approximately 0.66W.
The input capacitor requires an RMS current rating of at
least 0.75A at temperature, and C
of 0.1 for optimum efficiency.
SENSE
R
L
FB1
MIN
DS ON
(
and D
resistor, the short-circuit current through the
)
DS(ON)
550
OUT
FB2
DC I
kHz
4 2 2 5
.
(Figure 5) will reduce the diode dissipa-
is then given by:
= 0V, i.e., short circuit. With a 0.03
OUT
P
0 0288
= 250mW and its thermal resistance
.
0 03
P
.
.
P
2
1
resistor is used. For the
p
2 5 0 3
4 2 0 3
.
.
OUT
0 11
.
will require an ESR
.
.
2 00
.
A
= 25 C
DS(ON)
H