LTC1874 Linear Technology, LTC1874 Datasheet - Page 10

• Manufacturer Part Number: LTC1874
• Description: Dual Constant Frequency Current Mode Step-Down DC/DC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1874
Low Supply Operation
Although the controller can function down to approxi-
mately 2.0V, the maximum allowable output current is
reduced when V
amount of change as the supply is reduced down to 2V.
Also shown in Figure 3 is the effect of V
goes below 2.3V.
Setting Output Voltage
The controller develops a 0.8V reference voltage between
the feedback (V
selecting resistor R1, a constant current is caused to flow
through R1 and R2 to set the overall output voltage. The
regulated output voltage is determined by:
For most applications, an 80k resistor is suggested for R1.
To prevent stray pickup, locate resistors R1 and R2 close
to the LTC1874.
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 5. In a hard short (V
current will be reduced to approximately 50% of the
maximum output current.
10
V
OUT
FB1
and D
0 8 1
.
V
FB
IN
FB2
) terminal and ground (see Figure 4). By
decreases below 3V. Figure 3 shows the
U
between the output and the I
R
R
2
1
U
W
IN
on V
OUT
U
REF
= 0V), the
TH
as V
/RUN
IN
Figure 3. Line Regulation of V
105
100
95
90
85
80
75
Figure 5. Foldback Current Limiting
13
2.0
Figure 4. Setting Output Voltage
1/2 LTC1874
I
TH
1/2 LTC1874
/RUN1
2.2
GND1
V
FB1
GND1
INPUT VOLTAGE (V)
V
FB1
V
V
2.4
4
3
REF
ITH
4
3
R2
R1
2.6
R2
R1
1874F04
+
2.8
REF
V
OUT
and V
1874 F03
1874 F05
D
D
3.0
FB1
FB2
ITH
V
OUT