ltc1871ems-trpbf Linear Technology Corporation, ltc1871ems-trpbf Datasheet - Page 13

• Manufacturer Part Number: ltc1871ems-trpbf
• Description: Wide Input Range, No Rsense Current Mode Boost, Flyback And Sepic Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Boost Converter: Duty Cycle Considerations
For a boost converter operating in a continuous conduction
mode (CCM), the duty cycle of the main switch is:
where V
converters where the input voltage is close to the output
voltage, the duty cycle is low and for converters that develop
a high output voltage from a low voltage input supply,
the duty cycle is high. The maximum output voltage for a
boost converter operating in CCM is:
The maximum duty cycle capability of the LTC1871 is
typically 92%. This allows the user to obtain high output
voltages from low input supply voltages.
D =
V
O(MAX)
LOGIC CONTROL
Figure 8b. On/Off Control Using External Logic
D
V
EXTERNAL
O
is the forward voltage of the boost diode. For
V
+ V
=
O
(
+ V
1– D
D
V
IN(MIN)
– V
D
MAX
IN
RUN
SUPPLY
)
Figure 8a. Programming the Turn-On and Turn-Off Thresholds Using the RUN Pin
INPUT
– V
6V
1.248V
D
+
–
CAPACITOR
OPTIONAL
FILTER
–
+
1871 F08b
COMPARATOR
RUN
R2
R1
V
RUN
GND
IN
REFERENCE
μPOWER
6V
1.248V
Boost Converter: The Peak and Average Input Currents
The control circuit in the LTC1871 is measuring the input
current (either by using the R
or by using a sense resistor in the MOSFET source), so
the output current needs to be refl ected back to the input
in order to dimension the power MOSFET properly. Based
on the fact that, ideally, the output power is equal to the
input power, the maximum average input current is:
The maximum duty cycle, D
minimum V
The peak input current is:
I
I
IN(MAX)
IN(PEAK)
+
–
SUPPLY
INPUT
COMPARATOR
=
+
–
Figure 8c. External Pull-Up Resistor On
RUN Pin for “Always On” Operation
= 1+
IN
1– D
.
RUN
I
O(MAX)
1M
R2
MAX
2
•
START-UP
CONTROL
BIAS AND
1– D
V
RUN
GND
I
IN
O(MAX)
1871 F8a
MAX
MAX
DS(ON)
1.248V
6V
, should be calculated at
of the power MOSFET
+
–
LTC1871
COMPARATOR
RUN
1871 F08c
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