LTC1871 Linear Technology, LTC1871 Datasheet - Page 13

• Manufacturer Part Number: LTC1871
• Description: Wide Input Range/ No RSENSE Current Mode Boost/ Flyback and SEPIC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Application Circuits
A basic LTC1871 application circuit is shown in
Figure 1. External component selection is driven by the
characteristics of the load and the input supply. The first
topology to be analyzed will be the boost converter,
followed by SEPIC (single ended primary inductance
converter).
Boost Converter: Duty Cycle Considerations
For a boost converter operating in a continuous conduc-
tion 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.
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 reflected 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 peak input current is
I
I
D
V
IN MAX
IN PEAK
O MAX
(
(
(
D
V
)
O
is the forward voltage of the boost diode. For
)
)
V
O
1
V
I
1
O MAX
–
D
1
V
(
–
IN MIN
V
D
–
D
D
MAX
(
2
V
U
MAX
IN
)
•
)
1
I
O MAX
–
–
:
U
(
D
V
DS(ON)
MAX
D
)
W
of the power MOSFET
U
The maximum duty cycle, D
minimum V
Boost Converter: Ripple Current I
The constant ‘ ’ in the equation above represents the
percentage peak-to-peak ripple current in the inductor,
relative to its maximum value. For example, if 30% ripple
current is chosen, then
15% greater than the average.
For a current mode boost regulator operating in CCM,
slope compensation must be added for duty cycles above
50% in order to avoid subharmonic oscillation. For the
LTC1871, this ramp compensation is internal. Having an
internally fixed ramp compensation waveform, however,
does place some constraints on the value of the inductor
and the operating frequency. If too large an inductor is
used, the resulting current ramp ( I
to the internal ramp compensation (at duty cycles above
50%), and the converter operation will approach voltage
mode (ramp compensation reduces the gain of the current
loop). If too small an inductor is used, but the converter is
still operating in CCM (near critical conduction mode), the
internal ramp compensation may be inadequate to prevent
subharmonic oscillation. To ensure good current mode
gain and avoid subharmonic oscillation, it is recom-
mended that the ripple current in the inductor fall in the
range of 20% to 40% of the maximum average current. For
example, if the maximum average input current is 1A,
choose a I
between 0.2 and 0.4.
Boost Converter: Inductor Selection
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the inductor,
the inductor value can be determined using the following
equation:
where
L
I
L
:
V
IN MIN
I
(
L
•
IN
•
L
– 1
.
f
I
)
O MAX
between 0.2A and 0.4A, and a value ‘ ’
(
•
D
D
MAX
MAX
)
= 0.30, and the peak current is
MAX
, should be calculated at
L
) will be small relative
L
and the ‘ ’ Factor
LTC1871
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