LTC3872 Linear Technology, LTC3872 Datasheet - Page 7

• Manufacturer Part Number: LTC3872
• Description: No RSENSE Current Mode Boost DC/DC Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
Output Voltage Programming
The output voltage is set by a resistor divider according
to the following formula:
The external resistor divider is connected to the output
as shown in the Typical Application on the front page,
allowing remote voltage sensing.
Application Circuits
A basic LTC3872 application circuit is shown on the front
page of this datasheet. External component selection is
driven by the characteristics of the load and the input
supply.
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 LTC3872 has a built-in circuit that
allows the extension of the maximum duty cycle while
keeping the minimum switch off time unchanged. This
is accomplished by reducing the clock frequency when
the duty cycle is close to 80%. This function allows the
user to obtain high output voltages from low input supply
voltages. The shift of frequency with duty cycle is shown
in the Typical Performance Characteristics section.
D
V
O
=
=
⎛
⎜
⎝
1 2
D
V
.
O
is the forward voltage of the boost diode. For
V
V
+
O
•
V
⎛
⎝ ⎜
+
D
1
V
–
+
D
V
U
R
R
IN
2
1
⎞
⎠ ⎟
⎞
⎟
⎠
U
W
U
The Peak and Average Input Currents
The control circuit in the LTC3872 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:
Ripple Current I
The constant χ 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 χ = 0.30, and the peak current is
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
LTC3872, this ramp compensation is internal. Having an
internally fi xed 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 (continuous conduction mode),
the internal ramp compensation may be inadequate to
prevent subharmonic oscillation. To ensure good current
The peak input current is
I
I
IN MAX
IN PEAK
(
(
)
)
=
=
1
⎛
⎜
⎝
I
O MAX
–
1
(
L
+
D
and the χ Factor
MAX
χ
2
⎞
⎟
⎠
)
•
1
I
O MAX
–
:
(
D
DS(ON)
MAX
)
L
of the power MOSFET
) will be small relative
LTC3872
3872fa
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