LTC1871-1 LINER [Linear Technology], LTC1871-1 Datasheet - Page 16

• Manufacturer Part Number: LTC1871-1
• Description: High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC3780
Figure 11 is a basic LTC3780 application circuit. External
component selection is driven by the load requirement,
and begins with the selection of R
value. Next, the power MOSFETs are selected. Finally, C
and C
operation up to an input voltage of 36V.
Selection of Operation Frequency
The LTC3780 uses a constant frequency architecture and
has an internal voltage controlled oscillator. The switching
frequency is determined by the internal oscillator capacitor.
This internal capacitor is charged by a fi xed current plus
an additional current that is proportional to the voltage
applied to the PLLFLTR pin. The frequency of this oscillator
can be varied over a 2-to-1 range. The PLLFLTR pin can
be grounded to lower the frequency to 200kHz or tied to
2.4V to yield approximately 400kHz. When PLLIN is left
open, the PLLFLTR pin goes low, forcing the oscillator to
minimum frequency.
A graph for the voltage applied to the PLLFLTR pin vs
frequency is given in Figure 7. As the operating frequency
is increased the gate charge losses will be higher, reducing
effi ciency. The maximum switching frequency is approxi-
mately 400kHz.
16
OUT
Figure 7. Frequency vs PLLFLTR Pin Voltage
450
400
350
300
250
200
150
100
are selected. This circuit can be confi gured for
50
0
0
0.5
PLLFLTR PIN VOLTAGE (V)
1
1.5
SENSE
2
3780 F07
and the inductor
2.5
IN
Inductor Selection
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use of
smaller inductor and capacitor values. The inductor value
has a direct effect on ripple current. The inductor current
ripple ∆I
inductor current at boost mode V
the inductance terms in continuous mode are as follows:
where:
For high effi ciency, choose an inductor with low core loss,
such as ferrite and molypermalloy (from Magnetics, Inc.).
Also, the inductor should have low DC resistance to reduce
the I
current without saturating. To minimize radiated noise,
use a toroid, pot core or shielded bobbin inductor.
R
R
The current comparator threshold sets the peak of the
inductor current in boost mode and the maximum inductor
valley current in buck mode. In boost mode, the maximum
average load current at V
SENSE
SENSE
L
L
f is operating frequency, Hz
% Ripple is allowable inductor current ripple, %
V
V
V
I
I
OUT(MAX)
OUT MAX BOOST
BOOST
BUCK
IN(MIN)
IN(MAX)
OUT
2
R losses, and must be able to handle the peak inductor
(
Selection and Maximum Output Current
is chosen based on the required output current.
is output voltage, V
L
>
is typically set to 20% to 40% of the maximum
>
is minimum input voltage, V
,
is maximum input voltage, V
ƒ
is maximum output load current
V
V
ƒ
IN MIN
OUT
I
OUT MAX
(
I
OUT
)
=
(
)
2
(
⎛
⎝ ⎜
(
V
M M AX
R
160
IN MAX
(
)
SENSE
(
V
IN(MIN)
OUT
)
mV
Ripple V
)
Ripple V
IN(MIN)
is:
V
Δ
V
IN MIN
2
O
I
L
U U T
(
⎞
⎠ ⎟
)
. For a given ripple
IN MAX
)
V
)
100
(
IN
V
OUT
OUT
(
100
M M IN
2
)
)
H
H
,
3780fe