Datasheet

LTC3780
17
3780ff
For more information www.linear.com/LTC3780
applicaTions inForMaTion
Figure 11 is a basic LTC3780 application circuit. External
component selection is driven by the load requirement,
and begins with the selection of R
SENSE
and the inductor
value. Next, the power MOSFETs are selected. Finally, C
IN
and C
OUT
are selected. This circuit can be configured for
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 fixed 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
efficiency. The maximum switching frequency is approxi
-
mately 400kHz.
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
L
is typically set to 20% to 40% of the maximum
inductor current at boost mode V
IN(MIN)
. For a given ripple
the inductance terms in continuous mode are as follows:
L
VVV
I
BOOST
IN MIN OUT IN MIN
OUT
>
()
() ()
(
•– •
•
2
100
ƒ
MMAX OUT
BUCK
OUT IN MAXO
RippleV
H
L
VV V
)
()
•% •
,
•–
2
>
UUT
OUT MAXINMAX
IRippleV
H
()
•
••%•
() ()
100
ƒ
where:
f is operating frequency, Hz
% Ripple is allowable inductor current ripple, %
V
IN(MIN)
is minimum input voltage, V
V
IN(MAX)
is maximum input voltage, V
V
OUT
is output voltage, V
I
OUT(MAX)
is maximum output load current
For high efficiency, 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
2
R losses, and must be able to handle the peak
inductor current without saturation. To minimize radiated
noise, use a toroid, pot core or shielded bobbin inductor.
R
SENSE
Selection and Maximum Output Current
R
SENSE
is chosen based on the required output current.
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
IN(MIN)
is:
I
mV
R
I
V
OUT MAXBOOST
SENSE
L
IN
(, )
(
–•=






160
2
∆
MMIN
OUT
V
)
PLLFLTR PIN VOLTAGE (V)
0
0
OPERATING FREQUENCY (kHz)
50
150
200
250
1
2
2.5
450
3780 F07
100
0.5 1.5
300
350
400
Figure 7. Frequency vs PLLFLTR Pin Voltage