LTC3785 LINER [Linear Technology], LTC3785 Datasheet - Page 17

• Manufacturer Part Number: LTC3785
• Description: High Efficiency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
clock whose frequency is within range of the LTC3789’s
internal VCO. This is guaranteed to be between 200kHz
and 600kHz. A simplified block diagram is shown in
Figure 10.
If the external clock frequency is greater than the inter-
nal oscillator’s frequency, f
continuously from the phase detector output, pulling up
the filter network. When the external clock frequency is
less than f
the filter network. If the external and internal frequencies
are the same but exhibit a phase difference, the current
sources turn on for the amount of time corresponding to
the phase difference. The voltage on the filter network is
adjusted until the phase and frequency of the internal and
external oscillators are identical. At the stable operating
point, the phase detector output is high impedance and
the filter capacitor holds the voltage.
Typically, the external clock (on the MODE/PLLIN pin)
input high threshold is 1.6V, while the input low thresh-
old is 1V.
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
maximum inductor current in the boost region at V
OSCILLATOR
EXTERNAL
Figure 10. Phase-Locked Loop Block Diagram
OSC
MODE/
PLLIN
, current is sunk continuously, pulling down
L
FREQUENCY
DETECTOR
is typically set to 20% to 40% of the
DIGITAL
PHASE/
SYNC
OSC
2.4V
, then current is sourced
5V
10µA
FREQ
R
SET
VCO
IN(MIN)
3789 F10
.
For a given ripple the inductance terms in continuous
mode are as follows:
where:
For high efficiency, choose an inductor with low core loss,
such as ferrite. Also, the inductor should have low DC
resistance to reduce the I
handle the peak inductor current without saturating. To
minimize radiated noise, use a toroid, pot core or shielded
bobbin inductor.
C
In the boost region, input current is continuous. In the
buck region, input current is discontinuous. In the buck
region, the selection of input capacitor C
the need to filter the input square wave current. Use a low
ESR capacitor sized to handle the maximum RMS current.
For buck operation, the input RMS current is given by:
This formula has a maximum at V
I
is commonly used for design because even significant
deviations do not offer much relief. Note that ripple cur-
rent ratings from capacitor manufacturers are often based
on only 2000 hours of life which makes it advisable to
derate the capacitor.
RMS
IN
f is operating frequency, Hz
% Ripple is allowable inductor current ripple
V
V
V
I
I
L
L
OUT(MAX)
RMS
and C
IN(MIN)
IN(MAX)
OUT
BOOST
BUCK
= I
is output voltage, V
≈ I
OUT(MAX)
OUT
>
OUT(MAX)
>
is minimum input voltage, V
is maximum input voltage, V
is maximum output load current, A
f • I
Selection
V
V
f • I
IN(MIN)
OUT
OUT(MAX)
/2. This simple worst-case condition
OUT(MAX)
• V
•
(
2
V
V
OUT
• V
IN(MAX)
IN
2
• % Ripple • V
R losses, and must be able to
OUT
• % Ripple • V
•
– V
V
– V
V
OUT
IN
IN(MIN)
OUT
IN
– 1
LTC3789
)
= 2V
IN(MAX)
• 100
IN
OUT2
• 100
is driven by
OUT
, where
17
H
H,
3789f