LTC3785-1 Linear Technology, LTC3785-1 Datasheet - Page 9

• Manufacturer Part Number: LTC3785-1
• Description: Buck-Boost Controller
• Manufacturer: Linear Technology

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OPERATION
The input voltage, V
is given by:
the point at which the four switch region ends is given
by:
Boost Region (V
Switch A is always on and switch B is always off during
boost mode. When the error amp output voltage, V
proximately above 0.7V, switch pair C and D will alternately
switch to provide a boosted output voltage. This operation
is typical to a synchronous boost regulator. The maximum
duty cycle of the converter is limited to 90% typical.
Burst Mode OPERATION
During Burst Mode operation, the LTC3785-1 delivers
energy to the output until it is regulated and then goes
into a sleep state where the outputs are off and the IC
is consuming only 86μA. In Burst Mode operation, the
output ripple has a variable frequency component, which
is dependent upon load current
During the period where the converter is delivering en-
ergy to the output, the inductor will reach a peak current
determined by an on time, t
current for each cycle. The on time is given by:
where f is the oscillator frequency.
The peak current is given by:
So the peak current is independent of V
proportional to the f • L product optimizing the energy
transfer for various applications.
V
I
I
V
t
PEAK
PEAK
ON
IN
IN
= V
=
=
1
=
=
V
OUT
–
2 4 .
IN
V
f L
2 4
(
L
IN
•
•
.
300
(1 – D) = V
V
f
OUT
•
IN
t
ns f
ON
IN
< V
•
, where the four switch region begins
OUT
)
V
OUT
)
ON
(1 – 300ns • f) V
, and will terminate at zero
IN
and inversely
C
, is ap-
In Burst Mode operation the maximum output current is
given by:
Burst Mode operation is user-controlled by driving the
MODE pin high to enable and low to disable.
V
An internal P-channel low dropout regulator produces
4.35V at the V
the drivers and internal circuitry of the LTC3785-1. The
V
must be bypassed to ground with a minimum of 4.7μF
placed directly adjacent to the V
bypassing is necessary to supply the high transient cur-
rent required by the MOSFET gate drivers and to prevent
interaction between channels. If desired, the V
can be connected to V
provide higher gate drive in low input voltage applications.
The V
source directly (without a Schottky diode).
TOPSIDE MOSFET DRIVER SUPPLY (V
The external bootstrap capacitors connected to the V
and V
side MOSFET switches A and D. When the top MOSFET
switch A turns on, the switch node SW1 rises to V
the V
bottom MOSFET switch B turns on, the switch node SW1
drops low and the boost capacitor is charged through the
diode connected to V
turns on, the switch node SW2 rises to V
pin rises to approximately V
MOSFET switch C turns on, the switch node SW2 drops
low and the boost capacitor is charged through the diode
connected to V
100 times the gate charge required by the top MOSFET
switch A and D. In most applications a 0.1μF to 0.47μF ,
X5R or X7R dielectric capacitor is adequate.
CC
CC
I
OUT MAX BURST
pin regulator can supply a peak current of 100mA and
REGULATOR
BST2
CC
BST2
(
regulator can also be driven with an external 5V
pin rises to approximately V
pins supply the gate drive voltage for the top-
,
CC
CC
. The boost capacitors need to store about
pin from the V
)
≈
CC
f L
OUT
• •
. When the top MOSFET switch D
1 2
through a Schottky diode to
(
OUT
. •
V
OUT
IN
+ V
V
CC
supply pin. V
IN
+
LTC3785-1
and GND pins. Good
CC
V
. When the bottom
IN
IN
BST1
OUT
)
+ V
A
and the V
, V
CC
CC
. When the
CC
BST2
regulator
powers
IN
)
BST1
BST2
37851f
9
and