Datasheet

LTC3785
9
3785fc
operaTion
the buck-boost range, approximately 0.7V, the AC switch
pair completely phase out the BD pair, and the boost phase
begins at duty cycle, D4(SW).
The input voltage, V
IN
, where the four switch region begins
is given by:
V
IN
=
V
OUT
1– 300ns • f
( )
V
the point at which the four switch region ends is given
by:
V
IN
= V
OUT
(1 – D) = V
OUT
(1 – 300ns • f) V
Boost Region (V
IN
< V
OUT
)
Switch A is always on and switch B is always off during
boost mode. When the error amp output voltage, V
C
, is ap-
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 O
PERA
TION
During Burst Mode operation, the LTC3785 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
ON
, and will terminate at zero
current for each cycle. The on time is given by:
t
ON
=
2.4
V
IN
• f
where f is the oscillator frequency.
The peak current is given by:
I
PEAK
=
V
IN
L
• t
ON
I
PEAK
=
2.4
f •L
So the peak current is independent of V
IN
and inversely
proportional to the f • L product optimizing the energy
transfer for various applications.
In Burst Mode operation the maximum output current is
given by:
I
OUT(MAX,BURST)
≈
1.2 • V
IN
f •L • V
OUT
+ V
IN
( )
A
Burst Mode operation is user-controlled by driving the
MODE pin high to enable and low to disable.
V
CC
REGULATOR
An internal P-channel low dropout regulator produces
4.35V at the V
CC
pin from the V
IN
supply pin. V
CC
powers
the drivers and internal circuitry of the LTC3785. The V
CC
pin regulator can supply a peak current of 100mA and
must be bypassed to ground with a minimum of 4.7µF
placed directly adjacent to the V
CC
and GND pins. Good
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
CC
regulator
can be connected to V
OUT
through a Schottky diode to
provide higher gate drive in low input voltage applications.
The V
CC
regulator can also be driven with an external 5V
source directly (without a Schottky diode).
TOPSIDE
MOSFET DRIVER SUPPLY (V
BST1
, V
BST2
)
The external bootstrap capacitors connected to the V
BST1
and V
BST2
pins supply the gate drive voltage for the top-
side MOSFET switches A and D. When the top MOSFET
switch A turns on, the switch node SW1 rises to V
IN
and
the V
BST2
pin rises to approximately V
IN
+ V
CC
. When the
bottom MOSFET switch B turns on, the switch node SW1
drops low and the boost capacitor is charged through the
diode connected to V
CC
. When the top MOSFET switch D
turns on, the switch node SW2 rises to V
OUT
and the V
BST2
pin rises to approximately V
OUT
+ V
CC
. When the bottom
MOSFET switch C turns on, the switch node SW2 drops
low and the boost capacitor is charged through the diode
connected to V
CC
. The boost capacitors need to store about
100 times the gate charge required by the top MOSFET