LTC3785_15 Datasheet, PDF (9/20 Pages) Linear Technology – 10V, High Efficiency, Synchronous, No RSENSE Buck-Boost Controller

LTC3785
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
=
1–
V_OUT
(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 Operation
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:
( ) ^{IOUT(MAX,BURST)}
≈
f
•L
1.2 • V_IN
• 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
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