LTC3785 Datasheet, PDF (9/20 Pages) Linear Technology – 10V, High Effi ciency, Synchronous, No RSENSE Buck-Boost Controller

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OPERATIO
accordingly. When the V_C voltage reaches the edge of 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
If during the rectification phase (switch pair BD on) the
inductor current becomes discontinuous, then switch D is
turned off and a damping impedance is connected across
the inductor to prevent ringing.
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.
If during the rectification phase (switch pair AD on) the
inductor current becomes discontinuous then switch D is
turned off and a damping impedance is connected across
the inductor to prevent ringing.
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
LTC3785
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).
3785f
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