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LTC3785-1 Datasheet, PDF (9/20 Pages) Linear Technology – 10V, High Effi ciency, Buck-Boost Controller with Power Good
LTC3785-1
OPERATION
The input voltage, VIN, where the four switch region begins
is given by:
( ) VIN = 1–
VOUT
300ns • f
V
the point at which the four switch region ends is given
by:
VIN = VOUT(1 – D) = VOUT(1 – 300ns • f) V
Boost Region (VIN < VOUT)
Switch A is always on and switch B is always off during
boost mode. When the error amp output voltage, VC, 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 LTC3785-1 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, tON, and will terminate at zero
current for each cycle. The on time is given by:
tON
=
2.4
VIN •
f
where f is the oscillator frequency.
The peak current is given by:
IPEAK
=
VIN
L
•
tON
IPEAK
=
2.4
f •L
So the peak current is independent of VIN 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 • VIN
• VOUT +
VIN
A
Burst Mode operation is user-controlled by driving the
MODE pin high to enable and low to disable.
VCC REGULATOR
An internal P-channel low dropout regulator produces
4.35V at the VCC pin from the VIN supply pin. VCC powers
the drivers and internal circuitry of the LTC3785-1. The
VCC 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 VCC 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 VCC regulator
can be connected to VOUT through a Schottky diode to
provide higher gate drive in low input voltage applications.
The VCC regulator can also be driven with an external 5V
source directly (without a Schottky diode).
TOPSIDE MOSFET DRIVER SUPPLY (VBST1, VBST2)
The external bootstrap capacitors connected to the VBST1
and VBST2 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 VIN and
the VBST2 pin rises to approximately VIN + VCC. 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 VCC. When the top MOSFET switch D
turns on, the switch node SW2 rises to VOUT and the VBST2
pin rises to approximately VOUT + VCC. 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 VCC. The boost capacitors need to store about
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.
37851f
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