LTC3784_15 Datasheet, PDF (11/38 Pages) Linear Technology – 60V PolyPhase Synchronous Boost Controller

LTC3784
Operation
Main Control Loop
The LTC3784 uses a constant-frequency, current mode
step-up architecture with the two controller channels
operating out of phase. During normal operation, each
external bottom MOSFET is turned on when the clock for
that channel sets the RS latch, and is turned off when the
main current comparator, ICMP, resets the RS latch. The
peak inductor current at which ICMP trips and resets the
latch is controlled by the voltage on the ITH pin, which is
the output of the error amplifier EA. The error amplifier
compares the output voltage feedback signal at the VFB
pin (which is generated with an external resistor divider
connected across the output voltage, V_OUT, to ground), to
the internal 1.200V reference voltage. In a boost converter,
the required inductor current is determined by the load
current, V_IN and V_OUT. When the load current increases,
it causes a slight decrease in VFB relative to the reference,
which causes the EA to increase the ITH voltage until the
average inductor current in each channel matches the new
requirement based on the new load current.
After the bottom MOSFET is turned off each cycle, the
top MOSFET is turned on until either the inductor current
starts to reverse, as indicated by the current comparator,
IREV, or the beginning of the next clock cycle.
INTV_CC/EXTV_CC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV_CC pin.
When the EXTV_CC pin is tied to a voltage less than 4.8V,
the VBIAS LDO (low dropout linear regulator) supplies
5.4V from VBIAS to INTV_CC. If EXTV_CC is taken above
4.8V, the VBIAS LDO is turned off and an EXTV_CC LDO is
turned on. Once enabled, the EXTV_CC LDO supplies 5.4V
from EXTV_CC to INTV_CC. Using the EXTV_CC pin allows the
INTV_CC power to be derived from an external source, thus
removing the power dissipation of the VBIAS LDO.
Shutdown and Start-Up (RUN and SS Pins)
The two internal controllers of the LTC3784 can be shut
down using the RUN pin. Pulling this pin below 1.28V
shuts down the main control loops for both phases. Pull-
ing this pin below 0.7V disables both channels and most
internal circuits, including the INTV_CC LDOs. In this state,
the LTC3784 draws only 4μA of quiescent current.
NOTE: Do not apply a heavy load for an extended time while
the chip is in shutdown. The top MOSFETs are turned off
during shutdown and the output load may cause excessive
dissipation in the body diodes.
The RUN pin may be externally pulled up or driven directly
by logic. When driving the RUN pin with a low impedance
source, do not exceed the absolute maximum rating of
8V. The RUN pin has an internal 11V voltage clamp that
allows the RUN pin to be connected through a resistor to
a higher voltage (for example, V_IN), as long as the maxi-
mum current into the RUN pin does not exceed 100μA.
An external resistor divider connected to V_IN can set the
threshold for converter operation. Once running, a 4.5μA
current is sourced from the RUN pin allowing the user to
program hysteresis using the resistor values.
The start-up of the controller’s output voltage V_OUT is
controlled by the voltage on the SS pin. When the voltage
on the SS pin is less than the 1.2V internal reference, the
LTC3784 regulates the VFB voltage to the SS pin voltage
instead of the 1.2V reference. This allows the SS pin to
be used to program a soft-start by connecting an external
capacitor from the SS pin to SGND. An internal 10μA
pull‑up current charges this capacitor creating a voltage
ramp on the SS pin. As the SS voltage rises linearly from
0V to 1.2V (and beyond up to INTV_CC), the output voltage
rises smoothly to its final value.
Light Load Current Operation—Burst Mode Operation,
Pulse-Skipping or Continuous Conduction
(PLLIN/MODE Pin)
The LTC3784 can be enabled to enter high efficiency Burst
Mode operation, constant-frequency, pulse-skipping
mode or forced continuous conduction mode at low
load currents. To select Burst Mode operation, tie the
PLLIN/MODE pin to ground (e.g., SGND). To select
forced continuous operation, tie the PLLIN/MODE pin to
INTV_CC. To select pulse-skipping mode, tie the PLLIN/
MODE pin to a DC voltage greater than 1.2V and less
than INTV_CC – 1.3V.
For more information www.linear.com/LTC3784
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