LTC3787
OPERATION
INTV /EXTV Power
pull-up current charges this capacitor creating a voltage
ramp on the SS pin. As the SS voltage rises linearly from
CC
CC
Power for the top and bottom MOSFET drivers and most
0V to 1.2V (and beyond up to INTV ), the output voltage
CC
other internal circuitry is derived from the INTV pin.
CC
rises smoothly to its final value.
When the EXTV pin is tied to a voltage less than 4.8V,
CC
the VBIAS LDO (low dropout linear regulator) supplies
Light Load Current Operation—Burst Mode Operation,
Pulse-Skipping or Continuous Conduction
(PLLIN/MODE Pin)
5.4V from VBIAS to INTV . If EXTV is taken above
CC
CC
4.8V, the VBIAS LDO is turned off and an EXTV LDO is
CC
turned on. Once enabled, the EXTV LDO supplies 5.4V
CC
The LTC3787 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
from EXTV to INTV . Using the EXTV pin allows the
CC
CC
CC
INTV power to be derived from an external source, thus
CC
removing the power dissipation of the VBIAS LDO.
Shutdown and Start-Up (RUN and SS Pins)
The two internal controllers of the LTC3787 can be shut
down using the RUN pin. Pulling this pin below 1.28V
shuts down the main control loops for both phases.
Pulling this pin below 0.7V disables both controllers and
most internal circuits, including the INTV LDOs. In this
state, the LTC3787 draws only 8μA of quiescent current.
INTV . To select pulse-skipping mode, tie the PLLIN/
CC
MODE pin to a DC voltage greater than 1.2V and less
than INTV – 1.3V.
CC
CC
When the controller is enabled for Burst Mode opera-
tion, the minimum peak current in the inductor is set to
approximately 30% of the maximum sense voltage even
though the voltage on the ITH pin indicates a lower value.
If the average inductor current is higher than the required
current, the error amplifier EA will decrease the voltage
on the ITH pin. When the ITH voltage drops below 0.425V,
the internal sleep signal goes high (enabling sleep mode)
and both external MOSFETs are turned off.
NOTE: Do not apply a heavy load for an extended time
while the chip is in shutdown. The top MOSFETs will be
turnedoffduringshutdownandtheoutputloadmaycause
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 ), as long as the maxi-
mum current into the RUN pin does not exceed 100μA.
In sleep mode much of the internal circuitry is turned off
and the LTC3787 draws only 135μA of quiescent current.
In sleep mode the load current is supplied by the output
capacitor. As the output voltage decreases, the EA’s output
beginstorise. Whentheoutputvoltagedropsenough, the
sleep signal goes low and the controller resumes normal
operation by turning on the bottom external MOSFET on
the next cycle of the internal oscillator.
IN
An external resistor divider connected to V can set the
IN
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.
When the controller is enabled for Burst Mode operation,
the inductor current is not allowed to reverse. The reverse
currentcomparator(IR)turnsoffthetopexternalMOSFET
just before the inductor current reaches zero, preventing
it from reversing and going negative. Thus, the controller
operates in discontinuous current operation.
The start-up of the controller’s output voltage V
is
OUT
controlled by the voltage on the SS pin. When the voltage
on the SS pin is less than the 1.2V internal reference, the
LTC3787 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
3787fc
12
Linear Systems [ Linear Systems ]
