LTC3878
9
3878fa
operaTion
LTC1778 Compatibility
The LTC3878 is compatible with the LTC1778 in applica-
tions which
do not use the EXTV
CC
function. The LTC3878
offers improved gate drive and reduced dead time, which
allows higher efficiency than the LTC1778. On the LTC1778
Pin 9 is EXTV
CC
, but on the LTC3878 it is a no connect.
The other notable difference is that the shutdown latch-
off timer is removed. The LTC3878 should be a drop in,
pin-for-pin replacement in most applications that do not
use EXTV
CC
. The LTC3878 should be tested and verified
in each application without assuming compatibility. Con-
tact a
Linear applications expert to answer any questions
regarding LTC3878/LTC1778 compatibility.
Main Control Loop
The LTC3878 is a valley current mode controller IC for
use in DC/DC step-down converters. In normal continu-
ous operation,
the top MOSFET is turned on for a fixed
interval determined by a one-shot timer, OST. When the
top MOSFET is turned off, the bottom MOSFET is turned
on until the current comparator, I
CMP
, trips, restarting
the one-shot timer and initiating the next cycle. Inductor
valley current is measured by sensing the voltage between
the PGND and SW pins using the bottom MOSFET on-
resistance. The voltage on the I
TH
pin sets the compara-
tor threshold corresponding to inductor valley current.
The error amplifier EA adjusts this voltage by comparing
the feedback signal V
FB
from the output voltage to the
feedback reference voltage V
FBREF
. Increasing the load
current causes a drop in the feedback voltage relative
to the reference. The EA senses the feedback voltage
drop and adjusts the I
TH
voltage higher until the average
inductor current matches the load current.
With DC current loads less than 1/2 of the peak-to-peak
ripple the inductor current can drop to zero or become
negative. In discontinuous operation, negative inductor
current is detected and prevented by the current reversal
comparator I
REV
, which shuts off MB. Both switches
remain off with the output capacitor supplying the load
current until the EA moves the I
TH
voltage above the zero
current level (0.8V) to initiate another switching cycle.
When the FCB (forced continuous bar) pin is below the
internal FCB threshold reference, V
FCB
, the regulator is
forced to operate in continuous mode by disabling reversal
comparator, I
REV
, thereby allowing the inductor current to
become negative.
The continuous mode operating frequency can be deter
-
mined by
dividing the calculated duty cycle, V
OUT
/V
IN
,
by the fixed on-time. The OST generates an on-time
proportional to the ideal duty cycle, thus holding the
frequency approximately constant with changes in V
IN
.
The nominal frequency can be adjusted with an external
resistor, R
ON
.
Foldback current limiting is provided to protect against
low impedance shorts. If the controller is in current limit
and V
OUT
drops to less 50% of regulation, the current limit
set-point “folds back” to progressively lower values. To
recover from foldback current limit, the excessive load or
low impedance short needs to be removed.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both MT and MB. Releasing
the pin allows an internal 1.2µA current source to charge
up an external soft-start capacitor, C
SS
. When the RUN/
SS pin is less than 0.7V, the device is in the low power
shutdown condition with a nominal bias current of 18µA.
When RUN/SS is greater than 0.7V and less than 1.5V,
INTV
CC
and all internal circuitry are enabled while MT and
MB are forced off. Current-limited soft-start begins when
RUN/SS exceeds 1.5V. Normal operation at full current
limit is achieved at approximately 3V on RUN/SS. Foldback
current limit is defeated during soft-start.
