LTC3868 Datasheet, PDF (11/38 Pages) Linear Technology – Low IQ, Dual 2-Phase Synchronous Step-Down Controller

LTC3868
Operation (Refer to the Functional Diagram)
The LTC3868 uses a constant frequency, current mode
step-down architecture with the two controller channels
operating 180 degrees out of phase. During normal op-
eration, each external top 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 I_TH pin,
which is the output of the error amplifier, EA. The error
amplifier compares the output voltage feedback signal at
the V_FB pin (which is generated with an external resistor
divider connected across the output voltage, V_OUT , to
ground) to the internal 0.800V reference voltage. When the
load current increases, it causes a slight decrease in V_FB
relative to the reference, which causes the EA to increase
the I_TH voltage until the average inductor current matches
the new load current.
After the top MOSFET is turned off each cycle, the bottom
MOSFET is turned on until either the inductor current starts
to reverse, as indicated by the current comparator IR, 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 left open or tied to a voltage less than
4.7V, the V_IN LDO (low dropout linear regulator) supplies
5.1V from V_IN to INTV_CC. If EXTV_CC is taken above 4.7V,
the V_IN LDO is turned off and an EXTV_CC LDO is turned on.
Once enabled, the EXTV_CC LDO supplies 5.1V from EXTV_CC
to INTV_CC. Using the EXTV_CC pin allows the INTV_CC power
to be derived from a high efficiency external source such
as one of the LTC3868 switching regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap capacitor, C_B, which normally recharges during each
cycle through an external diode when the top MOSFET
turns off. If the input voltage, V_IN, decreases to a voltage
close to V_OUT , the loop may enter dropout and attempt
to turn on the top MOSFET continuously. The dropout
detector detects this and forces the top MOSFET off for
about one-twelfth of the clock period every tenth cycle to
allow C_B to recharge.
Shutdown and Start-Up (RUN1, RUN2
and SS1, SS2 Pins)
The two channels of the LTC3868 can be independently
shut down using the RUN1 and RUN2 pins. Pulling either of
these pins below 1.26V shuts down the main control loop
for that controller. Pulling both pins below 0.7V disables
both controllers and most internal circuits, including the
INTV_CC LDOs. In this state, the LTC3868 draws only 8µA
of quiescent current.
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), so long as the maximum
current into the RUN pin does not exceed 100µA.
The start-up of each controller’s output voltage, V_OUT , is
controlled by the voltage on the SS pin for that channel.
When the voltage on the SS pin is less than the 0.8V internal
reference, the LTC3868 regulates the V_FB voltage to the SS
pin voltage instead of the 0.8V 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
1µA pull-up current charges this capacitor creating a volt-
age ramp on the SS pin. As the SS voltage rises linearly
from 0V to 0.8V (and beyond up to the absolute maximum
rating of 6V), the output voltage V_OUT rises smoothly from
zero to its final value.
Short-Circuit Latchoff
After the controller has been started and been given ad-
equate time to ramp up the output voltage, the SS capaci-
tor is used in a short-circuit timeout circuit. Specifically,
once the voltage on the SS pin rises above 2V (the arming
threshold), the short-circuit timeout circuit is enabled (see
Figure 1). If the output voltage falls below 70% of its nomi-
nal regulated voltage, the SS capacitor begins discharg-
ing with a net 9µA pulldown current on the assumption
that the output is in an overcurrent and/or short-circuit
condition. If the condition lasts long enough to allow the
SS pin voltage to fall below 1.5V (the latchoff threshold),
the controller will shut down (latch off) until the RUN pin
voltage or the V_IN voltage is recycled.
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