LTC3858
11
3858fc
OPERATION
(Refer to the Functional Diagram)
The LTC3858 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 amplifi er, EA. The error
amplifi er 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 the 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 effi ciency external source such
as one of the LTC3858 switching regulator outputs.
Each top MOSFET driver is biased from the fl oating boot-
strap capacitor, C
B
, which normally recharges during each
switching 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 LTC3858 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 LTC3858 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 LTC3858 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 fi nal value.
Short-Circuit Latch-Off
After the controller has been started and been given
adequate time to ramp up the output voltage, the SS
capacitor is used in a short-circuit time-out circuit. Spe-
cifi cally, 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 nominal regulated voltage, the SS capacitor
begins discharging with a net 9µA pull-down current on
the assumption that the output is in an overcurrent and/or
short-circuit condition. If the condition lasts long enough
