Datasheet

LTC3858-1
11
38581fd
OPERATION
(Refer to the Functional Diagram)
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 LTC3858-1 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 LTC3858-1 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 LTC3858-1 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 on this pin. 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 LTC3858-1 regulates the V
FB
volt-
age 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 ca-
pacitor creating a voltage 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
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.
The delay time from when an short-circuit occurs until
the controller latches off can be calculated using the fol-
lowing equation:
t
LATCH
≈ C
SS
V
SS
– 1.5V
9µA
where V
SS
is the initial voltage (must be greater than 2V)
on the SS pin at the time the short-circuit occurs. Normally
the SS pin voltage will have been pulled up to the INTV
CC
voltage (5.1V) by the internal 1µA pull-up current.
Note that the two controllers on the LTC3858-1 have
separate, independent short-circuit latchoff circuits.
Latchoff can be overridden/defeated by connecting a
resistor 150k or less from the SS pin to INTV
CC
. This
resistor provides enough pull-up current to overcome the
9µA pull-down current present during a short-circuit. Note
that this resistor also shortens the soft-start period.