LTC3858 Linear Technology, LTC3858 Datasheet - Page 11

• Manufacturer Part Number: LTC3858
• Description: Synchronous Step-Down Controller
• Manufacturer: Linear Technology

operaTion
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
which is the output of the error amplifier, EA. The error
amplifier compares the output voltage feedback signal at
the V
divider connected across the output voltage, V
ground) to the internal 0.800V reference voltage. When the
load current increases, it causes a slight decrease in V
relative to the reference, which causes the EA to increase
the I
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
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
the EXTV
4.7V, the V
5.1V from V
the V
Once enabled, the EXTV
to INTV
to be derived from a high efficiency external source such
as one of the LTC3858 switching regulator outputs.
Each top MOSFET driver is biased from the floating boot-
strap capacitor, C
switching cycle through an external diode when the top
MOSFET turns off. If the input voltage, V
a voltage close to V
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
TH
CC
IN
FB
voltage until the average inductor current matches
LDO is turned off and the EXTV
/EXTV
pin (which is generated with an external resistor
CC
CC
. Using the EXTV
IN
pin is left open or tied to a voltage less than
LDO (low dropout linear regulator) supplies
IN
CC
to INTV
B
Power
B
to recharge.
, which normally recharges during each
OUT
(Refer to the Functional Diagram)
CC
CC
, the loop may enter dropout and
. If EXTV
LDO supplies 5.1V from EXTV
CC
pin allows the INTV
CC
is taken above 4.7V,
CC
LDO is turned on.
IN
, decreases to
CC
pin. When
CC
OUT
power
TH
pin,
, to
CC
FB
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
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
current into the RUN pin does not exceed 100µA.
The start-up of each controller’s output voltage, V
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
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
zero to its final 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-
cifically, 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
CC
LDOs. In this state, the LTC3858 draws only 8µA
IN
), so long as the maximum
OUT
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rises smoothly from
FB
LTC3858
voltage to the SS

OUT
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