LTC3861-1 Datasheet, PDF (10/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Pin Functions
PGOOD1 (Pin 27), PGOOD2 (Pin 14): Power Good Indi-
cator Output for Each Channel. Open-drain logic out that
is pulled to SGND when either channel output exceeds a
±10% regulation window, after the internal 30µs power
bad mask timer expires.
I_AVG (Pin 28): Average Current Output Pin. A capacitor
tied to ground from this pin stores a voltage proportional
to the instantaneous average current of the master when
multiple outputs are paralleled together in a master-slave
configuration. Only the master phase contributes infor-
mation to this average through an internal resistor when
in current sharing mode. The I_AVG pin ignores channels
configured for independent operation, hence the pin
should be connected to SGND when the controller drives
independent outputs.
SGND (Pin 29, Exposed Pad Pin 33): Signal Ground.
Pins 29 and 33 are electrically connected internally. The
exposed pad must be soldered to the PCB ground for
rated thermal performance. All soft-start, small-signal
and compensation components should return to SGND.
CONFIG (Pin 30): Line Feedforward Configuration Pin.
This pin allows the user to configure the multiplier to
achieve accurate modulator gain over varying V_IN and
switching frequencies. This pin can be connected to V_CC
or SGND. An internal resistor will pull this pin to SGND
when it is floated.
VINSNS (Pin 31): V_IN Sense Pin. Connects to the V_IN
power supply to provide line feedforward compensation.
A change in V_IN immediately modulates the input to the
PWM comparator and changes the pulse width in an in-
versely proportional manner, thus bypassing the feedback
loop and providing excellent transient line regulation. An
external lowpass filter can be added to this pin to prevent
noisy signals from affecting the loop gain.
TRACK/SS1 (Pin 32), TRACK/SS2 (Pin 9): Combined
Soft-Start and Tracking Inputs. For soft-start operation,
connecting a capacitor from this pin to ground will control
the voltage ramp at the output of the power supply. An
internal 2.5μA current source will charge the capacitor and
thereby control an extra input on the reference side of the
error amplifier. For tracking operation, this input allows the
start-up of a secondary output to track a primary output
according to a ratio established by a resistor divider from
the primary output to the secondary error amplifier track
pin. For coincident tracking of both outputs at start-up,
a resistor divider with values equal to those connected
to the secondary VSNSP pin from the secondary output
should be used to connect the secondary track input
from the primary output. This pin is internally clamped
to 1.2V, and is used to communicate over current events
in a master-slave configuration.
38611f
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