Datasheet

LTC3859
12
3859fa
PIN FUNCTIONS
TRACK/SS1, TRACK/SS2, SS3 (Pins 34, 16, 3/Pins 38,
20, 7): External Tracking and Soft-Start Input. For the buck
channels, the LTC3859 regulates the V
FB1,2
voltage to the
smaller of 0.8V, or the voltage on the TRACK/SS1,2 pin. For
the boost channel, the LTC3859 regulates the V
FB3
voltage
to the smaller of 1.2V, or the voltage on the SS3 pin. An
internal 1µA pull-up current source is connected to this pin.
A capacitor to ground at this pin sets the ramp time to fi nal
regulated output voltage. Alternatively, a resistor divider on
another voltage supply connected to the TRACK/SS pins
of the buck channels allow the LTC3859 buck outputs to
track the other supply during start-up.
I
TH1
, I
TH2
, I
TH3
(Pins 35, 15, 7/Pins 1, 19, 11): Error
Amplifi er Outputs and Switching Regulator Compensation
Points. Each associated channel’s current comparator trip
point increases.
V
FB1
, V
FB2
, V
FB3
(Pins 36, 14, 6/Pins 2, 18, 10): Receives
the remotely sensed feedback voltage for each controller
from an external resistive divider across the output.
SENSE1
+
, SENSE2
+
, SENSE3
+
(Pins 37, 13, 4/Pins 3, 17, 8):
The (+) Input to the Differential Current Comparators.
The I
TH
pin voltage and controlled offsets between the
SENSE
–
and SENSE
+
pins in conjunction with R
SENSE
set the
current trip threshold. For the boost channel, the SENSE3
+
pin supplies current to the current comparator.
SENSE1
–
, SENSE2
–
, SENSE3
–
(Pins 38, 12, 5/Pins 4,
16, 9): The (–) Input to the Differential Current Compara-
tors. When SENSE1,2
–
for the buck channels is greater
than INTV
CC
, then SENSE1,2
–
pin supplies current to the
current comparator.
PGND (Exposed Pad Pin 39): Driver Power Ground. Con-
nects to the sources of bottom (synchronous) N-channel
MOSFETs and the (–) terminal(s) of C
IN
. The exposed
pad must be soldered to the PCB for rated electrical and
thermal performance.
(QFN/TSSOP)