Datasheet

LTC3859
11
3859fa
PIN FUNCTIONS
FREQ (Pin 1/Pin 5): The Frequency Control Pin for the
Internal VCO. Connecting the pin to GND forces the VCO
to a fi xed low frequency of 350kHz. Connecting the pin
to INTV
CC
forces the VCO to a fi xed high frequency of
535kHz. Other frequencies between 50kHz and 900kHz
can be programmed using a resistor between FREQ and
GND. The resistor and an internal 20µA source current
create a voltage used by the internal oscillator to set the
frequency.
PLLIN/MODE (Pin 2/Pin 6): External Synchronization
Input to Phase Detector and Forced Continuous Mode
Input. When an external clock is applied to this pin, the
phase-locked loop will force the rising TG1 signal to be
synchronized with the rising edge of the external clock,
and the regulators operate in forced continuous mode.
When not synchronizing to an external clock, this input,
which acts on all three controllers, determines how the
LTC3859 operates at light loads. Pulling this pin to ground
selects Burst Mode operation. An internal 100k resistor to
ground also invokes Burst Mode operation when the pin is
fl oated. Tying this pin to INTV
CC
forces continuous inductor
current operation. Tying this pin to a voltage greater than
1.2V and less than INTV
CC
– 1.3V selects pulse-skipping
operation. This can be done by connecting a 100k resistor
from this pin to INTV
CC
.
SGND (Pin 8/Pin 12): Small Signal Ground common to
both controllers, must be routed separately from high
current grounds to the common (–) terminals of the C
IN
capacitors.
RUN1, RUN2, RUN3 (Pins 9, 10, 11/Pins 13, 14, 15):
Digital Run Control Inputs for Each Controller. Forcing
RUN1 below 1.7V and RUN2/RUN3 below 1.20V shuts
down that controller. Forcing all of these pins below 0.7V
shuts down the entire LTC3859, reducing quiescent current
to approximately 14µA.
OV3 (Pin 17/Pin 21): Overvoltage Open-Drain Logic
Output for the Boost Regulator. OV3 is pulled to ground
when the voltage on the V
FB3
pin is under 110% of its set
point, and becomes high impedance when V
FB3
goes over
110% of its set point.
INTV
CC
(Pin 22/Pin 26): Output of the Internal Linear Low
Dropout Regulator. The driver and control circuits are pow-
ered from this voltage source. Must be decoupled to PGND
with a minimum of 4.7µF ceramic or tantalum capacitor.
The INTV
CC
pin should also be connected to the DRV
CC
pin, and should not be used for any other purpose.
EXTV
CC
(Pin 23/Pin 27): External Power Input to an
Internal LDO Connected to INTV
CC
. This LDO supplies
INTV
CC
power, bypassing the internal LDO powered from
V
IN
whenever EXTV
CC
is higher than 4.7V. See EXTV
CC
Connection in the Applications Information section. Do
not exceed 14V on this pin.
V
BIAS
(Pin 24/Pin 28): Main Bias Input Supply Pin. A
bypass capacitor should be tied between this pin and the
SGND pin.
BG1, BG2, BG3 (Pins 29, 21, 25/Pins 33, 25, 29): High
Current Gate Drives for Bottom (Synchronous) N-Channel
MOSFETs. Voltage swing at these pins is from ground to
INTV
CC
.
BOOST1, BOOST2, BOOST3 (Pins 30, 20, 26/Pins 34,
24, 30): Bootstrapped Supplies to the Top Side Floating
Drivers. Capacitors are connected between the BOOST and
SW pins and Schottky diodes are tied between the BOOST
and INTV
CC
pins. Voltage swing at the BOOST pins is from
INTV
CC
to (V
IN
+ INTV
CC
).
SW1, SW2, SW3 (Pins 31, 19, 28/Pins 35, 23, 32):
Switch Node Connections to Inductors.
TG1, TG2, TG3 (Pins 32, 18, 27/Pins 36, 22, 31): High
Current Gate Drives for Top N-Channel MOSFETs. These
are the outputs of fl oating drivers with a voltage swing
equal to INTV
CC
superimposed on the switch node volt-
age SW.
PGOOD1 (Pin 33/Pin 37): Open-Drain Logic Output.
PGOOD1 is pulled to ground when the voltage on the V
FB1
pin is not within ±10% of its set point.
(QFN/TSSOP)