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LTC3891 Datasheet(PDF) 9 Page - Linear Technology

Part # LTC3891
Description  60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC3891 Datasheet(HTML) 9 Page - Linear Technology

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LTC3890-2
9
38902f
PIN FUNCTIONS
SENSE1, SENSE2(Pin 1, Pin 9): The (–) Input to the
Differential Current Comparators. When greater than
INTVCC – 0.5V, the SENSEpin supplies current to the
current comparator.
FREQ (Pin 2): The frequency control pin for the internal
VCO. Connecting the pin to GND forces the VCO to a fixed
low frequency of 350kHz. Connecting the pin to INTVCC
forces the VCO to a fixed high frequency of 535kHz.
Other frequencies between 50kHz and 900kHz can be
programmed using a resistor between FREQ and GND.
An internal 20μA pull-up current develops the voltage to
be used by the VCO to control the frequency.
PHASMD (Pin 3): Control Input to Phase Selector which
determines the phase relationships between control-
ler 1, controller 2 and the CLKOUT signal. Pulling this
pin to ground forces TG2 and CLKOUT to be out of phase
180° and 60° with respect to TG1. Connecting this pin to
INTVCC forces TG2 and CLKOUT to be out of phase 240°
and 120° with respect to TG1. Floating this pin forces TG2
and CLKOUT to be out of phase 180° and 90° with respect
to TG1. Refer to Table 1.
CLKOUT (Pin 4): Output clock signal available to daisy-
chain other controller ICs for additional MOSFET driver
stages/phases. The output levels swing from INTVCC to
ground.
PLLIN/MODE (Pin 5): 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. When not syn-
chronizing to an external clock, this input, which acts on
both controllers, determines how the LTC3890-2 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 floated. Tying this pin
to INTVCC forces continuous inductor current operation.
Tying this pin to a voltage greater than 1.2V and less than
INTVCC – 1.3V selects pulse-skipping operation.
SGND (Pins 6, Exposed Pad Pin 33): Small-signal ground
common to both controllers, must be routed separately
from high current grounds to the common (–) terminals
of the CIN capacitors. The exposed pad must be soldered
to PCB ground for rated thermal performance.
RUN1, RUN2 (Pin 7, Pin 8): Digital Run Control Inputs
for Each Controller. Forcing RUN1 below 1.16V or RUN2
below 1.20V shuts down that controller. Forcing both of
these pins below 0.7V shuts down the entire LTC3890-2,
reducing quiescent current to approximately 14μA.
INTVCC (Pin 19): Output of the Internal Linear Low Dropout
Regulator. The driver and control circuits are powered from
this voltage source. Must be decoupled to power ground
with a minimum of 4.7μF ceramic or other low ESR ca-
pacitor. Do not use the INTVCC pin for any other purpose.
EXTVCC (Pin 20): External Power Input to an Internal LDO
Connected to INTVCC. This LDO supplies INTVCC power,
bypassing the internal LDO powered from VIN whenever
EXTVCC is higher than 4.7V. See EXTVCC Connection in the
Applications Information section. Do not float or exceed
14V on this pin.
PGND (Pin 21): Driver Power Ground. Connects to the
sources of bottom (synchronous) N-channel MOSFETs
and the (–) terminal(s) of CIN.
VIN (Pin 22): Main Supply Pin. A bypass capacitor should
be tied between this pin and the signal ground pin.
BG1, BG2 (Pin 23, Pin 18): High Current Gate Drives
for Bottom (Synchronous) N-Channel MOSFETs. Voltage
swing at these pins is from ground to INTVCC.
BOOST1, BOOST2 (Pin 24, Pin 17): Bootstrapped Supplies
to the Topside Floating Drivers. Capacitors are connected
between the BOOST and SW pins and Schottky diodes are
tied between the BOOST and INTVCC pins. Voltage swing
at the BOOST pins is from INTVCC to (VIN + INTVCC).
SW1, SW2 (Pin 25, Pin 16): Switch Node Connections
to Inductors.


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