LTC3605A_15 Datasheet, PDF (6/22 Pages) Linear Technology – 20V, 5A Synchronous Step-Down Regulator

LTC3605A
Pin Functions
RT (Pin 1): Oscillator Frequency Programming Pin. Con-
nect an external resistor (between 200k to 40k) from RT
to SGND to program the frequency from 800kHz to 4MHz.
Since the synchronization range is ±30% of set frequency,
be sure that the set frequency is within this percentage
range of the external clock to ensure frequency lock.
V_ON (Pin 9): On-Time Voltage Input. Voltage trip point for
the on-time comparator. Tying this pin to the output volt-
age makes the on-time proportional to V_OUT and keeps the
switching frequency constant at different V_OUT. However,
when V_ON is <0.6V or >6V, then switching frequency will
no longer remain constant.
PHMODE (Pin 2): Control Input to Phase Selector. Deter-
mines the phase relationship between internal oscillator
and CLKOUT. Tie it to INTV_CC for 2-phase operation, tie it
to SGND for 3-phase operation, and tie it to INTV_CC/2 for
4-phase operation.
MODE (Pin 3): Operation Mode Select. Tie this pin to
INTV_CC to force continuous synchronous operation at
all output loads. Tying it to SGND enables discontinuous
mode operation at light loads. Tying this pin to INTV_CC/2
shuts off the internal clock during discontinuous intervals.
FB (Pin 4): Output Feedback Voltage. Input to the error
amplifier that compares the feedback voltage to the internal
0.6V reference voltage. This pin is normally connected to
a resistive divider from the output voltage.
TRACK/SS (Pin 5): Output Tracking and Soft-Start Pin.
Allows the user to control the rise time of the output volt-
age. Putting a voltage below 0.6V on this pin bypasses
the internal reference input to the error amplifier, instead
it servos the FB pin to the TRACK voltage. Above 0.6V,
the tracking function stops and the internal reference
resumes control of the error amplifier. There’s an internal
2.5µA pull-up current from INTV_CC on this pin, so putting
a capacitor here provides soft-start function.
ITH (Pin 6): Error Amplifier Output and Switching Regu-
lator Compensation Point. The current comparator’s trip
threshold is linearly proportional to this voltage, whose
normal range is from 0.3V to 1.8V. Tying this pin to IN-
TV_CC activates internal compensation and output voltage
positioning, raising V_OUT to 1.5% higher than the nominal
value at I_OUT = 0 and 1.5% lower at I_OUT = 5A.
RUN (Pin 7): Run Control Input. Enables chip operation
by tying RUN above 1.2V. Tying it below 1.1V shuts down
the part.
PGOOD (Pin 8): Output Power Good with Open-Drain
Logic. PGOOD is pulled to ground when the voltage on the
FB pin is not within ±10% of the internal 0.6V reference.
PGND (Pin 10, Exposed Pad Pin 25): Power Ground.
Return path of internal power MOSFETs. Connect this
pin to the negative terminals of the input capacitor and
output capacitor. The exposed pad must be soldered to
the PCB ground for electrical contact and rated thermal
performance.
SW (Pins 11 to 16): Switch Node Connection to External
Inductor. Voltage swing of SW is from a diode voltage
drop below ground to PV_IN.
PV_IN (Pins 17, 18): Power V_IN. Input voltage to the on-
chip power MOSFETs.
SV_IN (Pin 19): Signal V_IN. Filtered input voltage to the
on-chip 3.3V regulator. Connect a (1Ω to 10Ω) resistor
between SV_IN and PV_IN and bypass to GND with a 0.1µF
capacitor.
BOOST (Pin 20): Boosted Floating Driver Supply for Inter-
nal Top Power MOSFET. The (+) terminal of the bootstrap
capacitor connects here. This pin swings from a diode
voltage drop below INTV_CC up to PV_IN + INTV_CC.
INTV_CC (Pin 21): Internal 3.3V Regulator Output. The
internal power drivers and control circuits are powered
from this voltage. Decouple this pin to power ground with
a minimum of 1µF low ESR ceramic capacitor.
SGND (Pin 22): Signal Ground Connection.
CLKOUT (Pin 23): Output Clock Signal for PolyPhase
Operation. The phase of CLKOUT with respect to CLKIN
is determined by the state of the PHMODE pin. CLKOUT’s
peak-to-peak amplitude is INTV_CC to GND.
CLKIN (Pin 24): External Synchronization Input to Phase
Detector. This pin is internally terminated to SGND with 20k.
The phase-locked loop will force the top power NMOS’s
turn on signal to be synchronized with the rising edge of
the CLKIN signal.
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