LTC3605A
OPERATION
Main Control Loop
The LTC3605A is a current mode monolithic step-down
regulator. In normal operation, the internal top power
MOSFET is turned on for a fixed interval determined by a
one-shot timer, OST. When the top power MOSFET turns
off, the bottom power MOSFET turns on until the current
comparator, I
CMP
, trips, restarting the one-shot timer and
initiating the next cycle. Inductor current is determined
by sensing the voltage drop across the bottom power
MOSFET ’s VDS. The voltage on the ITH pin sets the com-
parator threshold corresponding to the inductor valley
current. The error amplifier, EA, adjusts this ITH voltage
by comparing the feedback signal, V
FB
, from the output
voltage with that of an internal 0.6V reference. If the load
current increases, it causes a drop in the feedback volt-
age relative to the internal reference. The ITH voltage then
rises until the average inductor current matches that of
the load current.
At low load current, the inductor current can drop to zero
and become negative. This is detected by current reversal
comparator, I
REV
, which then shuts off the bottom power
MOSFET, resulting in discontinuous operation. Both power
MOSFETs will remain off with the output capacitor supplying
the load current until the ITH voltage rises above the zero
current level (0.6V) to initiate another cycle. Discontinu-
ous mode operation is disabled by tying the MODE pin to
INTV
CC
, which forces continuous synchronous operation
regardless of output load.
The operating frequency is determined by the value of the
R
T
resistor, which programs the current for the internal
oscillator. An internal phase-lock loop servos the oscillator
frequency to an external clock signal if one is present on
the CLKIN pin. Another internal phase-lock loop servos the
switching regulator on-time to track the internal oscillator
to force constant switching frequency.
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback volt-
age, V
FB
, exits a ±10% window around the regulation
point. Continuous operation is forced during OV and UV
condition except during start-up when the TRACK pin is
ramping up to 0.6V.
Foldback current limiting is provided if the output is
shorted to ground. As V
FB
drops to zero, the maximum
sense voltage allowed across the bottom power MOSFET
is lowered to approximately 40% of the original value to
reduce the inductor valley current.
Pulling the RUN pin to ground forces the LTC3605A into
its shutdown state, turning off both power MOSFETs and
most of its internal control circuitry. Bringing the RUN pin
above 0.6V turns on the internal reference only, while still
keeping the power MOSFETs off. Further increasing the
RUN voltage above 1.25V turns on the entire chip.
INTV
CC
Regulator
An internal low dropout (LDO) regulator produces the
3.3V supply that powers the drivers and the internal bias
circuitry. The INTV
CC
can supply up to 100mA RMS and
must be bypassed to ground with a minimum of 1µF
ceramic capacitor. Good bypassing is necessary to supply
the high transient currents required by the power MOSFET
gate drivers. Applications with high input voltage and high
switching frequency will increase die temperature because
of the higher power dissipation across the LDO. Connect-
ing a load to the INTV
CC
pin is not recommended since it
will further push the LDO into its RMS current rating while
increasing power dissipation and die temperature.
V
IN
Overvoltage Protection
In order to protect the internal power MOSFET devices
against transient voltage spikes, the LTC3605A constantly
monitors the V
IN
pin for an overvoltage condition. When
V
IN
rises above 23.5V, the regulator suspends operation
by shutting off both power MOSFETs. Once V
IN
drops
below 21.5V, the regulator immediately resumes normal
operation. The regulator does not execute its soft-start
function when exiting an overvoltage condition.
3605af
8
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