LTC3605A_15 Datasheet, PDF (8/22 Pages) Linear Technology – 20V, 5A Synchronous Step-Down Regulator

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 de-
termined by sensing the voltage drop across the bottom
power MOSFET ’s VDS. The voltage on the ITH pin sets
the comparator 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
voltage 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.
RUN Threshold
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 1.2V 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.
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