LTC3600_15 Datasheet, PDF (10/28 Pages) Linear Technology – 15V, 1.5A Synchronous Rail-to-Rail Single Resistor Step-Down Regulator

LTC3600
Operation
Main Control Loop
The LTC3600 is a current mode monolithic step down
regulator. The accurate 50µA current source on the ISET pin
allows the user to use just one external resistor to program
the output voltage in a unity gain buffer fashion. In normal
operation, the internal top power MOSFET is turned on for
a fixed interval determined by a fixed 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 SW and PGND nodes of the bottom power
MOSFET. The voltage on the ITH pin sets the comparator
threshold corresponding to inductor valley current. The
error amplifier, EA, adjusts this ITH voltage by comparing
the V_OUT voltage with the voltage on ISET. If the load current
increases, it causes a drop in the V_OUT voltage relative to
V_ISET. 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 rever-
sal 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 (0.8V) to initiate another
cycle. Discontinuous mode operation is disabled by tying
the MODE pin to INTV_CC, which forces continuous syn-
chronous 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 oscilla-
tor as well as the current for the internal one-shot timer. An
internal phase-locked loop servos the switching regulator
on-time to track the internal oscillator to force constant
switching frequency. If an external synchronization clock is
present on the MODE/SYNC pin, the regulator on-time and
switching frequency would then track the external clock.
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output power good
feedback voltage V_PGFB exits a 7.5% window around the
regulation point. Continuous operation is forced during
an OV condition. To defeat the PGOOD function, simply
tie PGFB to INTV_CC.
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Pulling the RUN pin to ground forces the LTC3600 into
its shutdown state, turning off both power MOSFETs and
all of its internal control circuitry. Bringing the RUN pin
above 0.7V turns on the internal reference only, while still
keeping the power MOSFETs off. Further increasing the
RUN voltage above 1.5V turns on the entire chip.
INTV_CC Regulator
An internal low drop out (LDO) regulator produces the
5V supply that powers the drivers and the internal bias
circuitry. The INTV_CC can supply up to 50mA RMS and
must be bypassed to ground with a minimum of 1µF ce-
ramic 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 LTC3600 constantly
monitors the V_IN pin for an overvoltage condition. When
V_IN rises above 16V, the regulator suspends operation by
shutting off both power MOSFETs and discharges the ISET
pin voltage to ground. Once V_IN drops below 15V, the regu-
lator immediately resumes normal switching operation by
first charging up the ISET pin to its programmed voltage.
Programming Switching Frequency
Connecting a resistor from the RT pin to GND programs
the switching frequency from 200kHz to 4MHz according
to the following formula:
Frequency (Hz) = ^{3.6 • 1010 (1/ F)}
R_T (Ω)
For ease of use, the RT pin can be connected directly to
the INTV_CC pin for 1MHz operation. Do not float the RT pin.
The internal on-time phase-locked loop has a synchroni-
zation range of 30% around its programmed frequency.
Therefore, during external clock synchronization, the proper
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