LTC3603 Datasheet, PDF (9/22 Pages) Linear Technology – 2.5A, 15V Monolithic Synchronous Step-Down Regulator

LTC3603
OPERATION
Dropout Operation
When the input supply voltage decreases toward the output
voltage, the duty cycle increases toward the maximum
on-time. Further reduction of the supply voltage forces the
top switch to remain on for more than one cycle until it
attempts to stay on continuously. In order to replenish the
voltage on the floating BOOST supply capacitor, however,
the top switch is forced off and the bottom switch is forced
on for approximately 85ns every sixteen clock cycles. This
achieves an effective duty cycle that can exceed 99%. The
output voltage will then be primarily determined by the
input voltage minus the voltage drop across the upper
internal N-channel MOSFET and the inductor.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant-fre-
quency architectures by preventing subharmonic oscilla-
tions at duty cycles greater than 50%. It is accomplished
internally by adding a compensating ramp to the inductor
current signal at duty cycles in excess of 30%. Normally,
the maximum inductor peak current is reduced when
slope compensation is added. In the LTC3603, however,
slope compensation recovery is implemented to reduce
the variation of the maximum inductor peak current (and
therefore the maximum available output current) over the
range of duty cycles.
Short-Circuit Protection
When the output is shorted to ground, the inductor cur-
rent decays very slowly during a single switching cycle.
To prevent current runaway from occurring, a secondary
current limit is imposed on the inductor current. If the
inductor valley current increases to more than 4.5A, the
top power MOSFET will be held off and switching cycles
will be skipped until the inductor current is reduced.
Overtemperature and PV_IN Overvoltage Protection
When using the LTC3603 in an application circuit, care must
be taken not to exceed any of the ratings specified in the Ab-
solute Maximum Ratings section. As an added safeguard,
however, the LTC3603 does incorporate an overtemperature
shutdown feature. If the junction temperature reaches ap-
proximately 150°C, both power switches will be turned off
and the SW node will become high impedance. After the
part has cooled to below 115°C, it will restart. Similarly,
the LTC3603 contains an overvoltage shutdown feature
that monitors the voltage on the PV_IN pin. If this voltage
exceeds approximately 16.5V, both power switches will be
turned off until PV_IN voltage is reduced below 16V.
Voltage Tracking and Soft-Start
Some microprocessors and DSP chips need two power
supplies with different voltage levels. These systems often
require voltage sequencing between the core power supply
and the I/O power supply. Without proper sequencing,
latch-up failure or excessive current draw may occur that
could result in damage to the processor’s I/O ports or the
I/O ports of a supporting system device such as memory,
an FPGA or a data converter. To ensure that the I/O loads
are not driven until the core voltage is properly biased,
tracking of the core supply and the I/O supply voltage is
necessary.
Voltage tracking is enabled by applying a ramp voltage to
the TRACK/SS pin. When the voltage on the TRACK pin
is below 0.6V, the feedback voltage will regulate to this
tracking voltage. When the tracking voltage exceeds 0.6V,
tracking is disabled and the feedback voltage will regulate
to the internal reference voltage.
The TRACK/SS pin is also used to implement an external
soft-start function. A 1.2μA current is sourced from this
pin so that an external capacitor may be added to create
a smooth ramp. If this ramp is slower than the internal
1ms soft-start, then the output voltage will track this ramp
during start-up instead. Leave this pin floating to use the
internal 1ms soft-start ramp. Do not tie the TRACK/SS
pin to INTV_CC or to PV_IN.
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