LTC3416 Linear Technology, LTC3416 Datasheet - Page 7

• Manufacturer Part Number: LTC3416
• Description: 4A/ 4MHz/ Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Technology

OPERATIO
power MOSFET is turned off and the bottom power MOSFET
is switched on until either the overvoltage condition clears
or the bottom MOSFET’s current limit is reached.
Voltage Tracking
Some microprocessors, ASIC and DSP chips need two
power supplies with different voltage levels. These sys-
tems 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 supporting system devices
such as memory, FPGAs or data converters. To ensure
that the I/O loads are not driven until the core voltage is
properly biased, tracking of the core supply voltage and
the I/O supply voltage is necessary.
Voltage tracking is enabled by applying a voltage to the
TRACK pin. When the voltage on the TRACK pin is below
0.8V, the feedback voltage will regulate to this tracking
voltage. When the tracking voltage exceeds 0.8V, tracking
is disabled and the feedback voltage will regulate to the
internal reference voltage.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases toward the maxi-
mum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one
cycle, eventually reaching 100% duty cycle. The output
voltage will then be determined by the input voltage minus
the voltage drop across the internal P-channel MOSFET
and the inductor.
U
Low Supply Operation
The LTC3416 is designed to operate down to an input
supply voltage of 2.25V. One important consideration at
low input supply voltages is that the R
P-channel and N-channel power switches increases. The
user should calculate the power dissipation when the
LTC3416 is used at 100% duty cycle with low input
voltages to ensure that thermal limits are not exceeded.
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 40%. Normally,
the maximum inductor peak current is reduced when
slope compensation is added. In the LTC3416, however,
slope compensation recovery is implemented to keep the
maximum inductor peak current constant throughout the
range of duty cycles. This keeps the maximum output
current relatively constant regardless of duty cycle.
Short-Circuit Protection
When the output is shorted to ground, the inductor current
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 exceeds 7.8A, the top power
MOSFET will be held off and switching cycles will be
skipped until the inductor current is reduced.
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LTC3416
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