LTC3413IFE Datasheet, PDF (7/16 Pages) Linear Technology – Monolithic Synchronous Regulator

LTC3413
OPERATION
main switch to remain on for more than one cycle until it
reaches 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.
Low Supply Operation
The LTC3413 is designed to operate down to an SVIN input
supply voltage of 2.25V. One important consideration at low
input supply voltages is that the R_DS(ON) of the P-channel
and N-channel power switches increases. The user should
calculate the power dissipation when the LTC3413 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 frequency
architectures by preventing subharmonic oscillations 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 LTC3413, however, slope
compensation recovery is implemented to keep the
maximum inductor peak current constant throughout 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 greater than 5A, the top
power MOSFET will be held off and switching cycles will
be skipped until the inductor current is reduced.
Pre-Biased Load
It is important to sequence the start-up of the LTC3413
prior to any external circuitry that might drive the V_OUT pin.
If the V_OUT pin is externally driven to a voltage more than
10% (the OV threshold) above the desired V_OUT voltage,
the LTC3413 may enter a latched state where it no longer
switches. To avoid this scenario, the user should ensure
there is not a pre-biased load during start-up. This can
be accomplished by sequencing the LTC3413’s RUN pin
before the load’s supply.
APPLICATIONS INFORMATION
The basic LTC3413 application circuit is shown in Figure 1a.
External component selection is determined by the
maximum load current and begins with the selection of
the inductor value and operating frequency followed by
C_IN and C_OUT.
Operating Frequency
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequencies improves efficiency by
reducing internal gate charge losses but requires larger
inductance values and/or capacitance to maintain low
output ripple voltage.
The operating frequency of the LTC3413 is determined by
an external resistor that is connected between pin R_T and
ground. The value of the resistor sets the ramp current
that is used to charge and discharge an internal timing
capacitor within the oscillator and can be calculated by
using the following equation.
R_OSC
=
3.23
•
f
10¹¹ _(Ω)
–
10kΩ _
Although frequencies as high as 2MHz are possible, the
minimum on-time of the LTC3413 imposes a minimum
limit on the operating duty cycle. The minimum on-time
is typically 110ns. Therefore, the minimum duty cycle is
equal to 100 • 110ns • f (Hz).
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