LTC3815_15 Datasheet, PDF (21/42 Pages) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

LTC3815
Applications Information
Minimum Off-Time and Minimum On-Time
Considerations
The minimum off-time, t_OFF(MIN), is the smallest amount
of time that the LTC3815 is capable of turning on the bot-
tom power MOSFET, tripping the current comparator and
turning the power MOSFET back off. This time is generally
about 100ns. The minimum off-time limit imposes a maxi-
mum duty cycle of t_ON/(t_ON + t_OFF(MIN)). If the maximum
duty cycle is reached, due to a dropping input voltage for
example, then the output will drop out of regulation. The
minimum input voltage to avoid dropout is:
V_IN(MIN)
=
V_OUT
•
t_ON
+ t_OFF(MIN)
t_ON
Conversely, the minimum on-time is the smallest dura-
tion of time in which the top power MOSFET can be in
its “on” state. This time is typically 75ns. In continuous
mode operation, the minimum on-time limit imposes a
minimum duty cycle of:
DC_MIN = f • t_ON(MIN)
where t_ON(MIN) is the minimum on-time. As the equation
shows, reducing the operating frequency will alleviate the
minimum duty cycle constraint.
In the cases where the minimum duty cycle is surpassed,
the output voltage will still remain in regulation, but the
switching frequency will decrease from its programmed
value. This is an acceptable result in many applications, so
this constraint may not be of critical importance in most
cases. High switching frequencies may be used in the
design without any fear of severe consequences. As the
sections on inductor and capacitor selection show, high
switching frequencies allow the use of smaller board com-
ponents, thus reducing the size of the application circuit.
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple current:
ΔI_L
=
V_OUT
f •L
⎛
• _⎜1−
⎝
V_OUT
V_IN
⎞
⎟
⎠
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a trade-off between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current that
is about 30-40% of I_OUT(MAX). This is especially important
at low V_OUT operation where V_OUT is 1.8V or below. Care
must be given to choose an inductance value that will gen-
erate a big enough current ripple so that the chip’s valley
current comparator has enough signal-to-noise ratio to
force constant switching frequency. Meanwhile, also note
that the largest ripple current occurs at the highest V_IN. To
guarantee that ripple current does not exceed a specified
maximum, the inductance should be chosen according to:
L
=
f
•
V_OUT
ΔI_{L(MAX )}
⎛
• _{⎝⎜⎜}1−
V_OUT
V_IN(MAX
)
⎞
⎠⎟⎟
Once the value for L is known, the type of inductor must
be selected. Actual core loss is independent of core size
for a fixed inductor value, but is very dependent on the
inductance selected. As the inductance or frequency in-
creases, core losses decrease. Unfortunately, increased
inductance requires more turns of wire and therefore
copper losses will increase.
Ferrite designs have very low core losses and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard”, which means that LTC3815
inductance collapses abruptly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
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3815p
21