LTC3770_15 Datasheet, PDF (14/24 Pages) Linear Technology – Fast No RSENSE Step-Down Synchronous Controller with Margining, Tracking and PLL

LTC3770
APPLICATIONS INFORMATION
A reasonable starting point is to choose a ripple current
that is about 40% of I_OUT(MAX). 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. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mμ^® cores. A variety of inductors designed for
high current, low voltage applications are available from
manufacturers such as Sumida, Panasonic, Coiltronics,
Coilcraft and Toko.
Schottky Diode D1 Selection
The Schottky diode D1 shown in Figure 12 conducts dur-
ing the dead time between the conduction of the power
MOSFET switches. It is intended to prevent the body diode
of the bottom MOSFET from turning on and storing charge
during the dead time, which can cause a modest (about
1%) efficiency loss. The diode can be rated for about one
half to one fifth of the full load current since it is on for
only a fraction of the duty cycle. In order for the diode
to be effective, the inductance between it and the bottom
MOSFET must be as small as possible, mandating that
these components be placed adjacently. The diode can
be omitted if the efficiency loss is tolerable.
C_IN and C_OUT Selection
The input capacitance C_IN is required to filter the square
wave current at the drain of the top MOSFET. Use a low ESR
capacitor sized to handle the maximum RMS current.
I_RMS
≅
I_OUT(MAX)
V_OUT
V_IN
V_{IN – 1}
V_OUT
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT(MAX)/2. This simple worst-case condition
is commonly used for design because even significant
deviations do not offer much relief. Note that ripple current
ratings from capacitor manufacturers are often based on
only 2000 hours of life which makes it advisable to derate
the capacitor.
The selection of C_OUT is primarily determined by the ESR
required to minimize voltage ripple and load step transients.
The output ripple ΔV_OUT is approximately bounded by:
ΔV_OUT
≤
ΔI_L
⎛
⎜ESR
⎝
+
1
8fC_OUT
⎞
⎟
⎠
Since ΔI_L increases with input voltage, the output ripple
is highest at maximum input voltage. Typically, once the
ESR requirement is satisfied, the capacitance is adequate
for filtering and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic
and ceramic capacitors are all available in surface mount
packages. Special polymer capacitors offer very low ESR
but have lower capacitance density than other types.
Tantalum capacitors have the highest capacitance density
but it is important to only use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have significantly higher ESR, but
can be used in cost-sensitive applications providing that
consideration is given to ripple current ratings and long
term reliability. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient
and audible piezoelectric effects. The high Q of ceramic
capacitors with trace inductance can also lead to significant
ringing. When used as input capacitors, care must be taken
to ensure that ringing from inrush currents and switching
does not pose an overvoltage hazard to the power switches
and controller. To dampen input voltage transients, add
a small 5μF to 50μF aluminum electrolytic capacitor with
an ESR in the range of 0.5Ω to 2Ω. High performance
through-hole capacitors may also be used, but an additional
ceramic capacitor in parallel is recommended to reduce
the effect of their lead inductance.
3770fc
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