LTC3717 Datasheet, PDF (10/20 Pages) Linear Technology – Wide Operating Range, No RSENSE Step-Down Controller for DDR/QDR Memory Termination

LTC3717
APPLICATIO S I FOR ATIO
on-time t_ON of the top MOSFET switch. The on-time is set
by the current into the I_ON pin according to:
t_ON
=
(0.7V) _{(10pF )}
I_ION
Tying a resistor R_ON from V_IN to the I_ON pin yields an on-
time inversely proportional to V_IN. For a step-down con-
verter, this results in approximately constant frequency
operation as the input supply varies:
f
=
V_OUT
(0.7V) R_ON(10pF)
[H_Z ]
Because the voltage at the I_ON pin is about 0.7V, the
current into this pin is not exactly inversely proportional to
V_IN, especially in applications with lower input voltages.
A more exact equation taking in account the 0.7V drop on
the I_ON pin is:
f
=
V_OUT (V_IN – 0.7V)
(0.7V) R_ON(10pF)V_IN
[H_Z ]
To correct for this error, an additional resistor R_ON2
connected from the I_ON pin to the 5V INTV_CC supply will
further stabilize the frequency.
R_ON2
=
5V
0.7V
R_ON
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
∆I_L
=


V_OUT
fL


_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 tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
10
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
∆I_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 af-
ford 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 manu-
facturers such as Sumida, Panasonic, Coiltronics, Coilcraft
and Toko.
Schottky Diode D1 Selection
The Schottky diode D1 shown in Figure 1 conducts during
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
Kool Mµ is a registered trademark of Magnetics, Inc.
sn3717 3717fs