LTC3714_15 Datasheet, PDF (14/28 Pages) Linear Technology – Intel Compatible, Wide Operating Range, Step-Down Controller with Internal Op Amp

LTC3714
Applications Information
V_OUT
R_VON1
30k
R_VON2
100k
R_C
C_C
C_VON
0.01µF
V_ON
LTC3714
I_TH
3714 F04a
V_OUT
INTV_CC
R_VON1
3k
R_VON2
10k 10k
Q1
2N5087
C_VON
0.01µF
R_C
C_C
V_ON
LTC3714
I_TH
3714 F04b
(4a)
(4b)
Figure 4. Correcting Frequency Shift with Load Current Changes
Inductor Selection
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
ΔI_L
=
⎛
⎝⎜
V_OUT
fL
⎞⎠⎟⎛⎝⎜_1−
V_OUT
V_IN
⎞
⎟
⎠
Lower ripple current reduces cores 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
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
=
⎛
⎝⎜⎜
V_OUT
fΔI_{L(MAX )}
⎞⎛
⎠⎟⎟⎝⎜⎜^1−
V_OUT
V_{IN(MAX )}
⎞
⎠⎟⎟
Once the value for L is known, the type of inductor must be
selected. A variety of inductors designed for high current,
low voltage applications are available from manufacturers
such as Sumida and Panasonic.
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, causing 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 ef-
ficiency 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 de-
viations 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.
3714f
14