LTC3811_15 Datasheet, PDF (24/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
APPLICATIONS INFORMATION
For CCM operation, the inductor value can be chosen using
the following equation:
L
=
V_OUT
f • ΔI_L
⎡
⎢1–
⎣⎢
V_OUT
V_IN(MAX
)
⎤
⎥
⎦⎥
Choosing a larger value of ΔI_L allows the use of a lower
value inductor, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting the ripple current is 40% to 50% of the maximum
output current, or:
ΔI_L = 0.4 • I_OUT(MAX)
The inductor saturation current rating needs to be higher
than the peak inductor current during an overload condi-
tion. If I_OUT(MAX) is the maximum rated load current, then
the maximum overload current, I_MAX, would normally
be chosen to be some factor (e.g., 30%) greater than
I_OUT(MAX):
I_MAX = 1.3 • I_OUT(MAX)
I_{L(PK )}
=
I_MAX
+
1
2
•
ΔI_L
For a 40% ripple application, the minimum saturation
current rating of the inductor would therefore be:
I_L(PK) = 1.5 • I_O(MAX)
In other words, for an application with 40% inductor ripple
current and a maximum output current 30% greater than
the full load current, the inductor’s saturation current rat-
ing needs to be at least 1.5 times the maximum output
current.
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. High efficiency converters generally cannot afford
the core losses found in low cost powdered iron cores,
forcing the use of more expensive ferrite or molypermalloy
cores. Actual core loss is independent of core size for a fixed
inductor value, but is very dependent on the inductance
selected. As inductance increases, core losses go down.
Unfortunately, increased inductance requires more turns
of wire and therefore copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core materials exhibit “hard” saturation, meaning that
the inductance collapses abruptly when the peak current
capability is exceeded. This results in an abrupt increase
in inductor ripple current and output voltage ripple. Do
not allow the core to saturate!
Programming the Maximum Sense Voltage Using the
RNG Pin
The RNG pin can be used in two different ways in order to
program the maximum peak current sense voltage. The
easiest way to program the peak sense voltage is to tie
the RNG pin to either ground or INTV_CC. Connecting the
RNG pin to ground results in a 24mV peak sense voltage
and connecting it to INTV_CC programs in a 50mV peak
sense voltage. Alternately, an external resistor divider
from INTV_CC to ground can be used to set the RNG pin
between 0.6V and 2V, resulting in a nominal peak sense
voltage range of 24mV to 85mV. Figure 10 illustrates the
transfer function from the RNG pin to the peak sense
voltage, which closely follows the following equation for
0.6V < V_RNG < 2V:
V_SENSE(MAX) = 0.0436 • V_RNG – 0.0022
In general, the accuracy of the SENSE pin threshold will
scale with the peak sense voltage defined by the RNG
90
80
70
60
50
40
30
20
10
0
0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9
V_RNG VOLTAGE (V)
3811 G30
Figure 10. Maximum Current Sense Threshold
vs RNG Pin Voltage
3811f
24