NCV8855
http://onsemi.com
19
SMPS2 Diode Selection
The diode in SMPS2 provides the inductor current path
when the power switch turns off. This is known as the
non−synchronous diode or commutation diode. The peak
reverse voltage is equal to the maximum operating input
voltage. The peak conducting current is determined by the
internal current limit. The average current can be calculated
from:
I
D(avg)
+ IOUT2
ǒ
1 *
VOUT2
VIN_SW
Ǔ
(eq. 10)
However, the worse case diode average current occurs
during a short circuit condition. For a diode to survive an
indefinite short circuit condition, the current rating of the
diode should be equal to the maximum current limit which
is 3.6 A. Thus the MBRS4201T3 is the diode of choice.
Inductor Selection
Both mechanical and electrical considerations influence
the selection of an output inductor. From a mechanical
perspective, smaller inductor values generally correspond to
smaller physical size. Since the inductor is often one of the
largest components in SMPS system, a minimum inductor
value is particularly important in space−constrained
applications. From an electrical perspective, smaller
inductor values correspond to faster transient response. The
maximum current slew rate through the output inductor for
a buck regulator is given by:
Inductor Slew Rate +
dI
L
dt
+
V
L
L
(eq. 11)
Where I
L
is the inductor current, L is the output
inductance, and V
L
is the voltage drop across the inductor.
This equation indicates that larger inductor values limit the
regulator’s ability to slew current through the output
inductor in response to output load transients. Consequently,
output capacitors must supply (or store) sufficient charge to
maintain regulation while the inductor current “catches up”
to the load. This results in larger values of output capacitance
to maintain tight output voltage regulation.
In contrast, smaller values of inductance increase the
regulator’s maximum achievable slew rate and decrease the
necessary capacitance, at the expense of higher ripple
current.
In continuous conduction mode, the peak−to−peak ripple
current is calculated using the following equation:
I
PP
+ FSW
VOUT
L
ǒ
1 *
VOUT
VBATT
Ǔ
(eq. 12)
From this equation it is clear that the ripple current
increases as L decreases, emphasizing the trade−off between
dynamic response and ripple current.
For most applications, the inductor value falls in the range
between 2.2 mH and 22 mH. There are many magnetic
component vendors providing standard product
lines suitable for SMPS1 and SMPS2’s requirements.
TDK offers the RLF12545−PF series inductors, which are
recommended for the automotive radio application.
SMPS Output Capacitor Selection
The output capacitor is a basic component for the fast
response of the power supply. In fact, during load transient,
for first few microseconds they supply the current to the
load. The controller recognizes the load transient and
proceeds to increase the duty cycle to its maximum.
Neglecting the effect of the ESL, the output voltage has a
first drop due to the ESR of the bulk capacitor(s).
DVOUT
ǒ
ESR
Ǔ
+ DIOUT ESR
(eq. 13)
A lower ESR produces a lower DV during load transient.
In addition, a lower ESR produces a lower output voltage
ripple.
The voltage drop due to the output capacitor discharge can
be approximated using the following equation:
DVOUT
ǒ
discharge
Ǔ
+
(
DIOUT
)
2
L
2 COUT
ǒ
VIN
ǒ
min
Ǔ
D
MAX
* VOUT
Ǔ
(eq. 14)
where, D
MAX
is the maximum duty cycle value, which is
90%. Although the ESR effect is not in phase with the
discharging of the output voltage, DVOUT
(ESR)
can be
added to DVOUT
(discharge)
to give a rough indication of the
maximum DVOUT during a transient condition. Simulation
can also help determine the maximum DVOUT; however, it
will ultimately have to be verified with the actual load since
the ESL effect is dependent on layout and the actual load’s
di/dt.
SMPS Input Capacitor Selection
The primary consideration for selecting the input
capacitor is input RMS current. However, since there are
two SMPS running out−of−phase with each other,
calculating the input RMS current can be complicated. The
graphs below shows how the input RMS current is affected
by differing phase angles between SMPS1 and SMPS2. The
plot below was generated with VOUT1 at 5 V with a load of
2 A and an output inductor value of 10 mH, and VOUT2 at
8 V with a load of 4 A and an output inductor value of 10 mH.
1.00
1.20
1.40
1.60
1.80
2.00
2.20
2.40
2.60
2.80
3.00
0.00 60.00 120.00 180.00 240.00 300.00 360.00
Phase (VOUT1 vs VOUT2)
Irms
9.00
10.00
11.00
12.00
13.00
14.00
15.00
16.00
17.00
18.00
Figure 22. Irms vs Phase
