NCV898031
www.onsemi.com
13
2. Select Current Sense Resistor
Current sensing for peak current mode control and current
limit relies on the MOSFET current signal, which is
measured with a ground referenced amplifier. The easiest
method of generating this signal is to use a current sense
resistor from the source of the MOSFET to device ground.
The sense resistor should be selected as follows:
R
S
+
V
CL
I
CL
Where: R
S
: sense resistor [W]
V
CL
: current limit threshold voltage [V]
I
CL
: desire current limit [A]
3. Select Output Inductor
The output inductor controls the current ripple that occurs
over a switching period. A high current ripple will result in
excessive power loss and ripple current requirements. A low
current ripple will result in a poor control signal and a slow
current slew rate in case of load steps. A good starting point
for peak to peak ripple is around 20−40% of the inductor
current at the maximum load at the worst case V
IN
, but
operation should be verified empirically. The worst case V
IN
is half of V
OUT
, or whatever V
IN
is closest to half of V
IN
.
After choosing a peak current ripple value, calculate the
inductor value as follows:
L +
V
IN(WC)
2
D
WC
DI
L,max
f
s
V
OUT
Where: V
IN(WC)
: V
IN
value as close as possible to half of
V
OUT
[V]
D
WC
: duty cycle at V
IN(WC)
DI
L,max
: maximum peak to peak ripple [A]
The maximum average inductor current can be calculated as
follows:
I
L,avg
+
V
OUT
I
OUT(max)
V
IN(min)
The Peak Inductor current can be calculated as follows:
I
L,peak
+ I
L,avg
)
V
IN(min)
2
D
WC
Lf
s
V
OUT
Where: I
L,peak
: Peak inductor current value [A]
4. Select Output Capacitors
The output capacitors smooth the output voltage and
reduce the overshoot and undershoot associated with line
transients. The steady state output ripple associated with the
output capacitors can be calculated as follows:
V
OUT(ripple)
+
DI
OUT(max)
fC
OUT
)
ǒ
I
OUT(max)
1 * D
)
V
IN(min)
D
2fL
Ǔ
R
ESR
The capacitors need to survive an RMS ripple current as
follows:
I
Cout(RMS)
+ I
OUT
D
WC
DȀ
WC
)
D
WC
12
ǒ
DȀ
WC
L
R
OUT
T
SW
Ǔ
2
Ǹ
The use of parallel ceramic bypass capacitors is strongly
encouraged to help with the transient response.
5. Select Input Capacitors
The input capacitor reduces voltage ripple on the input to
the module associated with the ac component of the input
current.
I
Cin(RMS)
+
V
IN(WC)
2
D
WC
Lf
s
V
OUT
23
Ǹ
6. Select Feedback Resistors
The feedback resistors form a resistor divider from the
output of the converter to ground, with a tap to the feedback
pin. During regulation, the divided voltage will equal V
ref
.
The lower feedback resistor can be chosen, and the upper
feedback resistor value is calculated as follows:
R
upper
+ R
lower
ǒ
V
out
* V
ref
Ǔ
V
ref
The total feedback resistance (R
upper
+ R
lower
) should be
in the range of 1 kW – 100 kW.
7. Select Compensator Components
Current Mode control method employed by the
NCV898031 allows the use of a simple, Type II
compensation to optimize the dynamic response according
to system requirements.
8. Select MOSFET(s)
In order to ensure the gate drive voltage does not drop out
the MOSFET(s) chosen must not violate the following
inequality:
Q
g(total)
v
I
drv
f
s
Where: Q
g(total)
: Total Gate Charge of MOSFET(s) [C]
I
drv
: Drive voltage current [A]
f
s
: Switching Frequency [Hz]
The maximum RMS Current can be calculated as follows:
I
Q(max)
+ I
out
D
WC
Ǹ
DȀ
WC
The maximum voltage across the MOSFET will be the
maximum output voltage, which is the higher of the
maximum input voltage and the regulated output voltaged:
V
Q(max)
+ V
OUT(WC)
9. Select Diode
The output diode rectifies the output current. The average
current through diode will be equal to the output current:
I
D(avg)
+ I
OUT(max)
