NCV8870
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10
7. Select Compensator Components
Current Mode control method employed by the NCV8870
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
Ǹ
DȀ
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(max)
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)
Additionally, the diode must block voltage equal to the
higher of the output voltage and the maximum input voltage:
V
D(max)
+ V
OUT(max)
The maximum power dissipation in the diode can be
calculated as follows:
P
D
+ V
f(max)
I
OUT(max)
Where: P
d
: Power dissipation in the diode [W]
V
f(max)
: Maximum forward voltage of the diode [V]
10. Determine Feedback Loop Compensation Network
The purpose of a compensation network is to stabilize the
dynamic response of the converter. By optimizing the
compensation network, stable regulation response is
achieved for input line and load transients.
Compensator design involves the placement of poles and
zeros in the closed loop transfer function. Losses from the
boost inductor, MOSFET, current sensing and boost diode
losses also influence the gain and compensation
expressions. The OTA has an ESD protection structure
(R
ESD
≈ 502 W, data not provided in the datasheet) located
on the die between the OTA output and the IC package
compensation pin (VC). The information from the OTA
PWM feedback control signal (V
CTRL
) may differ from the
IC-VC signal if R
2
is of similar order of magnitude as R
ESD
.
The compensation and gain expressions which follow take
influence from the OTA output impedance elements into
account.
Type-I compensation is not possible due to the presence
of R
ESD
. The Figures 11 and 12 compensation networks
correspond to a Type-II network in series with R
ESD
.
The resulting control-output transfer function is an accurate
mathematical model of the IC in a boost converter topology.
The model does have limitations and a more accurate SPICE
model should be considered for a more detailed analysis:
• The attenuating effect of large value ceramic capacitors
in parallel with output electrolytic capacitor ESR is not
considered in the equations.
• The CCM Boost control-output transfer function
includes operating efficiency as a correction factor to
improve modeling accuracy under low input voltage
and high output current operating conditions where
operating losses becomes significant.
