NCV898032
www.onsemi.com
10
overtake the increased power transfer. If the converter is in
this range it will not be able to maintain output regulation.
If the following equation is not satisfied, the device will
skip pulses at high V
IN
:
D
min
f
s
w t
on(min)
Where: f
s
: switching frequency [Hz]
t
on(min)
: minimum on time [s]
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 between the MOSFET source and ground. The sense
resistor should be selected as follows:
R
SNS
+
V
CL
I
CL
Where: R
SNS
: sense resistor [W]
V
CL
: current limit threshold voltage [V]
I
CL
: desired current limit [A]
3. Select the Boost Inductor
The Boost inductor controls the current ripple that occurs
over a switching period. A discontinuous current ripple will
result in superior transient response and lower switching
noise at the expense of higher transistor conduction losses
and operating ripple current requirements. A low current
ripple will result in CCM operation having a slower response
current slew rate in case of load steps (e.g. introducing an
LED series dimming circuit). A good starting point is to
select components for DCM operation at V
in(min)
, but
operation should be verified empirically. Calculate the
maximum inductor value as follows:
L
max
+
ǒ
1 *
1
M
max
Ǔ
V
in(min)
2
ǒ
V
out
I
LED
Ǔ
2f
s
V
out
2
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
+
V
IN(min)
D
max
Lf
s
Where: I
L,peak
: Peak inductor current value [A]
4. Select Output Capacitor
The output capacitor smoothes the output voltage and
reduces 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)
+
I
LED
ǒ
1 * d (M
max
)
Ǔ
f
s
C
OUT
The capacitors must withstand an RMS ripple current as
follows:
I
Cout(RMS)
+ I
LED
2
) d(M
max
)
ǒ
I
L,pk
2
3
* I
L,pk
I
LED
Ǔ
Ǹ
A 1−2.2 mF ceramic capacitor is usually sufficient for high
brightness LED applications for f
s
= 2 MHz.
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)
+
ǒ
D(M
max
) ) d (M
max
)
3
Ǔ
I
L,pk
2
* I
L,avg
2
Ǹ
6. Select Feedback Resistors
The feedback resistor provides LED current sensing for
the feedback signal. It may be calculated as follows:
R
F1
+
V
ref
I
LED
7. Select Compensator Components
Current Mode control method employed by the
NCV898032 allows the use of a simple Type II
compensation to optimize the dynamic response according
to system requirements. A transconductance amplifier is
used, so compensation components must
be connected
between the compensation pin and ground.
8. Select MOSFET(s)
In order to ensure the gate drive voltage does not drop out,
the selected MOSFET 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
L,peak
D(M
max
)
3
Ǹ
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 voltage:
V
Q(max)
+ V
OUT(max)
