Datasheet
NCV8873
http://onsemi.com
9
APPLICATION INFORMATION
Design Methodology
This section details an overview of the component
selection process for the NCV8873 in discontinuous
conduction mode (DCM) Boost converter operation with a
high brightness LED (100−150 mA typical) string as a load.
LED current is used for the feedback signal. It is intended to
assist with the design process but does not remove all
engineering design work. Many of the equations make use
of the small ripple approximation. This process entails the
following steps:
1. Define Operational Parameters
2. Select Current Sense Resistor
3. Select Output Inductor
4. Select Output Capacitors
5. Select Input Capacitors
6. Select Feedback Resistors
7. Select Compensator Components
8. Select MOSFET(s)
9. Select Diode
1. Define Operational Parameters
Before beginning the design, define the operating
parameters of the application. These include:
V
IN(min)
: minimum input voltage [V]
V
IN(max)
: maximum input voltage [V]
V
OUT
: output voltage [V]
I
LED
: LED current [A]
I
CL
: desired typical cycle-by-cycle current limit [A]
V
ref
: NCV8873 feedback reference voltage = 0.2 V
I
L
: inductor current [A]
From this the ideal minimum and maximum duty cycles
can be calculated as follows:
M
min
+
V
out
V
in(max)
M
max
+
V
out
V
in(min)
R
out
+
V
out
I
LED
D
min
+
Lf
s
2R
out
ƪ
ǒ
2M
min
* 1
Ǔ
2
* 1
ƫ
Ǹ
D
max
+
Lf
s
2R
out
ƪ
(
2M
max
* 1
)
2
* 1
ƫ
Ǹ
d +
2V
out
2
V
in
R
out
I
L,peak
* D,
Where: (D + d) < 1 for DCM operation IL.
Both duty cycles will actually be slightly higher due to
power loss in the conversion. The exact duty cycles depend
on conduction and switching losses. If the maximum input
voltage is higher than the output voltage, the minimum duty
cycle will be a complex value. This is because a Boost
converter cannot have an output voltage lower than the input
voltage. In situations where the input voltage is higher than
the output, the output will follow the input (minus the diode
drop of the Boost diode) and the converter will not attempt
to switch.
If the inductor value is too large, continuous conduction
mode (CCM) operation will occur and a right-half-plane
(RHP) zero appears which can result in operation instability.
If the calculated D
max
is higher than the D
max
of the
NCV8873, the conversion will not be possible. It is
important for a Boost converter to have a restricted D
max
,
because while the ideal conversion ration of a Boost
converter goes up to infinity as D approaches 1, a real
converter’s conversion ratio starts to decrease as losses
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