MAX16831 Maxim Integrated Products, MAX16831 Datasheet - Page 14

• Manufacturer Part Number: MAX16831
• Description: High-Power LED Driver
• Manufacturer: Maxim Integrated Products

The minimum required inductance is a function of oper-
ating frequency, input-to-output voltage differential, and
the peak-to-peak inductor current (∆I
allows for a lower inductor value while a lower ∆I
requires a higher inductor value. A lower inductor value
minimizes size and cost, improves large-signal tran-
sient response but reduces efficiency due to higher
peak currents and higher peak-to-peak output ripple
voltage for the same output capacitance. On the other
hand, higher inductance increases efficiency by reduc-
ing the ripple current, ∆I
due to extra turns can exceed the benefit gained from
lower ripple current levels, especially when the induc-
tance is increased without also allowing for larger
inductor dimensions. A good compromise is to choose
∆I
saturating current is also important to avoid runaway
current during the output overload and continuous
short circuit. Select the I
mum peak current limit.
Buck configuration: In a buck configuration, the aver-
age inductor current does not vary with the input. The
worst-case peak current occurs at a high input voltage.
In this case, the inductance L for continuous conduc-
tion mode is given by:
where V
switching frequency, and V
Boost configuration: In the boost converter, the average
inductor current varies with line and the maximum aver-
age current occurs at low line. For the boost converter,
the average inductor current is equal to the input cur-
rent. In this case, the inductance L is calculated as:
where V
output voltage, and f
Buck-boost configuration: In a buck-boost converter,
the average inductor current is equal to the sum of the
input current and the load current. In this case, the
inductance L is:
High-Voltage, High-Power LED Driver with
Analog and PWM Dimming Control
14
L
equal to 30% of the full load current. The inductor
______________________________________________________________________________________
INMAX
INMIN
L
L
L
=
is the minimum input voltage, V
=
=
is the maximum input voltage, f
(
Applications Information
V
V
V
OUT
INMIN
OUT
V
V
INMAX
SW
V
OUT
+
OUT
×
V
×
is the switching frequency.
SAT
(
INMIN
V
(
INMAX
V
L
×
×
OUT
×
OUT
. However, resistive losses
f
SW
to be higher than the maxi-
f
V
SW
INMIN
)
×
Inductor Selection
is the output voltage.
-
×
×
-
f
V
SW
∆
V
INMIN
∆
I
OUT
L
I
L
×
∆
)
)
I
L
L
). Higher ∆I
OUT
SW
is the
is the
L
L
where V
output voltage, and f
The function of the output capacitor is to reduce the
output ripple to acceptable levels. The ESR, ESL, and
the bulk capacitance of the output capacitor contribute
to the output ripple. In most of the applications, the out-
put ESR and ESL effects can be dramatically reduced
by using low-ESR ceramic capacitors. To reduce the
ESL effects, connect multiple ceramic capacitors in
parallel to achieve the required bulk capacitance.
In a buck configuration, the output capacitance, C
calculated using the following equation:
where ∆V
In a boost configuration, the output capacitance, C
calculated as:
where I
In a buck-boost configuration, the output capacitance,
C
where V
the output current. Connect the output capacitor(s)
from the output to ground in a buck-boost configuration
(not across the load as for other configurations).
An input capacitor connected between UVEN and
ground must be used when configuring the MAX16831
as a buck converter. Use a low-ESR input capacitor
that can handle the maximum input RMS ripple current.
Calculate the maximum allowable RMS ripple using the
following equation:
In most of the cases, an additional electrolytic capaci-
tor should be added to prevent input oscillations due to
line impedances.
F
, is calculated as:
OUT
I
IN RMS
INMIN
OUT
R
(
is the maximum allowable output ripple.
is the output current.
C
C
is the voltage across the load and I
C
F
F
is the minimum input voltage, V
)
F
≥
≥
=
≥
∆
I
∆
OUT
(
V
V
(
V
V
R
R
OUT
SW
INMAX
×
× × ×
∆
×
2
2
(
V
is the switching frequency.
V
-
R
×
OUT
V
V
V
L V
×
OUT
INMIN
-
OUT
V
V
OUT
V
OUT
+
INMIN
INMAX
V
×
×
)
INMIN
(
Output Capacitor
× ×
I
V
)
×
OUT
INMIN
×
2
Input Capacitor
f
SW
V
×
OUT
)
I
OUT
f
×
SW
-
f
SW
V
2
OUT
OUT
)
OUT
is the
F
F
, is
, is
is