MAX16816 Maxim Integrated Products, MAX16816 Datasheet - Page 23

• Manufacturer Part Number: MAX16816
• Description: Programmable Switch-Mode LED Driver
• Manufacturer: Maxim Integrated Products

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:
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 ΔV
In a boost configuration, the output capacitance, C
calculated as:
where I
In a buck-boost configuration, the output capacitance,
C
F
, is calculated as:
C
OUT
INMIN
INMIN
C
F
R
C
F
L
is the maximum allowable output ripple.
L
≥
F
is the output current.
≥
=
≥
is the minimum input voltage, V
is the minimum input voltage, V
Δ
=
V
Δ
(
R
(
V
(
V
V
V
V
______________________________________________________________________________________
INMIN
R
OUT
OUT
INMAX
×
SW
SW
Programmable Switch-Mode LED Driver
×
Δ
2
V
2
V
O
(
×
−
V
+
R
is the switching frequency.
V
is the switching frequency.
with Analog-Controlled PWM Dimming
UT
x V
×
OUT
OUT
L
V
−
V
(
×
V
INMIN
INMIN
x f
OUT
OUT
×
V
V
OUT
x V
SW
OUT
+
V
INMAX
INMIN
V
)
)
Output Capacitor
×
−
INMIN
x I
)
x f
×
×
Δ
×
I
V
OUT
SW
2
INMIN
L
f
V
SW
×
×
)
OUT
x I
×
I
f
OUT
SW
Δ
)
f
SW
L
2
OUT
OUT
is the
is the
F
F
, is
, is
where V
the output current.
An input capacitor connected between V
ground must be used when configuring the MAX16816
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.
When using the MAX16816 in a boost or buck-boost
configuration, the input RMS current is low and the
input capacitance can be small (see the Typical
Operating Circuits ).
The MAX16816 can also be used in the absence of the
dimming MOSFET. In this case, the PWM dimming per-
formance is compromised but in applications that do
not require dimming the MAX16816 can still be used. A
short circuit across the load will cause the MAX16816
to disable the gate drivers and they will remain off until
the input power is recycled.
When selecting MOSFETs for switching, consider the
total gate charge, power dissipation, the maximum
drain-to-source voltage, and package thermal imped-
ance. The product of the MOSFET gate charge and
R
fying better performance. Select MOSFETs optimized
for high-frequency switching applications.
Typically, there are two sources of noise emission in a
switching power supply: high di/dt loops and high dv/dt
surfaces. For example, traces that carry the drain cur-
rent often form high di/dt loops. Similarly, the heatsink
of the MOSFET connected to the device drain presents a
high dv/dt source; therefore, minimize the surface area of
the heatsink as much as possible. Keep all PCB traces
carrying switching currents as short as possible to mini-
mize current loops. Use ground planes for best results.
DS(ON)
I
IN(RMS)
Operating the MAX16816 Without the
OUT
is a figure of merit, with a lower number signi-
Switching Power MOSFET Losses
is the voltage across the load and I
=
I
OUT
Layout Recommendations
×
V
OUT
V
INMIN
× (
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Dimming Switch
V
Input Capacitor
INMIN
- V
OUT
CC
OUT
)
and
23
is