MAX16818EVKIT+ Maxim Integrated Products, MAX16818EVKIT+ Datasheet - Page 21

• Manufacturer Part Number: MAX16818EVKIT+
• Description: KIT EVALUATION FOR MAX16818
• Manufacturer: Maxim Integrated Products

Specifications of MAX16818EVKIT+

• Current - Output / Channel: 1A
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 18V
• Features: Dimmable
• Voltage - Input: 6 ~ 28V
• Utilized Ic / Part: MAX16818
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Estimate the power loss (PD
and low-side MOSFETs using the following equations:
where Q
MOSFET’s total gate charge, on-resistance at maximum
operating temperature, rise time, and fall time, respectively.
For the buck regulator, D = V
(I
For example, from the typical specifications in the
Applications Information section with V
high-side and low-side MOSFET RMS currents are
0.77A and 0.63A, respectively, for a 1A buck regulator.
Ensure that the thermal impedance of the MOSFET
package keeps the junction temperature at least +25°C
below the absolute maximum rating. Use the following
equation to calculate the maximum junction tempera-
ture: T
the junction-to-ambient thermal impedance and ambi-
ent temperature, respectively.
To guarantee that there is no shoot-through from V
PGND, the MAX16818 produces a nonoverlap time of
35ns. During this time, neither high- nor low-side MOS-
FET is conducting, and since the output inductor must
maintain current flow, the intrinsic body diode of the
low-side MOSFET becomes the conduction path. Since
this diode has a fairly large forward voltage, a Schottky
diode (in parallel to the low-side MOSFET) diverts current
flow from the MOSFET body diode because of its lower
forward voltage, which, in turn, increases efficiency.
PD
PD
OUT
I
RMS HI
(
I
RMS LO
R
MOS HI
MOS LO
DS ON
- ∆I
J
(
−
−
−
−
= (PD
G
L
)
, R
/ 2) and I
=
x I
=
=
DS(ON)
=
RMS LO
MOS
(
(
⎛
⎜
⎝
(
+
I
Q
I
(
VALLEY
VALLEY
Q
V
G
(
−
IN
R
______________________________________________________________________________________
G
1.5MHz, 30A High-Efficiency, LED Driver
DS ON
x θ
, t
PK
x V
x V
2
x I
)
R
(
2
2
JA
, and t
= (I
OUT
DD
+
DD
+
) + T
)
I
MOS
OUT
PK
I
x I
x f
PK
x f
x t
2
RMS HI
F
2
SW
A
(
_) caused by the high-side
2
+
SW
+ ∆I
, where θ
R
are the upper-switching
+
LEDs
I
)
VALLEY
)
+
−
I
+
VALLEY
L
+
t
/ 2).
F
2
)
)
/ V
Buck Regulator
OUT
x f
with Rapid LED Current Pulsing
x I
JA
IN
x I
PK
SW
, I
= 7.8V, the
and T
)
PK
VALLEY
⎞
⎟
⎠
x
)
(
1
x
−
3
A
D
D
IN
3
)
are
to
=
Estimate the power loss (PD
FET using the following equations:
For a boost regulator in continuous mode, D = V
(V
+ ∆I
The voltage across the MOSFET:
where V
The output diode on a boost regulator must be rated to
handle the LED series voltage, V
have fast reverse-recovery characteristics and should
handle the average forward current that is equal to the
LED current.
For buck regulator designs, the discontinuous input
current waveform of the buck converter causes large
ripple currents in the input capacitor. The switching fre-
quency, peak inductor current, and the allowable peak-
to-peak voltage ripple reflected back to the source
dictate the capacitance requirement. Increasing switch-
ing frequency or paralleling out-of-phase converters
lowers the peak-to-average current ratio, yielding a
lower input capacitance requirement for the same LED
current. The input ripple is comprised of ∆V
by the capacitor discharge) and ∆V
ESR of the capacitor). Use low-ESR ceramic capacitors
with high-ripple-current capability at the input. Assume
the contributions from the ESR and capacitor discharge
are equal to 30% and 70%, respectively. Calculate the
input capacitance and ESR required for a specified ripple
using the following equation:
⎛
⎜
⎝
IN
I
V
RMS HI
L
IN
+ V
/ 2).
x I
−
LEDs
F
OUT
is the maximum forward voltage of the diode.
=
), I
PD
x t
ESR
(
(
2
VALLEY
I
FET
VALLEY
R
V
IN
MOSFET
+
=
t
=
F
(
)
2
Q
= (I
x f
⎛
⎜
⎝
G
+
I
OUT
SW
I
OUT
= V
x V
PK
∆
MOS
V
⎞
⎟ +
⎠
2
DD
LED
ESR
+
- ∆I
+
_) caused by the MOS-
(
∆
I
x f
R
VALLEY
2
Input Capacitors
+ V
L
I
DS ON
L
LED
SW
/ 2) and I
ESR
⎞
⎟
⎠
(
F
Boost Regulator
)
. It should also
+
)
(caused by the
x I
x I
PK
RMS HI
PK
Q
)
x
(caused
−
= (I
D
3
LEDs
2
OUT
)
21
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