MAX16818EVKIT+ Maxim Integrated Products, MAX16818EVKIT+ Datasheet - Page 19

• 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

This section provides some detail regarding the appli-
cation circuits in the Simplified Diagram and Figures
1–5. The discussion includes some description of the
topology as well as basic attributes.
The Simplified Diagram shows the MAX16818 providing
high-frequency, high-current pulses to the LEDs. The
basic topology must be a buck, since the inductor
always connects to the load in that configuration (in all
other topologies, the inductor disconnects from the
load at one time or another). The design minimizes the
current ripple by oversizing the inductor, which allows
for a very small (0.01µF) output capacitor. When MOS-
FET Q3 turns on, it diverts the current around the LEDs
at a very fast rate. Q3 also discharges the output
capacitor, but since the capacitor is so small, it does
not stress the MOSFET. Resistor R1 senses the LED/Q3
current and there is no reaction to the short that Q3
places across the LEDs. This design is superior in that
it does not attempt to actually change the inductor cur-
rent at high frequencies and yet the current in the LEDs
varies from zero to full in very small periods of time. The
efficiency of this technique is very high. Q3 must be
able to dissipate the LED current applied to its R
at some maximum duty cycle. If the circuit needs to
control extremely high currents, use paralleled
MOSFETs. PGOOD is low during LED pulsed-current
operation.
In Figure 1, the external components configure the
MAX16818 as a boost converter. The circuit applies the
input voltage to the inductor during the on-time, and
then during the off-time the inductor, which is in series
with the input capacitor, charges the output capacitor.
Because of the series connection between the input
voltage and the inductor, the output voltage can never
go lower than the input voltage. The design is nonsyn-
chronous, and since the current-sense resistor con-
nects to ground, the power supply can go to any output
voltage (above the input) as long as the components are
rated appropriately. R2 again provides the sense voltage
the MAX16818 uses to regulate the LED current.
The circuit in Figure 2 shows a step-up/step-down reg-
ulator. It is similar to the boost converter in Figure 1 in
that the inductor is connected to the input and the
MOSFET is essentially connected to ground. However,
rather than going from the output to ground, the LEDs
Application Circuit Descriptions
Applications Information
High-Frequency LED Current Pulser
______________________________________________________________________________________
1.5MHz, 30A High-Efficiency, LED Driver
Input-Referenced LED Driver
Boost LED Driver
with Rapid LED Current Pulsing
DS(ON)
span from the output to the input. This effectively
removes the boost-only restriction of the regulator in
Figure 1, allowing the voltage across the LEDs to be
greater than or less than the input voltage. LED current
sensing is not ground-referenced, so a high-side cur-
rent-sense amplifier is used to measure current.
Figure 3 shows the MAX16818 configured as a SEPIC
LED driver. While buck topologies require the output to
be lesser than the input, and boost topologies require
the output to be greater than the input, a SEPIC topolo-
gy allows the output voltage to be greater than, equal
to, or less than the input. In a SEPIC topology, the volt-
age across C1 is the same as the input voltage, and L1
and L2 are the same inductance. Therefore, when Q1
conducts (on-time), both inductors ramp up current at
the same rate. The output capacitor supports the output
voltage during this time. During the off-time, L1 current
recharges C1 and combines with L2 to provide current
to recharge C2 and supply the load current. Since the
voltage waveform across L1 and L2 are exactly the
same, it is possible to wind both inductors on the same
core (a coupled inductor). Although voltages on L1 and
L2 are the same, RMS currents can be quite different
so the windings may have a different gauge wire.
Because of the dual inductors and segmented energy
transfer, the efficiency of a SEPIC converter is some-
what lower than standard bucks or boosts. As in the
boost driver, the current-sense resistor connects to
ground, allowing the output voltage of the LED driver to
exceed the rated maximum voltage of the MAX16818.
Figure 4 depicts a buck/boost topology. During the on-
time with this circuit, the current flows from the input
capacitor, through Q1, L1, and Q3 and back to the
input capacitor. During the off-time, current flows up
through Q2, L1, D1, and to the output capacitor C1.
This topology resembles a boost in that the inductor sits
between the input and ground during the on-time.
However, during the off-time the inductor resides
between ground and the output capacitor (instead of
between the input and output capacitors in boost
topologies), so the output voltage can be any voltage
less than, equal to, or greater than the input voltage. As
compared to the SEPIC topology, the buck/boost does
not require two inductors or a series capacitor, but it
does require two additional MOSFETs.
In Figure 5, the input voltage can go from 7V to 28V and,
because of the ground-based current-sense resistor, the
output voltage can be as high as the input. The synchro-
Buck Driver with Synchronous Rectification
Ground-Referenced Buck/Boost LED Driver
SEPIC LED Driver
19