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1.5MHz, 30A High-Efficiency, LED Driver
with Rapid LED Current Pulsing
Differential Amplifier
The DIFF AMP facilitates remote sensing at the load
(Figure 7). It provides true differential LED current
(through the R LS sense resistor) sensing while rejecting
the common-mode voltage errors due to high-current
ground paths. The VEA provides the difference
between the differential amplifier output (DIFF) and the
desired LED current-sense voltage. The differential
amplifier has a bandwidth of 3MHz. The difference
between SENSE+ and SENSE- is regulated to 0.6V.
Connect SENSE+ to the positive side of the LED current-
sense resistor and SENSE- to the negative side of the
LED current-sense resistor (which is often PGND).
MOSFET Gate Drivers (DH, DL)
The high-side (DH) and low-side (DL) drivers drive the
gates of external n-channel MOSFETs (Figures 1–5).
The drivers’ 4A peak sink- and source-current capabili-
ty provides ample drive for the fast rise and fall times of
the switching MOSFETs. Faster rise and fall times result
in reduced cross-conduction losses. Due to physical
realities, extremely low gate charges and R DS(ON)
resistance of MOSFETs are typically exclusive of each
other. MOSFETs with very low R DS(ON) will have a high-
er gate charge and vice versa. Choosing the high-side
MOSFET (Q1) becomes a trade-off between these two
attributes. Applications where the input voltage is much
higher than the output voltage result in a low duty cycle
where conduction losses are less important than
switching losses. In this case, choose a MOSFET with
very low gate charge and a moderate R DS(ON).
Conversely, for applications where the output voltage is
near the input voltage resulting in duty cycles much
greater than 50%, the R DS(ON) losses become at least
equal, or even more important than the switching losses.
In this case, choose a MOSFET with very low R DS(ON)
and moderate gate charge. Finally, for the applications
where the duty cycle is near 50%, the two loss compo-
nents are nearly equal, and a balanced MOSFET with
moderate gate charge and R DS(ON) work best.
In a buck topology, the low-side MOSFET (Q2) typically
operates in a zero voltage switching mode, thus it does
not have switching losses. Choose a MOSFET with very
low R DS(ON) and moderate gate charge.
Size both the high-side and low-side MOSFETs to han-
dle the peak and RMS currents during overload condi-
tions. The driver block also includes a logic circuit that
provides an adaptive nonoverlap time to prevent shoot-
through currents during transition. The typical nonover-
lap time between the high-side and low-side MOSFETs
is 35ns.
BST
The MAX16818 uses V DD to power the low- and high-
side MOSFET drivers. The high-side driver derives its
power through a bootstrap capacitor and V DD supplies
power internally to the low-side driver. Connect a
0.47μF low-ESR ceramic capacitor between BST and
LX. Connect a Schottky rectifier from BST to V DD . Keep
the loop formed by the boost capacitor, rectifier, and IC
small on the PCB.
Protection
The MAX16818 includes output overvoltage protection
(OVP). During fault conditions when the load goes to
high impedance (opens), the controller attempts to
maintain LED current. The OVP protection disables the
MAX16818 whenever the voltage exceeds the thresh-
old, protecting the external circuits from undesirable
voltages.
Current Limit
The VEA output is clamped to 930mV with respect to
the common-mode voltage (V CM ). Average-current-
mode control has the ability to limit the average current
sourced by the converter during a fault condition. When
a fault condition occurs, the VEA output clamps to
930mV with respect to the common-mode voltage
(0.6V) to limit the maximum current sourced by the con-
verter to I LIMIT = 26.9mV / R S . The hiccup current limit
overrides the average current limit. The MAX16818
includes hiccup current-limit protection to reduce the
power dissipation during a fault condition. The hiccup
current-limit circuit derives inductor current information
from the output of the current amplifier. This signal is
compared against one half of V CLAMP(EA) . With no
resistor connected from the LIM pin to ground, the hic-
cup current limit is set at 90% of the full-load average
current limit. Use R EXT to increase the hiccup current
limit from 90% to 100% of the full load average limit.
The hiccup current limit can be disabled by connecting
LIM to SGND. In this case, the circuit follows the aver-
age current-limit action during overload conditions.
Overvoltage Protection
The OVP comparator compares the OVI input to the
overvoltage threshold. A detected overvoltage event
latches the comparator output forcing the power stage
into the OVP state. In the OVP state, the high-side
MOSFET turns off and the low-side MOSFET latches on.
Connect OVI to the center tap of a resistor-divider from
V LED to SGND. In this case, the center tap is compared
against 1.276V. Add an RC delay to reduce the sensitivity
of the overvoltage circuit and avoid nuisance tripping of
the converter. Disable the overvoltage function by con-
necting OVI to SGND.
18
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