MAX16818_15 Datasheet, PDF (18/25 Pages) Maxim Integrated Products – 1.5MHz, 30A High-Efficiency, LED Driver with Rapid LED Current Pulsing

MAX16818
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
capability 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
higher 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 switch-
ing 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 components 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 handle
the peak and RMS currents during overload conditions.
The driver block also includes a logic circuit that provides
an adaptive nonoverlap time to prevent shoot-through
currents during transition. The typical nonoverlap 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 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
converter 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
hiccup 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 average-
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 sensitiv-
ity of the overvoltage circuit and avoid nuisance tripping
of the converter. Disable the overvoltage function by
connecting OVI to SGND.
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