MAX16818_15 Datasheet, PDF (15/25 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
Detailed Description
The MAX16818 is a high-performance average-
current-mode PWM controller for high-power, high-
brightness LEDs (HB LEDs). Average current-mode
control is the ideal method for driving HB LEDs. This
technique offers inherently stable operation, reduces
component derating and size by accurately controlling
the inductor current. The device achieves high efficiency
at high current (up to 30A) with a minimum number of
external components. The high- and low-side drivers
source and sink up to 4A for lower switching losses while
driving high-gate-charge MOSFETs. The MAX16818's
CLKOUT output is 180° out-of-phase with respect to the
high-side driver. CLKOUT drives a second MAX16818
LED driver out of phase, reducing the input-capacitor
ripple current.
The MAX16818 consists of an inner average current loop
representing inductor current and an outer voltage loop
voltage-error amplifier (VEA) that directly controls LED
current. The combined action of the two loops results in
a tightly regulated LED current. The inductor current is
sensed across a current-sense resistor. The differential
amplifier senses LED current through a sense resistor
in series with the LEDs and the resulting sensed volt-
age is compared against an internal 0.6V reference at
the error-amplifier input. The MAX16818 will adjust the
LED current to within 1% accuracy to maintain emitted
spectrum of the light in HB LEDs.
IN, V_CC, and V_DD
The MAX16818 accepts either a 4.75V to 5.5V or 7V
to 28V input voltage range. All internal control circuitry
operates from an internally regulated nominal voltage of
5V (V_CC). For input voltages of 7V or greater, the internal
V_CC regulator steps the voltage down to 5V. The V_CC
output voltage is a regulated 5V output capable of
sourcing up to 60mA. Bypass the V_CC to SGND with
4.7µF and 0.1µF low-ESR ceramic capacitors for high-
frequency noise rejection and stable operation.
The MAX16818 uses V_DD to power the low-side and high-
side drivers. Isolate V_DD from V_CC with a 1Ω resistor and
put a 1µF capacitor in parallel with a 0.1µF capacitor to
ground to prevent high-current noise spikes created by
the driver from disrupting internal circuitry.
The TQFN is a thermally enhanced package and can
dissipate up to 2.7W. The high-power packages allow
the high-frequency, high-current converter to operate
from a 12V or 24V bus. Calculate power dissipation in
the MAX16818 as a product of the input voltage and the
total V_CC regulator output current (I_CC). I_CC includes
quiescent current (I_Q) and gate-drive current (I_DD):
P_D = V_IN x I_CC
I_CC = I_Q + [f_SW x (Q_G1 + Q_G2)]
where Q_G1 and Q_G2 are the total gate charge of the
low-side and high-side external MOSFETs at V_GATE =
5V, I_Q is 3.5mA (typ), and f_SW is the switching frequency
of the converter.
Undervoltage Lockout (UVLO)
The MAX16818 includes an undervoltage lockout with
hysteresis and a power-on-reset circuit for converter turn-
on. The UVLO rising threshold is internally set at 4.35V
with a 200mV hysteresis. Hysteresis at UVLO eliminates
chattering during startup.
Most of the internal circuitry, including the oscillator, turns
on when the input voltage reaches 4V. The MAX16818
draws up to 3.5mA of current before the input voltage
reaches the UVLO threshold.
Soft-Start
The MAX16818 has an internal digital soft-start for a
monotonic, glitch-free rise of the output current. Soft-start
is achieved by the controlled rise of the error amplifier
dominant input in steps using a 5-bit counter and a 5-bit
DAC. The soft-start DAC generates a linear ramp from 0
to 0.7V. This voltage is applied to the error amplifier at a
third (noninverting) input. As long as the soft-start voltage
is lower than the reference voltage, the system converges
to that lower reference value. Once the soft-start DAC
output reaches 0.6V, the reference takes over and the
DAC output continues to climb to 0.7V, assuring that it
does not interfere with the reference voltage.
Internal Oscillator
The internal oscillator generates a clock with the frequen-
cy proportional to the inverse of R_T. The oscillator fre-
quency is adjustable from 125kHz to 1.5MHz with better
than 8% accuracy using a single resistor connected from
RT/SYNC to SGND. The frequency accuracy avoids the
over-design, size, and cost of passive filter components
like inductors and capacitors. Use the following equation
to calculate the oscillator frequency:
For 120kΩ ≤ R_T ≤ 500kΩ:
R_T
=
6.25 x 10¹⁰
f_SW
For 40kΩ ≤ R_T ≤ 120kΩ
R_T
=
6.40 x 10¹⁰
f_SW
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