MAX16818_09 Datasheet, PDF (15/25 Pages) Maxim Integrated Products – 1.5MHz, 30A High-Efficiency, LED Driver with Rapid LED Current Pulsing

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 (HBLEDs). Average current-mode control is the
ideal method for driving HBLEDs. This technique offers
inherently stable operation, reduces component derat-
ing and size by accurately controlling the inductor cur-
rent. 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 dri-
ving 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 voltage is
compared against an internal 0.6V reference at the error-
amplifier input. The MAX16818 will adjust the LED cur-
rent to within 1% accuracy to maintain emitted spectrum
of the light in HBLEDs.
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 inter-
nal 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 resis-
tor 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 qui-
escent 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 frequen-
cy 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 refer-
ence takes over and the DAC output continues to climb
to 0.7V, assuring that it does not interfere with the refer-
ence voltage.
Internal Oscillator
The internal oscillator generates a clock with the fre-
quency proportional to the inverse of R_T. The oscillator
frequency is adjustable from 125kHz to 1.5MHz with
better than 8% accuracy using a single resistor con-
nected from RT/SYNC to SGND. The frequency accura-
cy avoids the over-design, size, and cost of passive
filter components like inductors and capacitors. Use
the following equation to calculate the oscillator fre-
quency:
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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