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1.5MHz, 30A High-Efficiency, LED Driver
with Rapid LED Current Pulsing
( ) × R S × V IN × R CF
Compensation
The main control loop consists of an inner current loop
(inductor current) and an outer LED current loop. The
MAX16818 uses an average current-mode control
scheme to regulate the LED current (Figure 7). The VEA
output provides the controlling voltage for the current
Boost:
f C _ buck =
9 . 488 mS V
2 π × L
source. The inner current loop absorbs the inductor pole
reducing the order of the LED current loop to that of a
single-pole system. The major consideration when
designing the current control loop is making certain that
f C _ boost =
A V × g m × R S × V LED × R CF
V RAMP × 2 π × L
the inductor downslope (which becomes an upslope at
the output of the CEA) does not exceed the internal
ramp slope. This is a necessary condition to avoid sub-
harmonic oscillations similar to those in peak current
which becomes:
f C _ boost =
( 9.488mS V ) × R S × V LED × R CF
2 π × L
mode with insufficient slope compensation. This requires
that the resistance, R CF , at the output of the CEA be lim-
ited, based on the following equation (Figure 6):
Buck:
For adequate phase margin, place the zero formed by
R CF and C CZ not more than 1/3 to 1/5 of the crossover
frequency. The pole formed by R CF and C CP may not
be required in most applications but can be added to
R CF ≤
V RAMP × f SW × L
A V × g m × R S × V LED
minimize noise at a frequency at or above the switching
frequency.
Power Dissipation
where V RAMP = 2V, g m = 550μS, and A V = 34.5.
The TQFN is a thermally enhanced package and can dis-
sipate about 2.7W. The high-power package makes the
Boost:
R CF ≤ 105 ×
f SW × L
R S × V LED
high-frequency, high-current LED driver possible to oper-
ate 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 ):
R CF ≤
V RAMP × f SW × L
A V × g m × R S × ( V LED ? V IN )
P D = V IN x I CC
I CC = I Q + [ f SW x ( Q G 1 + Q G 2 ) ]
R CF ≤ 1005 ×
f SW × L
R S × ( V LED ? V IN )
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 estimated from the Supply Current (I Q ) vs. Frequency
The crossover frequency of the inner current loop is
expressed as:
Buck:
graph in the Typical Operating Characteristics, and f SW
is the switching frequency of the LED driver. For boost
drivers, only consider one gate charge, Q G1 .
Use the following equation to calculate the maximum
f C _ buck =
A V × g m × R S × V IN × R CF
V RAMP × 2 π × L
power dissipation (P DMAX ) in the chip at a given ambi-
ent temperature (T A ):
P DMAX = 34.5 x (150 - T A ) mW.
When A V = 34.5, g m = 550μS, and V RAMP = 2V, this
becomes:
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