MAX16818_15 Datasheet, PDF (23/25 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
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 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 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 subharmonic oscillations
similar to those in peak current mode with insufficient slope
compensation. This requires that the resistance, R_CF, at
the output of the CEA be limited, based on the following
equation (Figure 6)
Buck:
R _CF
≤
A
V_RAMP × f_SW × L
V ^{× g}m ^{× R} S ^{× V}LED
where V_RAMP = 2V, gm = 550µS, and A_V = 34.5.
R _CF
≤
105
×
f_SW × L
R _S × V_LED
Boost:
R _CF
≤
A
V
×
V_RAMP × f_SW × L
g_m × R _S × (V_LED −
V_IN)
R _CF
≤
105
×
R_S
×
f_SW × L
(V_LED −
V_IN)
The crossover frequency of the inner current loop is
expressed:
Buck:
f_{C_buck}
=
A
V
×
g_m × R _S × V_IN × R _CF
V_RAMP × 2π × L
When A_V = 34.5, g_m = 550µS, and V_RAMP = 2V, this
becomes:
f_{C_buck}
=
(9.488mS
V) ×R_S ×
2π × L
V_IN
× R _CF
Boost:
f_{C_boost}
=
A
V
×
g_m ×R_S
V_RAMP
×
×
V_LED × R _CF
2π × L
f_{C_boost}
=
(9.488mS
V) ×R_S ×
2π × L
V_LED
× R _CF
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 minimize
noise at a frequency at or above the switching frequency.
Power Dissipation
The TQFN is a thermally enhanced package and can
dissipate about 2.7W. The high-power package makes the
high-frequency, high-current LED driver possible 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 estimated from the Supply Current (I_Q) vs. Frequency
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
power dissipation (P_DMAX) in the chip at a given ambient
temperature (T_A):
P_DMAX = 34.5 x (150 - T_A) mW.
www.maximintegrated.com
Maxim Integrated │  23