MAX16818_09 Datasheet, PDF (23/25 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
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 sub-
harmonic 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 lim-
ited, based on the following equation (Figure 6):
Buck:
R_CF
≤
V_RAMP × f_SW × L
A_V × g_m × R_S × V_LED
where V_RAMP = 2V, g_m = 550µS, and A_V = 34.5.
Boost:
R_CF
≤ 105 × ^{fSW × L}
R_S × V_LED
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 as:
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 × V_IN
2π ×L
× R_CF
Boost:
f_{C _ boost}
=
A_V
× g_m × R_S × V_LED × R_CF
V_RAMP × 2π × L
which becomes:
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 dis-
sipate about 2.7W. The high-power package makes the
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):
P_D = V_IN x I_CC
[ ] ^ICC ^{= 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 ambi-
ent temperature (T_A):
P_DMAX = 34.5 x (150 - T_A) mW.
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