MAX16818_09 Datasheet, PDF (21/25 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
Buck Regulator
Estimate the power loss (PD_MOS_) caused by the high-side
and low-side MOSFETs using the following equations:
PD_{MOS − HI} = (Q_G x V_DD x f_SW) +
⎛
⎝⎜
V_IN
x I_OUT
x (t_R
2
+
t_F)
x
f_SW
⎞
⎠⎟
+ (R_DS(ON) x I_{RMS − HI2})
where Q_G, R_DS(ON), t_R, and t_F are the upper-switching
MOSFET’s total gate charge, on-resistance at maximum
operating temperature, rise time, and fall time, respectively.
I_{RMS − HI} =
(I_VALLEY2
+
I_PK2
+
I_VALLEY
x
I_PK)
x
D
3
For the buck regulator, D = V_LEDs / V_IN, I_VALLEY =
(I_OUT - ∆I_L / 2) and I_PK = (I_OUT + ∆I_L / 2).
PD_{MOS − LO} = (Q_G x V_DD x f_SW) +
(R_DS(ON) x I_{RMS − LO2})
I_{RMS − LO} =
(I_VALLEY2
+
I_PK2
+
I_VALLEY
x
I_PK)
x
(1− D)
3
For example, from the typical specifications in the
Applications Information section with V_OUT = 7.8V, the
high-side and low-side MOSFET RMS currents are
0.77A and 0.63A, respectively, for a 1A buck regulator.
Ensure that the thermal impedance of the MOSFET
package keeps the junction temperature at least +25°C
below the absolute maximum rating. Use the following
equation to calculate the maximum junction tempera-
ture: T_J = (PD_MOS x θ_JA) + T_A, where θ_JA and T_A are
the junction-to-ambient thermal impedance and ambi-
ent temperature, respectively.
To guarantee that there is no shoot-through from V_IN to
PGND, the MAX16818 produces a nonoverlap time of
35ns. During this time, neither high- nor low-side MOS-
FET is conducting, and since the output inductor must
maintain current flow, the intrinsic body diode of the
low-side MOSFET becomes the conduction path. Since
this diode has a fairly large forward voltage, a Schottky
diode (in parallel to the low-side MOSFET) diverts current
flow from the MOSFET body diode because of its lower
forward voltage, which, in turn, increases efficiency.
Boost Regulator
Estimate the power loss (PD_MOS_) caused by the MOS-
FET using the following equations:
PD_FET = (Q_G x V_DD x f_SW) +
⎛
⎝⎜
V_IN
x I_OUT
x (t_R
2
+
t_F)
x
f_SW
⎞
⎠⎟
+
(R_DS(ON)
x I_{RMS − HI2})
I_{RMS − HI} =
(I_VALLEY2
+
I_PK2
+
I_VALLEY
x
I_PK )
x
D
3
For a boost regulator in continuous mode, D = V_LEDs /
(V_IN + V_LEDs), I_VALLEY = (I_OUT - ∆I_L / 2) and I_PK = (I_OUT
+ ∆I_L / 2).
The voltage across the MOSFET:
V_MOSFET = V_LED + V_F
where V_F is the maximum forward voltage of the diode.
The output diode on a boost regulator must be rated to
handle the LED series voltage, V_LED. It should also
have fast reverse-recovery characteristics and should
handle the average forward current that is equal to the
LED current.
Input Capacitors
For buck regulator designs, the discontinuous input
current waveform of the buck converter causes large
ripple currents in the input capacitor. The switching fre-
quency, peak inductor current, and the allowable peak-
to-peak voltage ripple reflected back to the source
dictate the capacitance requirement. Increasing switch-
ing frequency or paralleling out-of-phase converters
lowers the peak-to-average current ratio, yielding a
lower input capacitance requirement for the same LED
current. The input ripple is comprised of ∆V_Q (caused
by the capacitor discharge) and ∆V_ESR (caused by the
ESR of the capacitor). Use low-ESR ceramic capacitors
with high-ripple-current capability at the input. Assume
the contributions from the ESR and capacitor discharge
are equal to 30% and 70%, respectively. Calculate the
input capacitance and ESR required for a specified ripple
using the following equation:
ESR_IN
=
∆V_ESR
⎛⎝⎜_{IOUT +}
∆I_L
2
⎞
⎠⎟
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