MAX16818 Datasheet, PDF (21/26 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 - ΔI_L / 2) and I_PK = (I_OUT + ΔI_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 - Δ_L / 2) and I_PK = (I_OUT
+ ΔI_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
switching frequency or paralleling out-of-phase con-
verters lowers the peak-to-average current ratio, yield-
ing a lower input capacitance requirement for the same
LED current. The input ripple is comprised of ΔV_Q
(caused by the capacitor discharge) and ΔV_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%, respec-
tively. Calculate the input capacitance and ESR required
for a specified ripple using the following equation:
ESR_IN
=
ΔV_ESR
⎛⎝⎜_{IOUT +}
ΔI_L
2
⎞
⎠⎟
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