MAX16814_10 Datasheet, PDF (18/25 Pages) Maxim Integrated Products – Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller

Integrated, 4-Channel, High-Brightness LED
Driver with High-Voltage DC-DC Controller
Use the following equation to calculate the value of slope
compensation resistance, RSCOMP.
For boost configuration:
( ) _{RSCOMP =}
V_LED − 2V_{IN_MIN} × R_CS × 3
L_MIN × 50FA × f_SW × 4
For SEPIC and coupled-inductor boost-buck:
( ) _{RSCOMP =}
V_LED − V_{IN_MIN} × R_CS × 3
L_MIN × 50FA × f_SW × 4
where VLED and VIN_MIN are in volts, RSCOMP and RCS
are in ohms, LMIN is in henries and fSW is in hertz.
The value of the switch current-sense resistor, RCS, can
be calculated as follows:
For boost:
( ( ) ) _{0.396×0.9 = ILP ×RCS +}
D_MAX ×
V_LED − 2V_{IN_MIN}
4×L_MN ×f_SW
×R_CS × 3
For SEPIC and boost-buck:
( ( ) ) _{0.396×0.9 = ILP ×RCS+}
D_MAX ×
V_LED − V_{IN_MIN}
4×L_MN ×f_SW
×R_CS × 3
where 0.396 is the minimum value of the peak cur-
rent-sense threshold. The current-sense threshold also
includes the slope compensation component. The mini-
mum current-sense threshold of 0.396 is multiplied by
0.9 to take tolerances into account.
Output Capacitor Selection
For all the three converter topologies, the output capaci-
tor supplies the load current when the main switch is
on. The function of the output capacitor is to reduce the
converter output ripple to acceptable levels. The entire
output-voltage ripple appears across constant current-
sink outputs because the LED string voltages are stable
due to the constant current. For the MAX16814, limit
the peak-to-peak output voltage ripple to 200mV to get
stable output current.
The ESR, ESL, and the bulk capacitance of the output
capacitor contribute to the output ripple. In most of the
applications, using low-ESR ceramic capacitors can
dramatically reduce the output ESR and ESL effects.
To reduce the ESL and ESR effects, connect multiple
ceramic capacitors in parallel to achieve the required
bulk capacitance. To minimize audible noise during
PWM dimming, the amount of ceramic capacitors on the
output are usually minimized. In this case, an additional
electrolytic or tantalum capacitor provides most of the
bulk capacitance.
External MOSFET Selection
The external MOSFET should have a voltage rating suf-
ficient to withstand the maximum output voltage together
with the rectifier diode drop and any possible overshoot
due to ringing caused by parasitic inductances and
capacitances. The recommended MOSFET V_DS voltage
rating is 30% higher than the sum of the maximum output
voltage and the rectifier diode drop.
The recommended continuous drain current rating of the
MOSFET (ID), when the case temperature is at +70NC, is
greater than that calculated below:
ID_RMS
=


IL
2
AVG
×
D_MAX


×
1.3
The MOSFET dissipates power due to both switching
losses and conduction losses. Use the following equa-
tion to calculate the conduction losses in the MOSFET:
P_COND
=
IL
2
AVG
× D_MAX
× R_DS(ON)
where RDS(ON) is the on-state drain-to-source resistance
of the MOSFET.
Use the following equation to calculate the switching
losses in the MOSFET:
P_SW
=
IL _AVG
× V_LED2
2
× C_GD
× f_SW
×
1
_IGON
+
1
I_GOFF



where IGON and IGOFF are the gate currents of the
MOSFET in amperes, with VGS at the threshold voltage
in volts, when it is turned on and turned off, respectively.
CGD is the gate-to-drain MOSFET capacitance in farads.
Rectifier Diode Selection
Using a Schottky rectifier diode produces less forward
drop and puts the least burden on the MOSFET during
reverse recovery. A diode with considerable reverse-
recovery time increases the MOSFET switching loss.
Select a Schottky diode with a voltage rating 20% higher
than the maximum boost-converter output voltage and
current rating greater than that calculated in the follow-
ing equation:
I_D
=
1.2 ×IL _AVG
1− D_MAX
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