MAX16814_15 Datasheet, PDF (18/25 Pages) Maxim Integrated Products – Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller

MAX16814
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:
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
sufficient 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 VDS 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
( ( ) ) _{0.396×0.9 = ILP ×RCS +}
D_MAX ×
V_LED − 2V_{IN_MIN}
4×L_MN ×f_SW
×R_CS × 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:
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
P_COND =
IL
2
AVG
× D_MAX
× R_DS(ON)
where RDS(ON) is the on-state drain-to-source resistance
of the MOSFET.
where 0.396 is the minimum value of the peak cur- Use the following equation to calculate the switching
rent-sense threshold. The current-sense threshold also losses in the MOSFET:
includes the slope compensation component. The mini-
mum current-sense threshold of 0.396 is multiplied by
0.9 to take tolerances into account.
= P_SW
IL _AVG
× V_LED2
2
× C_GD
× f_SW
×
1
_IGON
+
1
I_GOFF



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
where IGON and IGOFF are the gate currents of the
MOSFET in amperes, when it is turned on and turned
off, respectively. CGD is the gate-to-drain MOSFET
capacitance in farads.
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.
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.
The ESR, ESL, and the bulk capacitance of the output Select a Schottky diode with a voltage rating 20% higher
capacitor contribute to the output ripple. In most of the than the maximum boost-converter output voltage and
applications, using low-ESR ceramic capacitors can current rating greater than that calculated in the follow-
dramatically reduce the output ESR and ESL effects. ing equation:
To reduce the ESL and ESR effects, connect multiple
ceramic capacitors in parallel to achieve the required
bulk capacitance. To minimize audible noise during
= I_D IL _AVG × (1 − D_MAX) x 1.2
PWM dimming, the amount of ceramic capacitors on the
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