MAX16838_11 Datasheet, PDF(16/21 Page) Maxim Integrated Products – Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller

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Integrated, 2-Channel, High-Brightness LED Driver

with High-Voltage Boost and SEPIC Controller

Output Capacitor Selection in

Boost Configuration

For the boost converter, the output capacitor

supplies the load current when the main switch is on.

The required output capacitance is high, especially at

higher duty cycles.

Calculate the output capacitor (COUT) using the follow-

ing equation:

COUT > (DMAX x ILED)/(VLED_P-P x fSW)

where VLED_P-P is the peak-to-peak ripple in the LED

supply voltage. Use a combination of low-ESR and high-

capacitance ceramic capacitors for lower output ripple

and noise.

Input Capacitor Selection in Boost Configuration

The input current for the boost converter is continuous

and the RMS ripple current at the input capacitor is low.

Calculate the minimum input capacitor CIN using the fol-

lowing equation:

CIN = ILP-P/(8 x fSW x VIN_P-P)

where VIN_P-P is the peak-to-peak input ripple voltage.

This equation assumes that input capacitors supply

most of the input ripple 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. 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 following equation:

ï£«

ID = 1.2 Ã ï£¬ï£¬ï£

ILAVG

1- DMAX

ï£¶

ï£·ï£·ï£¸ ( A )

Feedback Compensation

The voltage feedback loop needs proper compensa-

tion for stable operation. This is done by connecting

a resistor (RCOMP) and capacitor (CCOMP) in series

from COMP to SGND. RCOMP is chosen to set the high-

frequency integrator gain for fast transient response,

while CCOMP is chosen to set the integrator zero to main-

tain loop stability. For optimum performance, choose the

components using the following equations:

RCOMP

FP1Ã

fZRHP Ã RCS ÃILED

GMCOMP Ã VLED Ã (1â

DMAX )

where

fZRHP =

VLED(1â DMAX )2

2Ï Ã L ÃILED

is the right-half plane zero for the boost regulator.

RCS is the current-sense resistor in series with the

source of the internal switching MOSFET. ILED is the total

LED current that is the sum of the LED currents in both

the channels. VLED is the output voltage of the boost

regulator. DMAX is the maximum duty cycle that occurs

at minimum input voltage. GMCOMP is the transconduc-

tance of the error amplifier.

FP1 =

ILED

2 Ã Ï Ã VLED Ã COUT

is the output pole formed by the boost regulator.

Set the zero formed by RCOMP and CCOMP a decade

below the crossover frequency. Using the value of

RCOMP from above, the crossover frequency is at

fZRHP/5.

CCOMP =

2Ï Ã RCOMP Ã fZRHP

▷