MAX16838B Datasheet, PDF (16/21 Pages) Maxim Integrated Products – Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller

MAX16838B
Integrated, 2-Channel, High-Brightness LED Driver
with High-Voltage Boost and SEPIC Controller
Calculate the minimum inductance value L_MIN with the
inductor current ripple set to the maximum value:
L_MIN = V_{IN_MIN} x D_MAX/(f_SW x IL_P-P)
Choose an inductor that has a minimum inductance
greater than the calculated L_MIN and current rating
greater than IL_PEAK. The recommended saturation
current limit of the selected inductor is 10% higher than the
inductor peak current. The IL_P-P can be chosen to have a
higher ripple than 40%. Adjust the minimum value of the
inductance according to the chosen ripple. One fact that
must be noted is that the slope compensation is fixed and
has a 120mV peak per switching cycle. The dv/dt of the
slope-compensation ramp is 120f_SWV/µs, where f_SW is
in kHz. After selecting the inductance it is necessary to
verify that the slope compensation is adequate to prevent
subharmonic oscillations. In the case of the boost, the
following criteria must be satisfied:
120f_SW > R_CS (V_LED - 2V_{IN_MIN})/2L
where L is the inductance value in µH, R_CS is the current-
sense resistor value in Ω, V_{IN_MIN} is the minimum input
voltage in V, V_LED is the output voltage, and f_SW is the
switching frequency in kHz.
If the inductance value is chosen to keep the inductor in
discontinuous-conduction mode, the equation above does
not need to be satisfied.
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 out-
put capacitance is high, especially at higher duty cycles.
Calculate the output capacitor (C_OUT) using the following
equation:
C_OUT > (D_MAX x I_LED)/(V_{LED_P-P} x f_SW)
where V_{LED_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 C_IN using the
following equation:
C_IN = IL_P-P/(8 x f_SW x V_{IN_P-P})
where V_{IN_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:
I_D = IL _AVG (1 - D_MAX ) (A)
Feedback Compensation
The voltage-feedback loop needs proper compensation
for stable operation. This is done by connecting a resistor
(R_COMP) and capacitor (C_COMP) in series from COMP
to SGND. R_COMP is chosen to set the high-frequency
integrator gain for fast transient response, while C_COMP
is chosen to set the integrator zero to maintain loop
stability. For optimum performance, choose the
components using the following equations:
R _COMP
=
f_ZRHP × R _CS × I_LED
5 × FP1× GM_COMP × V_LED × (1−
D_MAX )
where
f_ZRHP =
V_LED(1− D_MAX )²
2π × L × I_LED
is the right-half plane zero for the boost regulator.
R_CS is the current-sense resistor in series with the
source of the internal switching MOSFET. I_LED is the total
LED current that is the sum of the LED currents in both
the channels. V_LED is the output voltage of the boost
regulator. D_MAX is the maximum duty cycle that occurs at
minimum input voltage. GM_COMP is the transconductance
of the error amplifier.
FP1 =
I_LED
2 × π × V_LED × C_OUT
is the output pole formed by the boost regulator.
Set the zero formed by R_COMP and C_COMP a decade
below the crossover frequency. Using the value of R_COMP
from above, the crossover frequency is at f_ZRHP/5:
C_COMP =
50
2π × R _COMP × f_ZRHP
www.maximintegrated.com
Maxim Integrated │  16