MAX16838 Datasheet, PDF (15/21 Pages) Maxim Integrated Products – Integrated, 2-Channel, High-Brightness LED Driver with High-Voltage Boost and SEPIC Controller

Integrated, 2-Channel, High-Brightness LED Driver
with High-Voltage Boost and SEPIC Controller
Enable (EN)
EN is a logic input that completely shuts down the
device when connected to logic-low, reducing the cur-
rent consumption of the device to less than 15FA (typ).
The logic threshold at EN is 1.24V (typ). The voltage at
EN must exceed 1.24V before any operation can com-
mence. There is a 71mV hysteresis on EN. The EN input
also allows programming the supply input UVLO thresh-
old using an external voltage-divider to sense the input
voltage as shown in Figure 3. Use the following equation
to calculate the value of R1EN and R2EN in Figure 3:
R1_EN =



V_ON
V_UVLOIN

− 1_

× R2_EN
where VUVLOIN is the EN rising threshold, 1.24V, and
VON is the desired input startup voltage. Choose an
R2EN between 10kI and 50kI. Connect EN to IN if not
used.
Current Foldback
The MAX16838 includes a current-foldback feature to
limit the input current at low VIN. Connect a resistor-
divider between IN, CFB, and SGND to set the current-
foldback threshold. When the voltage at CFB goes below
1.23V, then the LED current starts reducing proportion-
ally to VCFB.
This feature can be used for analog dimming of the LEDs,
too. Connect CFB to VCC to disable this feature.
Applications Information
Boost-Circuit Design
First, determine the required input supply voltage range,
the maximum voltage needed to drive the LED strings
including the minimum 1V across the constant LED cur-
rent sink (VLED), and the total output current needed to
drive the LED strings (ILED).
Calculate the maximum duty cycle (DMAX) using the fol-
lowing equation:
DMAX = (VLED + VD – VIN_MIN)/(VLED + VD)
where VD is the forward drop of the rectifier diode,
VIN_MIN is the minimum input supply voltage, and
VLED is the output voltage. Select the switching fre-
quency (fSW) depending on the space, noise, dynamic
response, and efficiency constraints.
MAX16838
EN
1.24V
VIN
R1EN
R2EN
Figure 3. Setting the MAX16838 Undervoltage Lockout
Threshold
Inductor Selection in Boost Configuration
Select the maximum peak-to-peak ripple on the inductor
current (ILP-P). Use the following equations to calculate
the maximum average inductor current (ILAVG) and
peak inductor current (ILPEAK):
ILAVG = ILED/(1 - DMAX)
Assuming ILP-P is 40% of the average inductor current:
ILP-P = ILAVG x 0.4
ILPEAK = ILAVG + ILP-P/2
Calculate the minimum inductance value LMIN with the
inductor current ripple set to the maximum value.
LMIN = VIN_MIN x DMAX/(fSW x ILP-P)
Choose an inductor that has a minimum inductance
greater than the calculated LMIN and current rating
greater than ILPEAK. The recommended saturation cur-
rent limit of the selected inductor is 10% higher than the
inductor peak current. The ILP-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 120fSWV/Fs, where
fSW is in kHz. After selecting the inductance it is neces-
sary to verify that the slope compensation is adequate
to prevent subharmonic oscillations. In the case of the
boost, the following criteria must be satisfied:
120fSW > RCS (VLED - 2VIN_MIN)/2L
where L is the inductance value in FH, RCS is the cur-
rent-sense resistor value in ω, VIN_MIN is the minimum
input voltage in V, VLED is the output voltage, and fSW 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.
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