Datasheet

15Maxim Integrated
Integrated, 2-Channel, High-Brightness LED Driver
with High-Voltage Boost and SEPIC Controller
MAX16838
Enable (EN)
EN is a logic input that completely shuts down the
device when connected to logic-low, reducing the
current consumption of the device to less than 15FA
(typ). The logic threshold at EN is 1.24V (typ). The volt-
age at EN must exceed 1.24V before any operation can
commence. There is a 71mV hysteresis on EN. The EN
input also allows programming the supply input UVLO
threshold using an external voltage-divider to sense the
input voltage, as shown in Figure 3. Use the following
equation to calculate the value of R1
EN
and R2
EN
in
Figure 3:

= −×


ON
EN EN
UVLOIN
V
R1 1 R2
V
where V
UVLOIN
is the EN rising threshold (1.24V) and
V
ON
is the desired input startup voltage. Choose an
R2
EN
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 V
IN
. 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 V
CFB
.
This feature can also be used for analog dimming of the
LEDs. Connect CFB to V
CC
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
current sink (V
LED
), and the total output current needed
to drive the LED strings (I
LED
).
Calculate the maximum duty cycle (D
MAX
) using the
following equation:
D
MAX
= (V
LED
+ V
D
– V
IN_MIN
)/(V
LED
+ V
D
)
where V
D
is the forward drop of the rectifier diode,
V
IN_MIN
is the minimum input supply voltage, and
V
LED
is the output voltage. Select the switching
frequency (f
SW
) depending on the space, noise, dynam-
ic response, and efficiency constraints.
Inductor Selection in Boost Configuration
Select the maximum peak-to-peak ripple on the inductor
current (IL
P-P
). Use the following equations to calculate
the maximum average inductor current (IL
AVG
) and
peak inductor current (IL
PEAK
):
IL
AVG
= I
LED
/(1 - D
MAX
)
Assuming IL
P-P
is 40% of the average inductor current:
IL
P-P
= IL
AVG
x 0.4
IL
PEAK
= IL
AVG
+ IL
P-P
/2
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 compensa-
tion is fixed and has a 120mV peak per switching cycle.
The dv/dt of the slope compensation ramp is 120f
SW
V/
Fs, 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 FH, R
CS
is the
current-sense resistor value in ω, V
IN_MIN
is the mini-
mum 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.
Figure 3. Setting the MAX16838 Undervoltage Lockout
Threshold
MAX16838
1.24V
EN
R1
EN
R2
EN
V
IN