MAX16838B Datasheet, PDF (15/21 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
Short-LED Detection
The device features two-level short-LED detection
circuitry. If a level 1 short is detected on any one of the
strings, FLT is asserted. A level 1 short is detected if the
difference between the total forward LED voltages of
the two strings exceeds 4.2V (typ). If a level 2 short is
detected on any one of the strings, the particular LED string
with the short is turned off after 6μs and FLT is asserted.
A level 2 short is detected if the difference between the
total forward LED voltages of the two strings exceeds
7.8V (typ). The strings are reevaluated on each DIM
rising edge and FLT is deasserted if the short is removed.
Short-LED detection is disabled when PWM pulse width
is less than 20 switching clock cycles (DIM pulse width
decreasing).
Enable (EN)
EN is a logic input that completely shuts down the device
when connected to logic-low, reducing the current con-
sumption of the device to less than 15μA (typ). The logic
threshold at EN is 1.24V (typ). The voltage 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 volt-
age, as shown in Figure 3. Use the following equation to
calculate the value of R1_EN and R2_EN in Figure 3:
= R1_EN



V_ON
V_UVLOIN
−

1_

×
R2 _EN
where V_UVLOIN is the EN rising threshold (1.24V) and
V_ON is the desired input startup voltage. Choose an R2_EN
between 10kΩ and 50kΩ. Connect EN to IN if not used.
Current Foldback
The device 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 proportionally to V_CFB.
This feature can also be used for analog dimming of the
LEDs. Connect CFB to V_CC to disable this feature.
MAX16838B
EN
1.24V
VIN
R1EN
R2EN
Figure 3. Setting the MAX16838B Undervoltage-Lockout
Threshold
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, dynamic 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
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