MAX16826_15 Datasheet, PDF (22/26 Pages) Maxim Integrated Products – Programmable, Four-String HB LED Driver with Output-Voltage Optimization and Fault Detection

MAX16826
Programmable, Four-String HB
LED Driver with Output-Voltage
Optimization and Fault Detection
Switching Noise Effects
on ADC Readings
Excessive switching noise can corrupt the ADC read-
ings on the current sink MOSFET drains. Proper PCB
layout is critical to minimize this noise. The output diode
should be selected appropriately. The capacitance and
reverse recovery characteristics of the output diode
contribute to ground noise. Diodes with lower capaci-
tance and lower reverse recovery time will result in
lower current spikes at the turn on edge of the boost
switch. The lower current spike will result in lower
ground noise. Another method to reduce the ground
noise is to add a gate resistor in series with the gate of
the switching MOSFET. For most applications the gate
resistor should be in the range of 4.7 to 10ohms. Care
must be taken in selecting the gate resistor value, due
to power dissipation increase on the boost transistor. A
higher resistor value reduces the switching noise, but
power dissipation due to switching losses will increase.
The ground noise should be measured by measuring
the voltage between the ground of the input capacitor
that is farthest from the IC on the PCB and the ground
of the MAX16826. The pins 1,2 and 3 are the ground
pins of the MAX16826. To prevent problems on the
ADC readings of the MAX16826 the ground noise mea-
sured from the IC ground to the input capacitor ground
should be less than 0.5V peak to peak on a wide band-
width scope using a wide bandwidth probe. A wide
bandwidth scope must have a bandwidth greater than
150MHz.
SEPIC Topology
The SEPIC power topology is very useful when the
input voltage is expected to be higher or lower than the
output voltage of the switching regulator stage as
required by the number of LEDs used in a single string.
The SEPIC topology is more complex than the simple
boost topology and it requires the use of two additional
energy storage components, L2 and C25, in Figure 9.
L1
C25
D1
VIN
Q1
R11
C26
R13
R15
C28
L2
R32
R33
R34
R17
R12
R14
R16
VOUT
R35
SYSTEM
µC
DIM
SDA
SCL
ENABLE
GND
C27
GND
GND GND
GND
GND
GND
GND
C29 _R18
IN
DL RSC CS
DIM1
DIM2
DIM3 ^{DIMMING INPUTS}
DIM4
MAX16826
COMP _FB
OVP
DR4
DR3
DR2
DR1
SDA
SCL
I²C INTERFACE
DL1
CS1
DL2
SYNC/EN
CS2
DL3
CS3
DL4
CS4
CSS
RTCT
VCC GND PGND
R19
C30
C33
C32
R27 R25 R23 R21
R20
Q5
R31
C44
R22
Q4
R30
C43
R26
R24
Q3
Q2
R29
R28
C42
C41
GND GND
GND
GND
GND GND GND
GND
Figure 9. SEPIC-Based LED Driver
www.maximintegrated.com
Maxim Integrated | 22