for thermal foldback function. The MAX16806 EV kit
also includes connection for the 5V-regulated output
and access to the on-board current-sense resistors.
PWM Dimming
The MAX16806 EV kit features wide-range dimming to
control the LED brightness by varying the duty cycle of a
PWM input signal, or by varying the amplitude of an ana-
log input voltage. Trim pot R3 generates the analog volt-
age needed to evaluate the analog-control PWM dimming
from the regulated 5V output, whereas the PWM signal
should be driven externally. For the analog-control PWM
dimming, an internal 200Hz ramp is compared against
the analog input and the output is chopped at 200Hz. The
peak value of the internal ramp can be adjusted through
I
2
C interface and is explained in the
Detailed Description
of Software
section
.
When an external PWM signal is used for dimming, the
chopping frequency is between 100Hz to 2kHz, and the
PWM amplitude is between 4V to 40V. An on-board
momentary switch provides active-low signal to the
switch input when pressed, and activates a bistable
latch in the MAX16806 to enable or disable the analog
dimming function. The momentary switch does not dis-
able dimming by the external PWM control signal or
dimming caused by thermal or LED current foldback.
Output Current Setting
Users select between three output current levels by set-
ting jumper J1 (see Table 1 for jumper settings). The
output current can be set to 150mA, 250mA, or 350mA.
The current-sense resistor is accessible through the
RSNS connector. The output current is adjusted by
removing R2 or R3, opening all the pins of J1, and con-
necting a resistor across RSNS with values calculated
using the following equation:
where RSNS is the external current-sense resistor,
0.198V is the factory-set current-sense reference, and
I
OUT
is the desired output current. If the current-sense
reference voltage is changed using binning adjustment,
the numerator of the above equation has to be modified
with the selected reference voltage.
Power Dissipation
Thermal shutdown turns off the device if power dissipa-
tion in the IC causes the junction temperature to reach
+155°C (typ). An external resistor can be added at the
input to the device or in series with LED to reduce the
power dissipation in the IC. The resistor’s power rating
should be higher than I
2
R. (I is the input current or LED
current, and R is the value of the added resistor.)
Use the following equation to calculate the maximum
LED current that can be drawn from the device without
causing a thermal shutdown:
where 2.760W is the maximum power dissipation capa-
city of the device when mounted on a multilayer board,
as per JEDEC specifications, with ambient temperature
below +70°C. V
IN
is the input-supply voltage and V
LED
is the total operating voltage of the LED string.
5V-Regulated Output
The +5V regulator is used to power other components
from the V5 connector. The 5V output supplies up to
0.5mA of current and is not disabled during PWM off.
Jumper Selection
Three-pin jumper J1 selects between three different out-
put current settings. Three-pin jumper J2 controls the EN
pin of the MAX16806 and enables or disables the device.
Three-pin jumper J3 selects between the on-board ana-
log voltage and external PWM signal for dimming. Close
CFD to disable the current foldback function. Table 1 lists
the jumper options.
Detailed Description of Software
The MAX16806 EV kit GUI software is used to
evaluate the features of the MAX16806 device that are
controllable through the I
2
C interface. The software uses
the CMAXQUSB+ board to generate the I
2
C interface
signals at 400KHz to communicate to the MAX16806
device. The software writes data into, as well as reads
data from, the dynamic registers and the nonvolatile
EEPROM registers of the MAX16806. Modifying contents
of the dynamic registers directly controls the respective
device function. Writing to EEPROM stores all the
dynamic register contents to the corresponding
EEPROM registers, which are copied to dynamic re-gis-
ters every time during power-up.
