Maxim Integrated MAX8790A Evaluation Kit MAX8790AEVKIT+ MAX8790AEVKIT+ Data Sheet

The MAX8790A provides a 100:1 dimming range
through analog or digital control methods. In both meth-
ods, the BRT input signal is translated into a control sig-
nal for each of the six current sources. By varying the
BRT duty cycle, the LED string current (I

LED

) is adjust-

ed between 0mA and the full-scale current setting. See
the 

Setting Full-Scale LED Current (I

LED(FS)

) section for

details on configuring the full-scale current. To select
the dimming mode, configure jumper JU3 as shown in
Table 5, and see the 

Direct DPWM Dimming Mode and

Analog + DPWM Dimming Mode sections for additional
information.

To enable DPWM dimming, configure jumper JU3 as
listed in Table 5, and ensure that a shunt is installed
across jumper JU5.

In direct DPWM dimming mode, I

LED

is a PWM signal

whose amplitude is equal to the full-scale current set-
ting. The duty cycle of the I

LED

signal is synchronized

with the BRT signal, such that at 50% BRT duty cycle
the I

LED

duty cycle are also 50%. While in direct DPWM

dimming mode, the internal phase-locked loop (PLL) is
not used and does not limit the BRT frequency. Refer to
the MAX8790A IC data sheet for information on deter-
mining the maximum allowable BRT frequency.

To enable analog + DPWM dimming, configure jumper
JU3 as listed in Table 5, and ensure that a shunt is not
installed across jumper JU5.

In analog dimming mode, I

LED

is a DC signal whose

amplitude is modulated by the duty cycle of the BRT
input signal. Therefore, at 50% BRT duty cycle, the I

LED

DC signal is set to 50% of the full-scale current. While in
analog dimming mode, the BRT frequency is limited to
the capture range of the internal PLL. The free-running
frequency of the PLL is set by R3 to 244Hz, providing a
capture range of 147Hz to 244Hz. Refer to the
MAX8790A IC data sheet for information on configuring
the PLL’s free-running frequency.

This analog control scheme holds true down to a BRT
duty cycle of 12.5%. Below 12.5%, the string currents
are modulated through a combination of analog and
digital dimming schemes. Refer to the MAX8790A IC
data sheet for further details.

As configured, the MAX8790A EV kit is assembled with
two strings of eight WLEDs (String1 and String2), but
can be reconfigured to drive up to six strings of eight
WLEDs. Each string has an associated 3-pin header
(JU6–JU11) and feedback pin (FB1–FB6). To evaluate
additional strings (String3–String6), the EV kit must be
reconfigured as follows:

1) Populate the String_ with either the WLED listed in

the 

Component List or its equivalent.

2) Cut the trace (solder side) between pins 2-3 of the

string’s associated header-footprint (JU8–JU11).

3) Install a 3-pin header (JU8–JU11) and configure its

shunt according to Table 6.

To evaluate the EV kit with off-board WLED strings, see
the 

Off-Board White LED String Configuration section.

Installed

Connected to VCC

20mA

Not installed*

Connected to GND

through R1 and R2

Adjustable from

15mA to 25mA

*

Default position.

1-2

Connected to VCC

Not installed (connected to GND through R3)

Analog + DPWM

2-3*

Connected to GND

Installed (connected to VCC)

Direct DPWM

*

Default position.