Rainbow Electronics MAX16825 - Manual (Page 9)

On-board pass elements minimize the need for external
components while providing ±5% LED current accura-
cy. Additional features include a +5V (±5%) regulated
output with 4mA output current capability and thermal
protection.
The MAX16824/MAX16825 use a feedback loop to lin-
early control the current at each output. The voltage
across each sense resistor is regulated to 200mV by
the internal feedback loop. The output current is set by
selecting the value of R
CS
.
+5V Regulator (REG)
The MAX16824/MAX16825 include a fixed +5V output
regulator that delivers up to 4mA of load current for
auxiliary low power applications throughout the 6.5V to
28V input voltage range. Connect a 1µF compensation
capacitor from REG to ground.
Thermal Protection
The MAX16824/MAX16825 enter a thermal shutdown
mode in the event of overheating. When the junction
temperature exceeds T
J = +165°C, the internal thermal
protection circuit turns off the series pass elements.
The MAX16824/MAX16825 recover from thermal shut-
down mode when the junction temperature drops by
23°C. The thermal protection does not operate if the
regulator (REG) is in a short-circuit condition.
4-Wire Serial Interface (MAX16825)
The MAX16825 features a 4-wire serial interface (DIN,
CLK, LE, OE) and a data output (DOUT) that allows the
use of a microcontroller to write brightness data to the
MAX16825. The serial-interface data word length is 3
bits (D0, D1, D2). The functions of the interface inputs
are as follows: DIN is the serial-data input, which must
be stable when it is sampled by the MAX16825 on the
rising edge of CLK. Data shifts in with the MSB first.
This means that data bit D2 is clocked in first, followed
by 2 more data bits (D1 and finishing with the LSB D0).
CLK is the serial-clock input, which shifts data at DIN
into the MAX16825’s 3-bit shift register on the rising
edge. LE is the latch-enable input that allows the trans-
fer of data from the MAX16825’s 3-bit shift register to its
3-bit latch when LE is high (transparent latch) and
latches the data on the falling edge of LE (Figure 4).
Output enable (OE) provides simultaneous control of
the output drivers. Driving OE high places outputs
OUT1, OUT2, and OUT3 into a high-impedance mode
without altering the contents of the output latches.
Driving OE low allows outputs OUT1, OUT2, and OUT3
to follow the state of the output latches.
OE is independent of the operation of the serial inter-
face. Data can be shifted into the serial-interface shift
register and latched regardless of the state of OE.
DOUT is the serial-data output that shifts data out from
the MAX16825’s 3-bit shift register on the rising edge of
CLK. Data at DIN propagates through the shift register
and appears at DOUT three clock cycles later.
Applications Information
Programming the LED Current
The MAX16824/MAX16825 use sense resistors to set the
output current in each channel. To set the LED current
for a particular channel, connect a sense resistor across
the corresponding current-sense input (CS_) and GND.
For better performance, connect the low side of the cur-
rent-sense resistors to the IC’s ground terminal and the
high side to the CS_ terminal with short traces. The value
of the sense resistor for a given desired current is calcu-
lated with the following equation:
where V CS_ is 200mV and I OUT_ is I LED (see the
Electrical Characteristics table).
Input Voltage Considerations
For proper operation, the minimum input voltage must
always be:
where V
CS_
is the voltage drop across the sense resis-
tor R CS _, V FT(MAX) is the total forward voltage of all
series connected LEDs and ΔV DO is the maximum
dropout voltage of the regulator. The minimum operat-
ing voltage of the device is 6.5V. If the device is operat-
ed below 6.5V, the output current may not meet the full
regulation specification (see the
Typical Operating
Characteristics ).
Pulse-Width Dimming (MAX16824)
The MAX16824 includes pulsed-width current dimming
inputs (PWM_) to control the LED brightness. An appli-
cation of up to 5kHz signal or less is recommended at
PWM_ input for proper operation. PWM_ also functions
as an active-high enable input for each output channel.
A logic-low at PWM_ turns off OUT_ and a logic-high
turns on OUT_.
V V V V
IN MIN CS FT MAX DO ( ) _ ( )
≥ + + Δ
R
V V
I A
CS
CS
OUT
_
_
_
( )
( )
( )
Ω =
MAX16824/MAX16825
High-Voltage, Three-Channel Linear
High-Brightness LED Drivers
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