Detailed Description
The MAX6978 LED driver comprises a 4-wire serial
interface driving eight constant-current sinking open-
drain output ports. The outputs drive LEDs in either sta-
tic or multiplex applications (Figure 1). The constant-
current outputs are guaranteed for current accuracy
not only with chip-supply voltage variations (5V ±10%
and 3V to 5.5V), but also over a realistic range of driver
output voltage drop (0.5V to 2.5V). The drivers use cur-
rent-sensing feedback circuitry (not simple current mir-
rors) to ensure very small current variations over the full
allowed range of output voltage (see the Typical
Operating Characteristics).
The 4-wire serial interface comprises an 8-bit shift reg-
ister and an 8-bit transparent latch. The shift register is
written through a clock input CLK and a data input DIN
and the data propagates to a data output DOUT. The
data output allows multiple drivers to be cascaded and
operated together. The contents of the 8-bit shift regis-
ter are loaded into the transparent latch through a
latch-enable input LE. The latch is transparent to the
shift register outputs when high, and latches the current
state on the falling edge of LE.
Each driver output is an open-drain constant-current
sink that should be connected to the cathode of either
a single LED or a series string of multiple LEDs. The
LED anode can be connected to a supply voltage of up
to 5.5V, independent of the MAX6978 supply, V+. The
constant-current capability is up to 55mA per output,
set for all eight outputs by an external resistor, R
SET
.
Initial Power-Up and Operation
An internal reset circuit clears the internal registers of
the MAX6978 on power-up. All outputs OUT0–OUT7,
therefore, initialize high impedance, and the interface
output DOUT initializes low, regardless of the initial
logic levels of the CLK, DIN, OE, and LE inputs.
4-Wire Serial Interface
The serial interface on the MAX6978 is a 4-wire serial
interface using four inputs (DIN, CLK, LE, OE) and a
data output (DOUT). This interface is used to write dis-
play data to the MAX6978. The serial-interface data
word length is 8 bits, D0–D7.
The functions of the five interface pins are as follows.
DIN is the serial-data input, and must be stable when it
is sampled on the rising edge of CLK. Data is shifted in,
MSB first. This means that data bit D7 is clocked in first,
followed by 7 more data bits, finishing with the LSB D0.
CLK is the serial-clock input, which shifts data at DIN
into the MAX6978 8-bit shift register on its rising edge.
LE is the latch load input of the MAX6978, which trans-
fers data from the MAX6978 8-bit shift register to its 8-bit
latch when LE is a high (transparent latch), and latches
the data on the LE falling edge of LE (Figure 2). The ris-
ing edge of LE loads fault data into the MAX6978 8-bit
shift register, replacing bits D6 and D5 with fault data.
This fault data is automatically shifted out through DOUT
when the next display data is shifted in.
The fourth input provides output-enable control of the
output drivers. OE is high to force outputs OUT0–OUT7
high impedance, without altering the contents of the
output latches, and low to enable outputs OUT0–OUT7
to follow the state of the output latches.
MAX6978
8-Port, 5.5V Constant-Current LED Driver with
LED Fault Detection and Watchdog
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Table 1. 4-Wire Serial-Interface Truth Table
