Ω

pF, typically 3pF

Ω and the rec-

allowed value, since it sets the display driver to the

a higher value to set the segment current to a lower

peak value where desired. The user must also ensure

that the peak current specifications of the LEDs con-

nected to the driver are not exceeded.

external capacitor used, but also the stray capacitance

from the OSC pin to GND. This capacitance is usually

in the 1pF to 5pF range, depending on the layout used.

LED Maximum Reverse Voltage

The display connection scheme used by the

MAX6950/MAX6951 puts LED segments in reverse bias

during a portion of the multiplexing time. The maximum

applied reverse bias voltage is the value of the supply

voltage, V+. It is therefore important to ensure that the

LEDs chosen are rated to withstand a reverse bias

equal to the maximum supply voltage applied to the

MAX6950/MAX6951.

Applications Information

Choosing Supply Voltage to Minimize

Power Dissipation

The MAX6950/MAX6951 drive a peak current of 40mA

into LEDs with a 2.4V forward-voltage drop when oper-

ated from a supply voltage of at least 3.0V. The mini-

mum voltage drop across the internal LED drivers is

therefore (3.0V - 2.4V) = 0.6V. If a higher supply volt-

age is used, the driver absorbs a higher voltage, and

the driver’s power dissipation increases accordingly.

However, if the LEDs used have a higher forward volt-

age drop than 2.4V, the supply voltage must be raised

accordingly to ensure that the driver always has at least

0.6V headroom.

The voltage drop across the drivers with a nominal +5V

supply (5.0V - 2.4V) = 2.6V is nearly 3 times the drop

across the drivers with a nominal 3.3V supply (3.3V -

2.4V) = 0.9V. In many systems, consumption is an

important design criterion, and the MAX6950/MAX6951

should be operated from the system’s 3.3V nominal

supply. In other designs, the lowest supply voltage may

be 5V. The issue now is to ensure the dissipation limit

for the MAX6950/MAX6951 is not exceeded. This can

be achieved by inserting a series resistor in the supply

to the MAX6950/MAX6951, ensuring that the supply

decoupling capacitors are still on the MAX6950/

MAX6951 side of the resistor. For example, consider

the requirement that the minimum supply voltage to a

MAX6951 must be 3.0V, and the input supply range is

8) = 335mA. Minimum input supply voltage is 4.75V.

Maximum series resistor value is (4.75V - 3.0V)/0.335A

= 5.2

Ω. We choose 4.7Ω ±10%. Worst-case resistor

dissipation is at maximum toleranced resistance, i.e.,

(0.335A)

resistor rating. The maximum MAX6951 supply voltage

is at maximum input supply voltage and minimum toler-

3.83V.

Low-Voltage Operation

The MAX6950/MAX6951 work over the +2.7V to +5.5V

supply range. The minimum useful supply voltage is

determined by the forward voltage drop of the LEDs at

by the driver output stages. The MAX6950/MAX6951

mum voltage. If the supply drops below this minimum

voltage, the driver output stages may brown out, and

Serially Interfaced, +2.7V to +5.5V,

5- and 8-Digit LED Display Drivers

______________________________________________________________________________________

15

d

d

g

a

b

f

c

e

Table 16. No-Decode Mode Data Bits and

Corresponding Segment Lines

REGISTER DATA

D7

D6

D5

D4

D3

D2

D1

D0

Segment line

dp

a

b

cd

e

f

g