Matrix LED Display Driver

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19

4MHz, which makes the blink frequencies 0.5Hz selec-

also sets the peak current to 40mA, which makes the

segment current adjustable from 2.5mA to 37.5mA in

2.5mA steps:

where:

pF, typically 2pF

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 OSC to GND. This capacitance is usually in the

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

Display-Test Register

The display-test register switches the drivers between

one of two modes: normal and display test. Display-test

mode turns all LEDs on by overriding, but not altering,

all control and digit registers (including the shutdown

register). In display-test mode, eight digits are scanned

and the duty cycle is 7/16 (half power). Table 22 lists

the display-test register format.

Applications Information

Choosing Supply Voltage to Minimize

Power Dissipation

The MAX6953 drives a peak current of 40mA into LEDs

with a 2.4V forward-voltage drop when operated from a

supply voltage of at least 3.0V. The minimum voltage

drop across the internal LED drivers is therefore (3.0V -

2.4V) = 0.6V. If a higher supply voltage is used, the dri-

ver absorbs a higher voltage, and the driver’s power

dissipation increases accordingly. However, if the LEDs

used have a higher forward voltage 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 most systems, consumption is an

important design criterion, and the MAX6953 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 that the dissipation limit for

the MAX6953 is not exceeded. This can be achieved

by inserting a series resistor in the supply to the

MAX6953, ensuring that the supply decoupling capaci-

tors are still on the MAX6953 side of the resistor. For

example, consider the requirement that the minimum

supply voltage to a MAX6953 must be 3.0V, and the

input supply range is 5V ±5%. Maximum supply current

is:

Minimum input supply voltage is 4.75V. Maximum

series resistor value is:

(4.75V - 3.0V) / 0.415A = 4.22Ω

We choose 3.3Ω ±5%. Worst-case resistor dissipation

(3.3Ω x 1.05) = 0.577W. We choose a 1W resistor rat-

ing. The maximum MAX6953 supply voltage is at maxi-

mum input supply voltage and minimum toleranced

REGISTER DATA

MODE

ADDRESS

CODE (HEX)

D7

D6

D5

D4

D3

D2

D1

D0

Normal operation

0x07

X

X

X

X

X

X

X

0

Display test

0x07

X

X

X

X

X

X

X

1

Table 22. Display-Test Register Format