MAX6955 데이터 시트보기 (PDF) - Maxim Integrated

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MAX6955

2-Wire Interfaced, 2.7V to 5.5V LED Display Driver with I/O Expander and Key Scan

Maxim Integrated 

MAX6955

2-Wire Interfaced, 2.7V to 5.5V

LED Display Driver with I/O

Expander and Key Scan

Choosing Supply Voltage to Minimize

Power Dissipation

The MAX6955 drives a peak current of 40mA into LEDs

with a 2.2V 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 -

2.2V) = 0.8V. 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 2.2V, the

supply voltage must be raised accordingly to ensure

that the driver always has at least 0.6V of headroom.

The voltage drop across the drivers with a nominal 5V

supply (5.0V - 2.2V) = 2.8V is nearly 3 times the drop

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

2.2V) = 1.1V. In most systems, consumption is an

important design criterion, and the MAX6955 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

MAX6955 is not exceeded. This can be achieved by

inserting a series resistor in the supply to the MAX6955,

ensuring that the supply decoupling capacitors are still

on the MAX6955 side of the resistor. For example, con-

sider the requirement that the minimum supply voltage

to a MAX6955 must be 3.0V, and the input supply

range is 5V ±5%. Maximum supply current is 35mA +

(40mA x 17) = 715mA. Minimum input supply voltage is

4.75V. Maximum series resistor value is (4.75V -

3.0V)/0.715A = 2.44Ω. We choose 2.2Ω ±5%. Worst-

case resistor dissipation is at maximum toleranced

resistance, i.e., (0.715A) 2 x (2.2Ω x 1.05) = 1.18W. The

maximum MAX6955 supply voltage is at maximum

input supply voltage and minimum toleranced resis-

tance, i.e., 5.25V - (0.715A x 2.2Ω x 0.95) = 3.76V.

Low-Voltage Operation

The MAX6955 works over the 2.7V to 5.5V supply

range. The minimum useful supply voltage is deter-

mined by the forward-voltage drop of the LEDs at the

peak current ISEG, plus the 0.8V headroom required by

the driver output stages. The MAX6955 correctly regu-

lates ISEG with a supply voltage above this minimum

voltage. If the supply drops below this minimum volt-

age, the driver output stages can brown out, and be

unable to regulate the current correctly. As the supply

voltage drops further, the LED segment drive current

becomes effectively limited by the output driver's on-

resistance, and the LED drive current drops. The char-

acteristics of each individual LED in a display digit are

well matched, so the result is that the display intensity

dims uniformly as supply voltage drops out of regula-

tion and beyond.

Computing Power Dissipation

The upper limit for power dissipation (PD) for the

MAX6955 is determined from the following equation:

PD = (V+ x 35mA) + (V+ - VLED) (DUTY x ISEG x N)

where:

V+ = supply voltage

DUTY = duty cycle set by intensity register

N = number of segments driven (worst case is 17)

VLED = LED forward voltage at ISEG

ISEG = segment current set by RSET

PD = Power dissipation, in mW if currents are in mA

Dissipation example:

ISEG = 30mA, N = 17, DUTY = 15/16,

VLED = 2.4V at 30mA, V+ = 3.6V

PD = 3.6V (35mA) + (3.6V - 2.4V)(15/16 x

30mA x 17) = 0.700W

Thus, for a 36-pin SSOP package (TJA = 1/0.0118 =

+85°C/W from Operating Ratings), the maximum

allowed ambient temperature TA is given by:

TJ(MAX) = TA + (PD x TJA) = +150°C

= TA + (0.700 x +85°C/W)

So TA = +90.5°C. Thus, the part can be operated safely

at a maximum package temperature of +85°C.

Power Supplies

The MAX6955 operates from a single 2.7V to 5.5V

power supply. Bypass the power supply to GND with a

0.1µF capacitor as close to the device as possible. Add

a 47µF capacitor if the MAX6955 is not close to the

board’s input bulk decoupling capacitor.

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