Datasheet

MAX6953
2-Wire Interfaced, 2.7V to 5.5V, 4-Digit 5
7
Matrix LED Display Driver
______________________________________________________________________________________ 21
Low-Voltage Operation
The MAX6953 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 I
SEG
, plus the 0.6V headroom required by
the driver output stages. The MAX6953 correctly regu-
lates I
SEG
with a supply voltage above this minimum
voltage. If the supply drops below this minimum volt-
age, the driver output stages may 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 5
7 matrix digit
are well matched, so the result is that the display inten-
sity dims uniformly as supply voltage drops out of regu-
lation and beyond. The MAX6953 operates down to 2V
supply voltage (although most displays are very dim at
this voltage), providing that the MAX6953 is powered
up initially to at least 2.7V to trigger the device's internal
reset, and also that the I
2
C interface is constrained to
100kbps.
Computing Power Dissipation
The upper limit for power dissipation (PD) for the
MAX6953 is determined from the following equation:
P
D
= (V+
15mA) + (V+ - V
LED
) (DUTY
I
SEG
N)
where:
V+ = supply voltage
Duty = duty cycle set by intensity register
N = number of segments driven (worst case is 10)
V
LED
= LED forward voltage
I
SEG
= segment current set by R
SET
P
D
= power dissipation, in mW if currents are in mA
Dissipation example:
I
SEG
= 40mA, N = 10, Duty = 15 / 16, V
LED
= 2.4V at 40mA, V+ = 3.6V
P
D
= 3.6V (15mA) + (3.6V - 2.4V)
(15 / 16
40mA
10) = 0.504W
Thus, for a 36-pin SSOP package (T
JA
= 1 / 0.0118 =
+85°C/W from operating ratings), the maximum allowed
ambient temperature T
A
is given by:
T
J(MAX)
= T
A
+ (P
D
T
JA
) = +150°C =
T
A
+ (0.504
+85°C/W)
So T
A
= +107°C. Thus, the part can be operated safely
at a maximum package temperature of +85°C.
Power Supplies
The MAX6953 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 MAX6953 is not close to the
board's input bulk decoupling capacitor.
Board Layout
When designing a board, use the following guidelines:
1) The R
SET
connection to the ISET pin is a high-
impedance node, and sensitive to layout. Place
R
SET
right next to the ISET pin and route R
SET
directly to these pins with very short tracks.
2) Ensure that the track from the ground end of R
SET
routes directly to GND pin 19 (PDIP package) or
GND pin 17 (SSOP package), and that this track is
not used as part of any other ground connection.
Chip Information
TRANSISTOR COUNT: 44,078
PROCESS: CMOS