Table 23. Display-Test Register Format
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 MAX6952 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
MAX6952 is not exceeded. This can be achieved by
inserting a series resistor in the supply to the MAX6952,
ensuring that the supply decoupling capacitors are still
on the MAX6952 side of the resistor. For example, con-
sider the requirement that the minimum supply voltage
to a MAX6952 must be 3.0V, and the input supply
range is 5V ±5%.
Maximum supply current is:
12mA + (40mA x 10) = 412mA
Minimum input supply voltage is 4.75V.
Maximum series resistor value is:
(4.75V - 3.0V) / 0.412A = 4.25Ω
We choose 3.3Ω ±5%. Worst-case resistor dissipation
is at maximum toleranced resistance, i.e., (0.412A) 2 x
(3.3Ω
✕ 1.05) = 0.577W. We choose a 1W resistor rat-
ing. The maximum MAX6952 supply voltage is at maxi-
mum input supply voltage and minimum toleranced
resistance, i.e., 5.25V - (0.412A x 3.3Ω ✕ 0.95) = 3.97V.
Low-Voltage Operation
The MAX6952 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 MAX6952 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 MAX6952 operates down to
2.5V supply voltage (although most displays are very
dim at this voltage), provided that the MAX6952 is pow-
ered up initially to at least 2.7V to trigger the device's
internal reset.
Computing Power Dissipation
The upper limit for power dissipation (PD) for the
MAX6952 is determined from the following equation:
P
D
= (V+ ✕ 12mA) + (V+ - V
LED
) (DUTY x 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 (12mA) + (3.6V - 2.4V)(15 / 16 ✕ 40mA ✕ 10)
= 0.493W
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
+ (0.493 ✕ +85°C/W)
So, T
A
= +108°C. Thus, the part can be operated safely
at a maximum package temperature of +85°C.
Power Supplies
The MAX6952 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 MAX6952 is not close to the
board’s input bulk decoupling capacitor.
