lm2756tmx National Semiconductor Corporation, lm2756tmx Datasheet - Page 12

• Manufacturer Part Number: lm2756tmx
• Description: Multi-display Inductorless Led Driver With 32 Exponential Dimming Steps In ?smd
• Manufacturer: National Semiconductor Corporation

www.national.com
to 32 different levels. GroupB and GroupC(D1C) have 8 ana-
log current levels.
Please refer to the I
datasheet for detailed instructions on how to adjust the bright-
ness control registers.
LED CURRENT RAMPING
The LM2756 provides an internal LED current ramping func-
tion that allows the GroupA LEDs to turn on and turn off
gradually over time. The target current level is set in the
GroupA Brightness Control Register (0xA0). The total ramp-
up/ramp-down time is determind by the GroupA brightness
level (0-31) and the user configurable ramp step time.
Bits RS1 and RS2 in the Ramp Step Time Register (0x20) set
the ramp step time to the following four times: '00' = 100µsec.,
'01' = 25msec., '10' = 50msec., '11' = 100msec.
The LM2756 will always ramp-up (upon enable) and ramp-
down (upon disable) through the brightness levels until the
target level is reached. At the default setting of '00', the
LM2756's current ramping feature looks more like a current
step rather than a current ramp. The following table gives the
approximate ramp-up/ramp-down times if the GroupA bright-
ness register is set to full-scale, or brightness code 31.
MAXIMUM OUTPUT CURRENT, MAXIMUM LED
VOLTAGE, MINIMUM INPUT VOLTAGE
The LM2756 can drive 8 LEDs at 22.5mA each (GroupA ,
GroupB, GroupC) from an input voltage as low as 3.2V, so
long as the LEDs have a forward voltage of 3.6V or less (room
temperature).
The statement above is a simple example of the LED drive
capability of the LM2756. The statement contains the key ap-
plication parameters that are required to validate an LED-
drive design using the LM2756: LED current (I
of active LEDs (N
mum input voltage (V
The equation below can be used to estimate the maximum
output current capability of the LM2756:
I
the other LED Groups.
R
losses of the charge pump that result in voltage droop at the
pump output V
is proportional to the total output current of the charge pump,
the loss parameter is modeled as a resistance. The output
resistance of the LM2756 is typically 2.4Ω (V
25°C). In equation form:
V
ADDITIONAL
OUT
VOUT
I
I
LED_MAX
LED_MAX
Ramp Code
– Output resistance. This parameter models the internal
= (1.5 × V
RS1-RS0
Brightness Ramp-Up/Ramp-Down Times
00
01
10
11
is the additional current that could be delivered to
= [(1.5 x V
= [(1.5 x V
OUT
IN
[(N
. Since the magnitude of the voltage droop
) – [(N
x
), LED forward voltage (V
x
[(N
2
x R
IN-MIN
C Compatible Interface section of this
R
IN
IN
x
OUT
Ramp Step
A
OUT
x 2.4Ω) + k
) - V
) - V
× I
).
100ms
]
100µs
25ms
50ms
Time
LEDA
) + k
LED
LED
(eq. 2)
HRx
+ N
- (I
- (I
] (eq. 1)
ADDITIONAL
HRx
ADDITIONAL
B
× I
]
LEDB
Total Ramp
LED
IN
+ N
3.2ms
Time
× R
LEDx
0.8s
1.6s
3.2s
× 2.4Ω)] /
= 3.6V, T
), and mini-
C
OUT
), number
× I
LEDC
)] /
A
) ×
=
12
k
mum voltage required to be present across the current sinks
for them to regulate properly. This minimum voltage is pro-
portional to the programmed LED current, so the constant has
units of mV/mA. The typical k
In equation form:
The "I
R
ing for I
minimum input voltage and LED forward voltage. Output cur-
rent capability can be increased by raising the minimum input
voltage of the application, or by selecting an LED with a lower
forward voltage. Excessive power dissipation may also limit
output current capability of an application.
Total Output Current Capability
The maximum output current that can be drawn from the
LM2756 is 180mA. Each driver Group has a maximum allot-
ted current per Dxx sink that must not be exceeded.
The 180mA load can be distributed in many different config-
urations. Special care must be taken when running the
LM2756 at the maximum output current to ensure proper
functionality.
PARALLEL CONNECTED AND UNUSED OUTPUTS
Connecting the outputs in parallel does not affect internal op-
eration of the LM2756 and has no impact on the Electrical
Characteristics and limits previously presented. The available
diode output current, maximum diode voltage, and all other
specifications provided in the Electrical Characteristics table
apply to this parallel output configuration, just as they do to
the standard LED application circuit.
All Dx current sinks utilize LED forward voltage sensing cir-
cuitry to optimize the charge-pump gain for maximum effi-
ciency. Due to the nature of the sensing circuitry, it is not
recommended to leave any of the DxA (D1A-D4A, D53, D62)
pins open if diode GroupA is going to be used during normal
operation. Leaving DxA pins unconnected will force the
charge-pump into 3/2× mode over the entire V
ing any efficiency gain that could have been achieved by
switching to 1× mode at higher input voltages.
If the D1B or D1C drivers are not going to be used, make sure
that the ENB and ENC bits in the general purpose register are
set to '0' to ensure optimal efficiency.
The D53 and D62 pins can be completely shutdown through
the general purpose register by writing a '1' to the SD53 or
SD62 bits.
Care must be taken when selecting the proper R
The current on any DxX pin must not exceed the maximum
current rating for any given current sink pin.
POWER EFFICIENCY
Efficiency of LED drivers is commonly taken to be the ratio of
power consumed by the LEDs (P
the input of the part (P
input current is equal to the charge pump gain times the output
HR
OUT
– Headroom constant. This parameter models the mini-
equation (eq. 2) with the k
LED-MAX
DRIVER TYPE
LEDx
(V
Typical Headroom Constant Values
. Maximum LED current is highly dependent on
VOUT
D1C
" equation (eq. 1) is obtained from combining the
DxA
DxB
k
HRA
– V
= k
LEDx
HRB
IN
). With a 3/2× - 1× charge pump, the
) > k
= k
HR
HRC
HRx
of the LM2756 is 3.25mV/mA.
HRx
MAXIMUM Dxx CURRENT
× I
= 3.25 mV/mA
LED
equation (eq. 3) and solv-
LEDx
30mA per DxA Pin
30mA per DxB Pin
) to the power drawn at
30mA
(eq. 3)
IN
range negat-
SET
value.