MAXIMUM OUTPUT CURRENT, MAXIMUM LED

VOLTAGE, MINIMUM INPUT VOLTAGE

The LM27966 can drive 6 LEDs at 30mA each (Main Display

and D

the LEDs have a forward voltage of 3.6V or less (room

temperature).

The statement above is a simple example of the LED drive

capabilities of the LM27966. The statement contains the key

application parameters that are required to validate an LED-

drive design using the LM27966: LED current (I

ber of active LEDs (N

minimum input voltage (V

The equation below can be used to estimate the maximum

output current capability of the LM27966:

I

[(N

I

Ω)] /

[(N

Ω)+k

I

AUX LED.

R

nal losses of the charge pump that result in voltage droop at

the pump output P

droop is proportional to the total output current of the charge

pump, the loss parameter is modeled as a resistance. The

output resistance of the LM27966 is typically 2.75

Ω (V

3.6V, T

V

(eq.

2)

k

mum voltage required to be present across the current

sources for them to regulate properly. This minimum voltage

is proportional to the programmed LED current, so the con-

stant has units of mV/mA. The typical k

8mV/mA. In equation form:

(V

(eq. 3)

Typical Headroom Constant Value

k

The "I

the R

solving for I

on minimum input voltage and LED forward voltage. Output

current 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

LM27966 is 180mA. Each driver bank has a maximum allot-

ted current per Dx sink that must not be exceeded.

MAXIMUM Dx CURRENT

30mA

The 180mA load can be distributed in many different con-

figurations. Special care must be taken when running the

LM27966 at the maximum output current to ensure proper

functionality.

PARALLEL CONNECTED AND UNUSED OUTPUTS

Outputs D1-5 may be connected together to drive one or two

LEDs at higher currents. In such a configuration, all five

parallel current sinks (Main Display) of equal value can drive

a single LED. The LED current programmed for Main Display

should be chosen so that the current through each of the

outputs is programmed to 20% of the total desired LED

current. For example, if 60mA is the desired drive current for

a single LED, R

through each of the current sink inputs is 12mA.

Connecting the outputs in parallel does not affect internal

operation of the LM27966 and has no impact on the Electri-

cal Characteristics and limits previously presented. The

available diode output current, maximum diode voltage, and

all other specifications provided in the Electrical Character-

istics table apply to this parallel output configuration, just as

they do to the standard 5-LED application circuit.

Main Display utilizes LED forward voltage sensing circuitry

on each Dxx pin to optimize the charge-pump gain for maxi-

mum efficiency. Due to the nature of the sensing circuitry, it

is not recommended to leave any of the Dx (D1-D4) pins

unused if either diode bank is going to be used during normal

operation. Leaving Dx pins unconnected will force the

charge-pump into 1.5x mode over the entire V

gating any efficiency gain that could be achieve by switching

to 1x mode at higher input voltages.

If D5 is not used, it is recommended that the driver pin be

grounded and the general purpose register bit EN-D5 be set

to 0 to ensure proper gain transitions.

Care must be taken when selecting the proper R

The current on any Dx pin must not exceed the maximum

current rating for any given current sink pin.

POWER EFFICIENCY

The efficiency of LED drivers is commonly taken to be the

ratio of power consumed by the LEDs (P

drawn at the input of the part (P

pump, the input current is equal to the charge pump gain

times the output current (total LED current). The efficiency of

the LM27966 can be predicted as follows:

P

(V

P

P

E=(P

It is also worth noting that efficiency as defined here is in part

dependent on LED voltage. Variation in LED voltage does

not affect power consumed by the circuit and typically does

not relate to the brightness of the LED. For an advanced

analysis, it is recommended that power consumed by the

circuit (V

POWER DISSIPATION

The power dissipation (P

can be approximated with the equations below. P

power generated by the 1.5x/1x charge pump, P

power consumed by the LEDs, T

ture, and

θ

for the LLP-24 package. V

LM27966, V

number of LEDs and I

P

P

(V

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