anode outputs and a group of 3 common cathode outputs.

There is an ON/OFF control pin for each group (ENA and

ENB).

The DC current through the LEDs is programmed with an

external resistor. Changing currents on-the-fly can be

achieved with the use of digital pulse (PWM) signals.

ENABLE PINS: ENA, ENB

The LM27961 has 2 enable pins. Both are active-high logic

(HIGH = ON). There are internal pull-down resistors (300k

Ω

typ.) that are connected internally between each of the en-

able pins and GND.

ENA and ENB can both enable and disable the part. When

shutdown mode. All internal circuitry is OFF and the part

consumes very little supply current when the LM27961 is

shutdown. When the voltage on either ENx pin is high

corresponding output current drivers are active.

ENA and ENB are used to turn the output currents ON and

OFF. ENA activates/deactivates the four GroupA outputs

(D1A-D4A). ENB activates/deactivates the three GroupB

outputs (D1B-D3B).

SETTING LED CURRENTS

The output currents of the LM27961 can be set to a desired

value simply by connecting an appropriately sized resistor

(R

R

sets the current for the GroupB outputs. The output currents

(LED currents) are proportional to the current that flows out

of the I

greater than the I

internal amplifier sets the voltage of the I

(typ.). Placing a resistor between I

the I

above are simplified in the equations below:

I

R

MAXIMUM OUTPUT CURRENT, MAXIMUM LED

VOLTAGE, MINIMUM INPUT VOLTAGE

The LM27961 can drive 4 LEDs at 15mA each from an input

voltage as low as 2.7V, so long as the LEDs have a forward

voltage of 3.5V or less (room temperature).

The statement above is a simple example of the LED drive

capabilities of the LM27961. The statement contains the key

application parameters that are required to validate an LED-

drive design using the LM27961: LED current (I

ber of active LEDs (N), LED forward voltage (V

minimum input voltage (V

The equation below can be used to estimate the total output

current capability of the LM27961:

I

I

Ω) + 22mV/mA)

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 LM27961 is typically 2.7

Ω (V

3.0V, 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

22mV/mA. In equation form:

(V

(eq. 3)

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.

PARALLEL Dx OUTPUTS FOR INCREASED CURRENT

CAPABILITY

Outputs D1A through D4A, or D1B through D3B may be

connected together in any combination to drive higher cur-

rents through fewer LEDs. For example in Figure 1, outputs

D1A and D2A are connected together to drive one LED. D3A

and D4A are connected to drive a second LED.

With this configuration, two parallel current sources of equal

value provide current to one of the LEDs. R

therefore be chosen so that the current through each output

is programmed to 50% of the desired current through the

parallel connected LED. For example, if 40mA is the desired

drive current for the parallel connected LED, R

be selected so that the current through each of the outputs is

20mA. Other combinations of parallel outputs may be imple-

mented in similar fashions, such as in Figure 2.

20127919

FIGURE 1. Two Parallel Connected LEDs

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