Note: DxA3-DxA0: Register Sets Current Level Supplied to DxA LED drivers

DxB3-DxB0: Register Sets Current Level Supplied to DxB LED drivers

Full-Scale Current set externally by the following equation:

Brightness Level Segments = 1/16th of Fullscale

Note: DKEY1-DKEY0: Sets Brightness for DKEY pin (KEYPAD Driver).

11=Fullscale

Bit7 to Bit 2: Not Used

Full-Scale Current set externally by the following equation:

Brightness Level are= 100% (Fullscale), 70%, 40%, 20%

Application Information

SETTING LED CURRENT

The current through the LEDs connected to DxA, DxB and

DKEY can be set to a desired level simply by connecting an

appropriately sized resistor (R

the LM27964 and GND. The DxA and DxB LED currents are

proportional to the current that flows out of the I

I

I

current that flows out of the I

times greater than the I

the internal amplifiers set the voltage of the I

1.25V (typ.). Separate R

each I

equations below:

I

R

I

R

Once the desired R

LM27964 has the ability to internally dim the LEDs by Pulse

Width Modulating (PWM) the current. The PWM duty cycle is

set through the I

BankA and BankB current sinks (DxA and DxB) can be

dimmed to 16 different levels/duty-cycles (1/16th of full-scale

to full-scale). The internal PWM frequency for BankA and

BankB

is

a

fixed

10kHz

(LM27964SQ-I)

or

23kHz

(LM27964SQ-A) depending on the option.

The DKEY current sink uses an analog current scaling

method to control LED brightness. The brightness levels are

100% (Fullscale), 70%, 40%, and 20%. When connecting

multiple LEDs in parallel to the DKEY current sink, it is

recommended that ballast resistors be placed in series with

the LEDs. The ballast resistors help reduce the affect of LED

forward voltage mismatch, and help equalize the diode cur-

rents. Ballast resistor values must be carefully chosen to

ensure that the current source headroom voltage is sufficient

to supply the desired current.

Please refer to the I

datasheet for detailed instructions on how to adjust the

brightness control registers.

MAXIMUM OUTPUT CURRENT, MAXIMUM LED

VOLTAGE, MINIMUM INPUT VOLTAGE

The LM27964 can drive 4 LEDs at 30mA each (BankA) and

12 keypad LEDs at 5mA each (60mA total at DKEY) 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

capabilities of the LM27964. The statement contains the key

application parameters that are required to validate an LED-

drive design using the LM27964: 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 LM27964:

I

[(N

I

Ω)] /

[(N

Ω)+k

I

to the other LED banks.

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 LM27964 is typically 2.75

Ω (V

3.6V, T

20138105

FIGURE 7. Brightness Control Register Description

Internal Hex Address: A0h

20138104

FIGURE 8. Brightness Control Register Example

20138110

FIGURE 9. Internal Hex Address: B0h

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