V

I

(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

12mV/mA. In equation form:

(V

(eq. 3)

Typical Headroom Constant Values

k

k

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

LM27964 is 180mA. Each driver bank has a maximum allot-

ted current per Dxx sink that must not be exceeded.

DRIVER TYPE

MAXIMUM Dxx CURRENT

DxA

30mA per DxA Pin

DxB

30mA per DxB Pin

DKEY

80mA

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

figurations. Special care must be taken when running the

LM27964 at the maximum output current to ensure proper

functionality.

PARALLEL CONNECTED OUTPUTS

Outputs D1A-4A or D1B-D2B may be connected together to

drive one or two LEDs at higher currents. In such a configu-

ration, all four parallel current sinks (BankA) of equal value

can drive a single LED. The LED current programmed for

BankA should be chosen so that the current through each of

the outputs is programmed to 25% 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 15mA. Similarly, if

two LEDs are to be driven by pairing up the D1A-4A inputs

(i.e D1A-2A, D3A-4A), R

the current through each current sink input is 50% of the

desired LED current. The same RSETx selection guidelines

apply to BankB diodes.

Connecting the outputs in parallel does not affect internal

operation of the LM27964 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 4-LED application circuit.

Both BankA and BankB utilize LED forward voltage sensing

circuitry on each Dxx pin to optimize the charge-pump gain

for maximum efficiency. Due to the nature of the sensing

circuitry, it is not recommended to leave any of the DxA or

DxB pins unused if either diode bank is going to be used

during normal operation. Leaving DxA and/or DxB pins un-

connected will force the charge-pump into 3/2x mode over

the entire V

be achieve by switching to 1x mode at higher input voltages.

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 current (total LED current). The efficiency of the

LM27964 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

LM27964, V

number of LEDs and I

P

P

I

(V

T

θ

The junction temperature rating takes precedence over the

ambient temperature rating. The LM27964 may be operated

outside the ambient temperature rating, so long as the junc-

tion temperature of the device does not exceed the maxi-

mum operating rating of 100˚C. The maximum ambient tem-

perature rating must be derated in applications where high

power dissipation and/or poor thermal resistance causes the

junction temperature to exceed 100˚C.

THERMAL PROTECTION

Internal thermal protection circuitry disables the LM27964

when the junction temperature exceeds 170˚C (typ.). This

feature protects the device from being damaged by high die

temperatures that might otherwise result from excessive

power dissipation. The device will recover and operate nor-

mally when the junction temperature falls below 165˚C (typ.).

It is important that the board layout provide good thermal

conduction to keep the junction temperature within the speci-

fied operating ratings.

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