design using the LM27951: LED current (I

active LEDs (N), LED forward voltage (V

input voltage (V

The equation below can be used to estimate the total output

current capability of the LM27951:

I

I

Ω) + 12mV/mA)

R

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

the pump output V

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 LM27951 is typically 3.3

Ω (V

3.0V, T

V

(eq. 2)

k

mum voltage required across the current sources for proper

regulation. This minimum voltage is proportional to the pro-

grammed LED current, so the constant has units of mV/mA.

The typical k

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 LEDs with a

lower forward voltage. Excessive power dissipation may also

limit output current capability of an application.

CAPACITOR SELECTION

The LM27951 requires 4 external capacitors for proper op-

eration. Surface-mount multi-layer ceramic capacitors are

recommended. These capacitors are small, inexpensive and

Ω

typ.). Tantalum capacitors, OS-CON capacitors, and alumi-

num electrolytic capacitors are not recommended for use

with the LM27951 due to their high ESR, as compared to

ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R

temperature characteristic are preferred for use with the

LM27951. These capacitors have tight capacitance toler-

-55˚C to 85˚C).

Capacitors with Y5V or Z5U temperature characteristic are

generally not recommended for use with the LM27951. Ca-

pacitors with these temperature characteristics typically

have wide capacitance tolerance (+80%, -20%) and vary

significantly over temperature (Y5V: +22%, -82% over -30˚C

to +85˚C range; Z5U: +22%, -56% over +10˚C to +85˚C

range). Under some conditions, a nominal 1µF Y5V or Z5U

capacitor could have a capacitance of only 0.1µF. Such

detrimental deviation is likely to cause Y5V and Z5U capaci-

tors to fail to meet the minimum capacitance requirements of

the LM27951.

The voltage rating of the output capacitor should be 10V or

more. All other capacitors should have a voltage rating at or

above the maximum input voltage of the application.

PARALLEL DX OUTPUTS FOR INCREASED CURRENT

DRIVE

Outputs D

two LEDs at higher currents. In a one LED configuration, all

four parallel current sources of equal value are connected

together to drive a single LED. The LED current pro-

grammed should be chosen such that the current provided

from each of the outputs is programmed to 25% of the total

desired LED current. For example, if 60mA is the desired

drive current for the single LED, R

such that the current out of each current source is 15mA.

Similarly, if two LEDs are to be driven by pairing up the D

1-4

outputs (i.e D

the current out of each current source output is 50% of the

desired LED current.

Connecting the outputs in parallel does not affect the internal

operation of the LM27951 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.

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). For a simple approxima-

tion, the current consumed by internal circuitry can be ne-

glected and the efficiency of the LM27951 can be predicted

as follows:

P

P

P

E=(P

Neglecting I

tion, but this impact will be no more than a few percentage

points when several LEDs are driven at full power. It is also

worth noting that efficiency as defined here is in part depen-

dent 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 analy-

sis, it is recommended that power consumed by the circuit

(V

THERMAL PROTECTION

Internal thermal protection circuitry disables the LM27951

when the junction temperature exceeds 150˚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 140˚C (typ.).

It is important that the board layout provide good thermal

conduction to keep the junction temperature within the speci-

fied operating ratings.

POWER DISSIPATION

The power dissipation (P

ture (T

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

the power consumed by the LEDs, T

perature, and

θ

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