AAT3194

High Efficiency 1.5X Fractional Charge Pump For White LED Applications

PRODUCT DATASHEET

10

3194.2008.05.1.1

www .analogictech.com

EN/SET

Code

1

2

3

OFF

OFF

t

HI

t

LO

t

OFF

Figure 2: EN/SET Timing Diagram.

ON/1

50ns minimum to enable

23456

n

(n < =32)

OFF

Figure 3: Enable / Disable / LED Brightness Level Set Data Input.

LED Selection

The AAT3194 is designed to drive white LEDs with for-

ward voltages to 4.2V. Since the D1:D4 output current

sources are matched with negligible voltage depen-

dence, the LED brightness will be matched regardless of

their forward voltage matching.

Charge Pump Efficiency

The AAT3194 uses a fractional charge pump. The effi-

ciency (η) can be simply defined as a linear voltage

regulator with an effective output voltage that is equal to

one and one half times the input voltage. Efficiency (η)

for an ideal 1.5X charge pump can typically be expressed

as the output power divided by the input power:

η =

In addition, with an ideal 1.5X charge pump, the output

current may be expressed as 2/3 of the input current.

The expression to define the ideal efficiency (η) can be

rewritten as:

-or-

η(%) = 100

⎛⎞

⎝⎠

For a charge pump with an output of 5V and a nominal

input of 3.5V, the theoretical efficiency is 95%. Due to

internal switching losses and IC quiescent current con-

sumption, the actual efficiency can be measured at

93%. These figures are in close agreement for output

load conditions from 1mA to 100mA.

Efficiency will

decrease as load current drops below 0.05mA or when

the level of the power supply of IN approaches OUT.

Refer to the Typical Characteristics section of this

datasheet for measured plots of efficiency versus input

voltage and output load current for the given charge

pump output voltage options.

Power Efficiency and Device Evaluation

The charge pump efficiency discussion in the previous

section only accounts for efficiency of the charge pump

section itself. Due to the unique circuit architecture and

design of the AAT3194, it is very difficult to measure

efficiency in terms of a percent value comparing input

power over output power. Since the device outputs are

pure constant current sources, it is difficult to measure

the output voltage for a given output (D1 to D4) to

derive an output power measurement. For any given

application, white LED forward voltage levels can differ,

yet the output drive current will be maintained as a con-

stant. This makes quantifying output power a difficult

task when taken in the context of comparing to other

white LED driver circuit topologies. A better way to

quantify total device efficiency is to observe the total

input power to the device for a given LED current drive

level. The best white LED driver for a given application

should be based on trade-offs of size, external compo-

nents count, reliability, operating range, and total energy

usage...not just “% efficiency.”