AAT3113 datasheet(10/15 Pages) AAT

10 / 15 page

AAT3113, AAT3114/14A

High Efficiency 1.5X Fractional Charge

Pumps For White LED Applications

3113.2006.07.1.9

EN/SET Serial Interface

The current source output magnitude is controlled

by the EN/SET pin using AnalogicTech’s Simple

Serial Control (S2Cwire) interface. The interface

records rising edges of the EN/SET pin and

decodes them into 32 individual current level set-

tings each 1dB apart. Code 32 is full scale, and

Code 1 is full scale attenuated by 31dB. The mod-

ulo 32 interface wraps states back to state 1 after

the 32nd clock, so 1dB of attenuation is achieved by

clocking the EN/SET pin 31 times (see graph titled,

“IDIODE Response -9dB to -10dB”). The counter can

be clocked at speeds up to 1MHz, so intermediate

states are not visible.

The first rising edge of

EN/SET enables the IC and initially sets the output

LED current to -31dB, the lowest setting equal to

525µA. Once the final clock cycle is input for the

desired brightness level, the EN/SET pin is held

high to maintain the device output current at the

programmed level. The device is disabled 500µs

after the EN/SET pin transitions to a logic low state.

The EN/SET timing is designed to accommodate a

wide range of data rates. After the first rising edge

of EN/SET, the charge pump is enabled and reach-

es full capacity after the soft-start time (TSS).

During the soft-start time, multiple clock pulses

may be entered on the EN/SET pin to set the final

output current level with a single burst of clocks.

Alternatively, the EN/SET clock pulses may be

entered one at a time to gradually increase the

LED brightness over any desired time period. A

constant current is sourced as long as EN/SET

remains in a logic high state. The current source

outputs are switched off after EN/SET has

remained in a low state for at least the tOFF timeout

period (see Figure 1).

LED Selection

AAT3113 and AAT3114/14A devices are designed

to drive white LEDs with forward voltages to 4.2V.

Since the D1:D6 output current sources are

matched with negligible voltage dependence, the

LED brightness will be matched regardless of their

forward voltage matching.

Charge Pump Efficiency

The AAT3113 and AAT3114/14A devices are frac-

tional charge pumps. The efficiency (

η) can be sim-

ply 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:

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-

For a charge pump with an output of 5 volts and a

nominal input of 3.5 volts, the theoretical efficiency

is 95%. Due to internal switching losses and IC

quiescent current consumption, the actual efficien-

cy can be measured at 93%. These figures are in

close agreement for output load conditions from

1mA to 100mA.

Efficiency will decrease as load

η(%) = 100

VOUT

1.5VIN

⎛⎞

⎝⎠

η = POUT = VOUT · IOUT = VOUT

VIN · 1.5IOUT

1.5VIN

η =

POUT

Figure 1: EN/SET Timing Diagram.

EN/SET

Code

OFF

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