AAT3170_08 datasheet(12/15 Pages) ANALOGICTECH

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AAT3170

High Current LED Flash Driver Charge Pump IC

ChargePumpTM

PRODUCT DATASHEET

3170.2007.11.1.2

www .analogictech.com

When enabling constant voltage mode, the LED channels

F1 and F2 are automatically disabled so current does not

flow through the flash LED during constant voltage

mode. Because of this, very little programming is

required. The user just addresses the voltage mode reg-

ister and programs it to voltage mode.

LED Selection

The AAT3170 is designed to drive high-intensity white

LEDs. It is particularly suitable for LEDs with an operat-

ing forward-voltage in the range of 4.2V to 1.5V.

The charge pump device can also drive other loads that

have similar characteristics to white LEDs. For various

load types, the AAT3170 provides a high-current, pro-

grammable ideal constant current source.

Device Power Efficiency

The AAT3170 power conversion efficiency depends on

the charge pump mode. By definition, device efficiency

is expressed as the output power delivered to the LED

divided by the total input power consumed.

η =

POUT

When the input voltage is sufficiently greater than the

LED forward voltage, the device optimizes efficiency by

operating in 1X mode. In 1X mode, the device is working

as a bypass switch and passing the input supply directly

to the output. The power conversion efficiency can be

approximated by:

η =

≈

VF · ILED

VIN · IIN

VIN

Due to the very low 1X mode quiescent current, the

input current nearly equals the current delivered to the

LED. Further, the low-impedance bypass switch intro-

duces negligible voltage drop from input to output.

The AAT3170 further maintains optimized performance

and efficiency by detecting when the input voltage is not

sufficient to sustain LED current. The device automati-

cally switches to 1.5X mode when the input voltage

drops too low in relation to the LED forward voltage.

In 1.5X mode, the output voltage can be boosted to 3/2

the input voltage. The 3/2 conversion ratio introduces a

corresponding 1/2 increase in input current. For ideal

conversion, the 1.5X mode efficiency is given by:

η =

VF · ILED

VIN · 1.5IIN

1.5 · VIN

Similarly, when the input falls further, such that 1.5X

mode can no longer sustain LED current, the device will

automatically switch to 2X mode. In 2X mode, the out-

put voltage can be boosted to twice the input voltage.

The doubling conversion ratio introduces a correspond-

ing doubling of the input current. For ideal conversion,

the 2X mode efficiency is given by:

η =

VF · ILED

VIN · 2IIN

2 · VIN

Capacitor Selection

Careful selection of the four external capacitors (CIN, C1,

C2, and COUT) is important because they will affect turn-

on time, output ripple, and transient performance.

Optimum performance will be obtained when low equiva-

lent series resistance (ESR) (<100mΩ) ceramic capaci-

tors are used. A value of 1μF for the flying capacitors is

a good starting point when choosing capacitors. If the

LED current sinks are only programmed for light current

levels, then the capacitor size may be decreased.

Ceramic composition capacitors are highly recommended

over all other types of capacitors for use with the

AAT3170. Ceramic capacitors offer many advantages

over their tantalum and aluminum electrolytic counter-

parts. A ceramic capacitor typically has very low ESR, is

lowest cost, has a smaller PCB footprint, and is non-

polarized. Low ESR ceramic capacitors help maximize

charge pump transient response.

Equivalent Series Resistance

ESR is an important characteristic to consider when

selecting a capacitor. ESR is a resistance internal to a

capacitor that is caused by the leads, internal connec-

tions, size or area, material composition, and ambient

temperature.

Capacitor ESR is typically measured in

milliohms for ceramic capacitors and can range to more

than several ohms for tantalum or aluminum electrolytic

capacitors.

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