AAT3129

High Efficiency 1X/1.5X/2X Charge Pump

for RGB Color LED Applications

12

3129.2007.01.1.4

Mode Register Settings - Address 5

LED Current

The resulting DC current for each of the R, G, and B channels depends on the overall system settings. Using the

overall settings, the current for a particular channel is calculated as

B - 1

4 · (T - 1) + (R - 1) + (G - 1) + (B - 1) + CHLS_ON

I

G - 1

4 · (T - 1) + (R - 1) + (G - 1) + (B - 1) + CHLS_ON

I

R - 1

4 · (T - 1) + (R - 1) + (G - 1) + (B - 1) + CHLS_ON

I

M Setting

Action

1

R, G, B, T Registers Latched Independently

2

R, G, B Registers Synchronized to T Register Latch

CHLS_ON is the number of channels programmed

to operate. Whenever a channel is programmed to

Data = 1, that channel is programmed to off and

must be removed from the CHLS_ON count.

Capacitor Selection

Careful selection of the four external capacitors

will affect turn-on time, output ripple, and transient

performance. Optimum performance will be

obtained when low equivalent series resistance

(ESR) ceramic capacitors are used. In general, low

ESR may be defined as less than 100m

Ω. For best

overall performance, 1μF capacitors are recom-

mended. Depending on ripple requirements, the

input and output capacitor values can be increased

or decreased accordingly. If lower ripple is desired,

increase the capacitor value. The peak-to-peak rip-

ple seen at the input or output increases or

decreases with capacitor size.

Capacitor Characteristics

Ceramic composition capacitors are highly recom-

mended over all other types of capacitors for use

with the AAT3129. Ceramic capacitors offer many

advantages over their tantalum and aluminum

electrolytic counterparts. A ceramic capacitor typi-

cally has very low ESR, is lowest cost, has a small-

er PCB footprint, and is non-polarized. Low ESR

ceramic capacitors help maximize charge pump

transient response. Since ceramic capacitors are

non-polarized, they are not prone to incorrect con-

nection damage.

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 connections, size or area, material compo-

sition, and ambient temperature. Capacitor ESR is

typically measured in milliohms for ceramic capac-

itors and can range to more than several ohms for

tantalum or aluminum electrolytic capacitors.