AAT4295 Datasheet, PDF (12/16 Pages) Advanced Analogic Technologies – Single/Dual RGB Controller

ture, where T_A = 85°C. Typical conditions are cal-
culated under normal ambient conditions, where T_A
= 25°C. At 25°C ambient, the AAT4295 is capable
of dissipating 444.4mW of power and the AAT4297
is capable of dissipating 625mW of power. At 85°C
ambient, the AAT4295 is capable of dissipating
177.8mW of power and the AAT4297 can dissipate
250mW.
The power dissipation of any given MOSFET
switch is limited by its respective on resistance
(R_DS). The R_DS of any given MOSFET switch is
controlled by the applied gate voltage to the switch,
which is set by the applied V_CC supply and the
ambient operating temperature. Switch RDS for
the AAT4295 or AAT4297 may be estimated by
using the R_DS versus Temperature curve in the
Typical Characteristics section of this datasheet.
The maximum current of any given switch can be
calculated for a given operating temperature and
V_CC supply level. The corresponding R_DS is deter-
mined by use of the R_DS vs. Temperature curve for
the given V_CC.
Given the maximum package power dissipation
and operating temperature, the maximum current
through any switch or combination of switches can
be calculated using the following formula:
1
I_SWITCH(MAX)
=
⎛ P_D(MAX)⎞
⎝ R_DS ⎠
2
Example: If all the switches on an AAT4295 were
closed simultaneously, each switch could handle
up to 271mA of current at 25°C for total of 813mA.
For the same set of operating conditions at 25°C,
the AAT4297 can handle up to 208mA per switch
for a total of 1.25A for all six switches. If the load
current for a desired application exceeds the rec-
ommended current at a given temperature, two or
more switches may be operated in parallel as long
as the overall power dissipation of the device pack-
age is not exceeded. If different current levels are
passed through different switches on a given
device, then one should total up the power dissipa-
tion for each switch and assure the sum of the
power dissipation does not exceed the power rat-
ing for the package.
12
AAT4295/97
Single/Dual RGB Controller
Application Circuits
Today, many mobile phones and similar products
contain RGB LED fashion lighting, LCD display and
sub-display, as well as keypad backlighting and
photo flash LEDs. Due to the nature of common
anode RGB LEDs, the AAT4295 and AAT4297
make ideal low-cost lighting control solutions. In
general, most types of LEDs can be controlled via a
low-side MOSFET switch and current limiting bal-
last resistor. The following application circuits
(Figures 4 through 7) show voltage boosting charge
pumps to power RGB and flash LEDs. However, if
a voltage or current source is already available in a
given product design, the charge pump circuit block
may be replaced with the existing power source
solution. Since both the AAT4295 and AAT4297
require only one GPIO line from the system micro-
controller to enable and disable all the switches via
the EN/SET input, these solutions can provide a
simple way to add lighting solutions to existing
design platforms.
Driving LED Loads
When driving LEDs with a voltage source, series
ballast resistors must be used to limit the LED for-
ward current. The LED current will vary with supply
voltage and LED forward voltage. Most types of
LEDs have forward voltage specifications ranging
from 2.0V to 5.0V. When controlling an LED of any
type with a low-side MOSFET switch, the neces-
sary series ballast resistor value can be calculated
from the following formula:
R_BALLAST =
(V_IN - V_F)
I_LED
- R_DS(ON)
Where:
R_BALLAST is the value of resistor to be placed in
series with the LED (Ω).
V_IN is the input supply voltage to the device (V).
V_F is the forward voltage of the LED (V).
R_DS(ON) is the resistance of the switch when it is
turned on (Ω).
I_LED is the desired operating current of the LED (A).
4295.2006.03.1.3