AAT3151B Datasheet, PDF (11/15 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X Charge Pump For White LED Applications

AAT3151B
High Efficiency 1X/1.5X Charge Pump
For White LED Applications
Data
1
2
3
4
Max Current
20mA Max Scale
30mA Max Scale
15mA Max Scale
Low Current Mode¹
Table 3: Max Current Register Settings—
Address 4.
Data
1
2
3
4
D1-D4 (mA)
0.05
0.5
1
2
Table 4: Low Current Register Settings—
Address 5.
Disabled Current Sinks
Current sink inputs that are not used should be dis-
abled. To disable and properly terminate unused
current sink inputs, they must be tied to V_OUT.
Properly terminating unused current sink inputs is
important to prevent the charge pump modes from
prematurely activating. When properly terminated,
only a small sense current flows for each disabled
channel. The sense current for each disabled chan-
nel is 120µA.
The AAT3151B is also equipped with an "auto-dis-
able" feature to protect against an LED failure
condition.
Thermal Protection
The AAT3151B has a thermal protection circuit that
will shut down the charge pump if the die tempera-
ture rises above the thermal limit, as is the case
during a short-circuit of the VOUT pin.
Applications Information
LED Selection
The AAT3151B is specifically intended for driving
white LEDs. However, the device design will allow
the AAT3151B to drive most types of LEDs with for-
ward voltage specifications ranging from 2.0V to
4.3V. LED applications may include main and sub-
LCD display backlighting, camera photo-flash appli-
cations, color (RGB) LEDs, infrared (IR) diodes for
remotes, and other loads benefiting from a regulat-
ed output current generated from a varying input
voltage. Since the D1 to D4 constant current sinks
are matched with negligible voltage dependence,
the LED brightness will be matched regardless of
the specific LED forward voltage (V_F) levels.
In some instances (e.g., in high luminous output
applications such as photo flash), it may be neces-
sary to drive high-V_F type LEDs. The low dropout
current sinks in the AAT3151B make it capable of
driving LEDs with forward voltages as high as 4.3V
at full current from an input supply as low as 3.0V.
Current sinks can be paralleled to drive high-cur-
rent LEDs without complication.
Device Switching Noise Performance
The AAT3151B operates at a fixed frequency of
approximately 1MHz to control noise and limit har-
monics that can interfere with the RF operation of
cellular telephone handsets or other communica-
tion devices. Back-injected noise appearing on the
input pin of the charge pump is 20mV peak-to-
peak, typically ten times less than inductor-based
DC/DC boost converter white LED backlight solu-
tions. The AAT3151B soft-start feature prevents
noise transient effects associated with inrush cur-
rents during start-up of the charge pump circuit.
Power Efficiency and Device Evaluation
The charge pump efficiency discussion in the follow-
ing sections only accounts for the efficiency of the
charge pump section itself. Due to the unique circuit
architecture and design of the AAT3151B, it is very
difficult to measure efficiency in terms of a percent
value comparing input power over output power.
1. Low Current mode requires the Max Current register to be set to Low Current Mode. Low Current mode is unaffected by the settings
for Addresses 1-3.
3151B.2007.09.1.0
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