AAT3192 Datasheet, PDF (13/17 Pages) Advanced Analogic Technologies – Low-Cost, 2-/3-Channel Charge-Pump LED Drivers

associated with inrush currents during start-up of
the charge pump circuit.
Shutdown
Since the current switches are the only power
returns for all loads, there is no leakage current
when all sink switches are disabled. To activate the
shutdown operation, the EN/SET input for the
AAT3192/93-1/2/3 should be strobed low for longer
than t_OFF (500μs). For the 3192/93-4 PWM options,
shutdown operation is enabled when the EN/PWM
input is strobed low longer than 1ms.In this state,
the AAT3192/93 typically draws less than 1μA from
the input. Registers are reset to 0 in shutdown.
Power Efficiency and Device Evaluation
The charge pump efficiency discussion in the follow-
ing sections accounts only for efficiency of the
charge pump section itself. Due to the unique circuit
architecture and design of the AAT3192/93, it is very
difficult to measure efficiency in terms of a percent
value comparing input power over output power.
Since the AAT3192/93 outputs are pure constant
current sinks and typically drive individual loads, it
is difficult to measure the output voltage for a given
output to derive an overall output power measure-
ment. For any given application, white LED forward
voltage levels can differ, yet the output drive cur-
rent will be maintained as a constant.
This makes quantifying output power a difficult task
when taken in the context of comparing to other
white LED driver circuit topologies. A better way to
quantify total device efficiency is to observe the total
input power to the device for a given LED current
drive level. The best white LED driver for a given
application should be based on trade-offs of size,
external component count, reliability, operating
range, and total energy usage...not just % efficiency.
The AAT3192/93 efficiency may be quantified under
very specific conditions and is dependent upon the
input voltage versus the output voltage across the
loads applied to outputs D1 through D3 (or D2 in the
case of the AAT3192) for a given constant current
setting. Depending on the combination of V_IN and
voltages sensed at the current sinks, the device will
operate in load switch mode. When any one of the
voltages sensed at the current sinks nears dropout,
the device will operate in 2X charge pump mode.
3193.2007.05.1.0
AAT3192/93
Low-Cost, 2-/3-Channel
Charge-Pump LED Drivers
Each of these modes will yield different efficiency
values. Refer to the following two sections for expla-
nations for each operational mode.
1X Mode Efficiency
The AAT3192/93 1X mode is operational at all
times and functions alone to enhance device power
conversion efficiency when V_IN is higher than the
voltage across the load. When in 1X mode, voltage
conversion efficiency is defined as output power
divided by input power.
An expression for the ideal efficiency (η) in 1X
charge-pump mode can be expressed as:
η = ^POUT = ^{VF · ILED} ≅ ^VF
P_IN V_IN · I_OUT
V_IN
-or-
η
(%)
=
100
·
⎛
⎝
V_{F ⎞}
V_IN⎠
2X Charge Pump Mode Efficiency
The AAT3192/93 contains a charge pump which
will boost the input supply voltage in the event
where V_IN is less than the voltage required to sup-
ply the output. The efficiency (η) can be simply
defined as a linear voltage regulator with an effec-
tive output voltage that is equal to one and two
times the input voltage. Efficiency (η) for an ideal
2X charge pump can typically be expressed as the
output power divided by the input power.
η=
P_F
P_IN
In addition, with an ideal 2X charge pump, the out-
put current may be expressed as 1/3 of the input
current. The expression to define the ideal efficien-
cy (η) can be rewritten as:
η = ^POUT = ^{VF · ILED} = ^VF
P_IN V_IN · 2I_OUT 2V_IN
-or-
η
(%)
=
100
·
⎛ ^VF ⎞
⎝^2VIN⎠
For a charge pump with an output of 5V and a nomi-
nal input of 3.5V, the theoretical efficiency is 71%.
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