AAT3132 Datasheet, PDF (11/14 Pages) Advanced Analogic Technologies – High Efficiency 1.5X Fractional Charge Pump for White LED Applications

EN/SET Timing Diagram
EN/SET
Code OFF
AAT3132/32A
High Efficiency 1.5X Fractional Charge
Pump for White LED Applications
t_HI
t_LO
t_OFF
1
2
3
OFF
Power Efficiency and Device
Evaluation
The charge pump efficiency discussion in the fol-
lowing sections only accounts for efficiency of the
charge pump section itself. Due to the unique circuit
architecture and design of the AAT3132/32A, it is
very difficult to measure efficiency in terms of a per-
cent value comparing input power over output
power.
Since the AAT3132/32A outputs are pure constant
current sources and typically drive individual loads, it
is difficult to measure the output voltage for a given
output (D1 to D4) to derive an overall output power
measurement. For any given application, white LED
forward voltage levels can differ, yet the output drive
current 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 applica-
tion should be based on trade-offs of size, external
components count, reliability, operating range, and
total energy usage...not just % efficiency.
The AAT3132/32A efficiency may be quantified
under very specific conditions and is dependent
upon the input voltage versus the output voltage
seen across the loads applied to outputs D1
through D4 for a given constant current setting.
The efficiency (η) can be simply defined as a linear
voltage regulator with an effective output voltage
that is equal to one and one half times the input
voltage. Efficiency (η) for an ideal 1.5X charge
pump can typically be expressed as the output
power divided by the input power.
3132.2005.12.1.4
η
=
P_OUT
P_IN
In addition, with an ideal 1.5X charge pump, the
output current may be expressed as 2/3 of the
input current. The expression to define the ideal
efficiency (η) can be rewritten as:
η = ^POUT =
P_IN
V_OUT × I_OUT
V_IN × 1.5I_OUT
=
V_OUT
1.5V_IN
-or-
η(%)
=
100
⎛
⎝
V_OUT ⎞
1.5V_IN⎠
For a charge pump with an output of 5V and a nom-
inal input of 3.5V, the theoretical efficiency is 95%.
Due to internal switching losses and IC quiescent
current consumption, the actual efficiency can be
measured at 93%. These figures are in close agree-
ment for output load conditions from 1mA to 100mA.
Efficiency will decrease as load current drops below
0.05mA or when level of V_IN approaches V_OUT.
Refer to the Typical Characteristics section of this
datasheet for measured plots of efficiency versus
input voltage and output load current for the given
charge pump output voltage options.
Capacitor Selection
Careful selection of the four external capacitors
C_IN, C₁, C₂, and C_OUT is important because they will
affect turn-on time, output ripple, and transient per-
formance. Optimum performance will be obtained
when low equivalent series resistance (ESR)
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