AAT3159 Datasheet, PDF (11/15 Pages) Advanced Analogic Technologies – High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications

ChargePump^TM
PRODUCT DATASHEET
AAT3159
High Efficiency 1X/1.5X/2X Charge Pump for White LED Applications
Additionally, a digital-to-analog converter can be used
with the SET pin to control the brightness level. The
result is like the equation above, where V_IO is replaced
with V_DAC:
I_LED(LO)
=
I_SET
⎛ 0.6V
⎝R₁ // R₂
-
V_DAC⎞
R₂ ⎠
For cases where PWM dimming is preferred, the PWM
signal can be applied directly to the SET resistor, as
shown in Figure 3. In order for the LED current to go to
zero, the voltage level of the PWM signal must be higher
than the SET pin voltage of 0.6V.
PWM
R1
AAT 3159
SET
Figure 3: SET Pin Configuration for
PWM Dimming Control.
Device Power Efficiency
The AAT3159’s power conversion efficiency depends on
the charge pump mode of operation. By definition,
device efficiency is expressed as the output power deliv-
ered to the LEDs divided by the total input power
consumed.
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ... + V_LED4
V_IN · I_IN
·
I_LED4
When the input voltage is sufficiently greater than the
LED forward voltages, the device optimizes efficiency by
operating in 1X mode. In 1X mode, the device is working
as a bypass switch and passing the input supply directly
to the output. By simplifying the conditions such that the
LEDs have uniform V_F, the power conversion efficiency
can be approximated by:
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ... + V_LED4
V_IN · I_IN
·
I_LED4
η
=
4
·
V_LEDX · I_LEDX
V_IN · I_IN
;
x
=
1,
2,
3
or
4
and
I_IN
=
4
·
I_LEDX
η
=
V_LEDX
V_IN
Due to the very low 1X mode quiescent current, the
input current nearly equals the total output current
delivered to the LEDs. Further, the low resistance bypass
switch introduces a negligible voltage drop from input to
output.
The AAT3159 further maintains optimized performance
and efficiency by detecting when the input voltage is not
sufficient to sustain the LED bias current. The device auto-
matically switches to 1.5X mode when the input voltage
drops too low in relation to the LED forward voltages.
In 1.5X mode, the output voltage can be boosted to 1.5X
the input voltage. The 1.5X conversion ratio introduces a
corresponding 0.5X increase in input current. For ideal
conversion, the 1.5X mode efficiency is given by:
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ... + V_LED4
V_IN · I_IN
·
I_LED4
η
=
4
·
V_LEDX · I_LEDX
V_IN · I_IN
;
x
=
1,
2,
3
or
4
and
I_IN
=
1.5(4
·
I_LEDX)
η
=
V_LEDX
1.5V_IN
Similarly, when the input falls further, such that 1.5X
mode can no longer sustain the LED bias currents, the
AAT3159 will automatically switch to 2X mode. In 2X
mode, the output voltage can be boosted to 2X the input
voltage. The 2X conversion ratio introduces a corre-
sponding 1X increase in input current. For ideal conver-
sion, the 2X mode efficiency is given by:
η
=
P_LEDs
P_IN
=
V_LED1
·
I_LED1 + ... + V_LED4
V_IN · I_IN
·
I_LED4
η
=
4
·
V_LEDX · I_LEDX
V_IN · I_IN
;
x
=
1,
2,
3
or
4
and
I_IN
=
2(4
·
I_LEDX)
η=
V_LEDX
2V_IN
3159.2007.10.1.0
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