AAT3177 Datasheet, PDF (11/16 Pages) Advanced Analogic Technologies – High Current, High Efficiency Charge Pump with Auto-Timer

ChargePump^TM
PRODUCT DATASHEET
AAT3177
High Current, High Efficiency Charge Pump with Auto-Timer
Next, choose R₂ based on the desired flash level LED
current:
R₂
=
R₁
·
R₁ · 600mA · 187kΩ
I_LED (flash) - 600mA · 187kΩ
The current and resistance values used in the equations
come from the conditions placed on the I_DX parameter of
the Electrical Characteristics table.
For examples of standard 1% values where the LED flash
current level is targeted for 700mA, see Table 3.
R1 (kΩ)
920
732
649
562
R2 (kΩ)
193
205
210
223
ILED Torch
(mA)
122
153
173
200
ILED Flash
(mA)
703
701
707
703
Table 3: Open-Drain I/O Example Resistor Values.
If the I/O port must be configured as an open-drain PMOS
type output, the appropriate equations can be generated
from these same concepts. As done in the previous exam-
ple, the necessary values can then be calculated.
As a reference, the equations applicable to the PMOS
case are:
600mA · 187kΩ
R₁ = _{ILED (flash)}
V_IO
0.7
-
1
R₂
=
1
R₁
-
I_LED (torch)
600mA · 187kΩ
The value to use for V_IO must come from the I/O supply
voltage used in the system. 0.7V is the typical value of
the V_RSET parameter found in the Electrical Characteristics
section of this datasheet.
Device Power Efficiency
The AAT3177 power conversion efficiency depends on
the charge pump mode. By definition, device efficiency
is expressed as the output power delivered to the LED
divided by the total input power consumed.
η
=
P_OUT
P_IN
When the input voltage is sufficiently greater than the
LED forward voltage, 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. The power conversion efficiency can be
approximated by:
η
=
V_F · I_LED
V_IN · I_IN
≈
V_F
V_IN
Due to the very low 1X mode quiescent current, the
input current nearly equals the current delivered to the
LED. Further, the low-impedance bypass switch intro-
duces negligible voltage drop from input to output.
The AAT3177 further maintains optimized performance
and efficiency by detecting when the input voltage is not
sufficient to sustain LED current. The device automati-
cally switches to 1.5X mode when the input voltage
drops too low in relation to the LED forward voltage.
In 1.5X mode, the output voltage can be boosted to 3/2
the input voltage. The 3/2 conversion ratio introduces a
corresponding 1/2 increase in input current. For ideal
conversion, the 1.5X mode efficiency is given by:
η
=
V_F · I_LED
V_IN · 1.5I_IN
=
V_F
1.5 · V_IN
Similarly, when the input falls further, such that 1.5X
mode can no longer sustain LED current, the device will
automatically switch to 2X mode. In 2X mode, the out-
put voltage can be boosted to twice the input voltage.
The doubling conversion ratio introduces a correspond-
ing doubling of the input current. For ideal conversion,
the 2X mode efficiency is given by:
η=
V_F · I_LED
V_IN · 2I_IN
=
V_F
2 · V_IN
LED Selection
The AAT3177 is designed to drive high-intensity white
LEDs. It is particularly suitable for LEDs with an operat-
ing forward voltage in the range of 4.2V to 1.5V.
The charge pump device can also drive other loads that
have similar characteristics to white LEDs. For various
load types, the AAT3177 provides a high-current, pro-
grammable ideal constant current source.
3177.2008.06.1.0
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