AAT2856_08 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – High Current Charge Pump with Dual LDO for Backlight Applications

PRODUCT DATASHEET
AAT2856
ChargePump^TM High Current Charge Pump with Dual LDO for Backlight Applications
Applications Information
LDO Output Voltage Programming
The output voltages for LDOA and LDOB are programmed
by an external resistor divider network. As shown below,
the selection of R₁ and R₂ is a straight forward matter.
R₁ is chosen by considering the tradeoff between the
feedback network bias current and resistor value. Higher
resistor values allow stray capacitance to become a
larger factor in circuit performance whereas lower resis-
tor values increase bias current and decrease efficiency.
OUT(A/B)
R2(A/B)
VOUT(A/B)
FB(A/B)
V_REF(A/B) = 1.2V
R1(A/B)
To select appropriate resistor values, first choose R₁ such
that the feedback network bias current is less than 10μA.
Then, according to the desired V_OUT, calculate R₂ according
to the equation below. An example calculation follows.
An R₁ value of 120K is chosen, resulting in a small feed-
back network bias current of 1.2V/120K = 10μA. The
desired output voltage is 1.8V. From this information, R₂
is calculated from the equation below.
R₂
=
R₁(V_OUT - 1.2V)
1.2V
The result is R₂ = 60K. Since 60K is not a standard
1%-value, 60.4K is selected. From this example calcula-
tion, for V_OUT = 1.8V, use R₁ = 120K and R₂ = 60.4K.
Example output voltages and corresponding resistor val-
ues are provided in Table 8.
Selection of set resistor values outside of the typical
application must be carefully evaluated to ensure that the
application’s performance requirements can still be met.
R₂ Standard 1% Values (R₁ = 120K)
V_OUT (V)
2.8
2.5
2.0
1.8
1.5
R₂ (Ω)
160K
130K
79.6K
60.4K
30.1K
Table 8: Example Output Voltages and
Corresponding Resistor Values.
Device Power Efficiency
The AAT2856 power conversion efficiency depends on
the charge pump mode. By definition, device efficiency
is expressed as the output power delivered to the LEDs
divided by the total input power consumed.
η
=
P_OUT
P_IN
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=
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 total output current delivered
to the LEDs. Further, the low-resistance bypass switch
introduces negligible voltage drop from input to output.
The AAT2856 further maintains optimized performance
and efficiency by detecting when the input voltage is not
sufficient to sustain LED drive 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 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
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2856.2008.02.1.3