AAT1168 Datasheet, PDF (19/22 Pages) Advanced Analog Technology, Inc. – TRIPLE-CHANNEL TFT LCD POWER SOLUTION WITH OPERATIONAL AMPLIFIERS

Advanced Analog Technology, Inc.
May 2008
AAT1168/1168A/1168B
SW
C9
0.1µF
U3
BAT 54 S
C10
0.1µF
OUT2 28
IN 2 27
R9
10k Ω
VREF 2
R8
62kΩ
C12
0.1µF
R7
6.8k Ω
Q2
MMBT4401
C11
V_OUT2
1µF _-6V/30mA
Figure 9. The Negative LDO Driver
Example 4:
For system design
V_OUT3 = 25V, R₅ = 200k Ω , R₆ = 10k Ω ,
V_OUT2 = −6V, R₈ = 62k Ω , R₉ = 10k Ω
Flying Capacitors
Increasing the flying capacitor ( C₅ , C₇ , C₉ ) values
can lower output voltage ripples. The 1µF ceramic
capacitors works well in positive LDO driver. A 0.1µF
ceramic capacitor works well in negative LDO driver.
LDO Driver Diode
To achieve high efficiency, a Schottky diode should be
used. BAT54S (Figure 8 and 9) has fast recovery time
and low forward voltage for best efficiency.
LDO Driver Base-Emitter Resistors
For AAT1168, the minimum drive current for positive
and negative LDO driver are 1mA, thus the minimum
base-emitter resistance can be calculated by the
following equation:
R_4(min) ≥ V_BE(max) /((I_OUT3(min) − I_C ) / hfe_(min) )
R_7(min) ≥ V_BE(max) /((I_OUT2(min) − I_C ) / hfe_(min) )
Table 4 Pass Transistor Specifications
MMBT4401 MMBT4403
V_BE(max)
0.65V
0.5V
hfe_(min)
130
90
DIODES Product, Case: SOT23
Example 5:
Output current of V_OUT3 and V_OUT2 are 30mA, the
minimum base-emitter resistor can be calculated as
R_4(min) ≥ 0.5 /(( 1mA − 30mA ) / 90) ≥ 750 Ω
R_7(min) ≥ 0.65 /(( 1mA − 30mA ) /130) ≥ 845 Ω
The minimum value can be used, however, the larger
value has the advantage of reducing quiescent current.
So we choose 6.8k Ω to be R₄.
Charge Pump Output Capacitor
Using low ESR ceramic capacitor to reduce the output
voltage ripple is recommended. With ceramic capacitor,
output voltage ripple is dominated by the capacitance
value. The minimum capacitance value can be
calculated by the following equation:
Cout ≥ ^Iload
2V_ripplef_s
Example 6:
The output voltage ripple of V_OUT3 and VGL is under 1%,
the minimum capacitance value can be calculated as
Cout(V_OUT3 )
≥
30mA
η2 × 250mV ×1.19MHz
≈
0.1µF
Cout(V_GL )
≥
30mA
η2 × 60mV ×1.19MHz
≈
0.33µF
η : Efficiency, about 60% at charge pump circuit
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