Advanced Analog Technology, Inc
.
May 2008
AAT1168/1168A/1168B
C9
0.1µF
C10
0.1µF
Table 4 Pass Transistor Specifications
SW
U3
BAT 54S
MMBT4401
0.65V
MMBT4403
0.5V
V
BE(max)
R7
6.8kΩ
hfe
Q2
MMBT4401
130
90
(min)
OUT2
IN2
28
27
R8
62kΩ
DIODES Product, Case: SOT23
VOUT2
-6V/30mA
R9
10kΩ
C11
1µF
Example 5:
VREF
2
C12
0.1µF
Output current of VOUT3 and VOUT2 are 30mA, the
minimum base-emitter resistor can be calculated as
Figure 9. The Negative LDO Driver
Example 4:
For system design
R
R
≥
0.5/((1mA − 30mA )/90) ≥ 750
Ω
4
7
(min)
≥ 0.65/((1mA − 30mA )/130) ≥ 845
Ω
(min)
V
OUT3 = 25V,
R
R
= 200k
Ω
,
,
R
= 10k
6
Ω
,
5
8
The minimum value can be used, however, the larger
value has the advantage of reducing quiescent current.
VOUT2 = −6V,
= 62k
Ω
R = 10kΩ
9
So we choose 6.8k
Ω to be R4.
Flying Capacitors
Increasing the flying capacitor (
C
,
C
, C ) values
7 9
5
Charge Pump Output Capacitor
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.
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:
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.
Iload
Cout ≥
2Vripple s
f
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:
Example 6:
The output voltage ripple of VOUT3 and VGL is under 1%,
the minimum capacitance value can be calculated as
30mA
Cout(VOUT3 ) ≥
≈
0.1µF
R
R
≥
V
/((I
−I )/hfe
(
min
)
4
7
BE(max)
OUT3
C
(min)
(min)
(min)
η2× 250mV ×1.19MHz
)
30mA
≥ V
/((I
−I )/hfe
C
)
BE(max)
OUT2
(
min
(min)
Cout(V ) ≥
GL
≈ 0.33µF
)
η2× 60mV ×1.19MHz
η
: Efficiency, about 60% at charge pump circuit
–
–
–
Advanced Analog Technology, Inc. –
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