Advanced Analog Technology, Inc.
April 2007
AAT1164/AAT1164B/AAT1164C
SW
C5
1µF
used. BAT54S (Figure 8 and 9) has fast recovery time
and low forward voltage for best efficiency.
VOUT1
C6
1µF
U1
BAT54S
13.3V/300mA
LDO Driver Base-Emitter Resistors
For AAT1164, the minimum drive current for positive
and negative LDO drivers are 1mA, thus the minimum
base-emitter resistance can be calculated by the
following equation:
R4
6.8kΩ
Q1
MMBT4403
OUT3 26
C7
1µF
U2
BAT54S
SW
R5
200 kΩ
VOUT3
25V/30mA
IN3 25
C8
1µF
R6
10kΩ
R4
R7
≥ VBE(max) / ((IOUT3(min) −IC ) / hfe(min))
(min)
(min)
VOUT3
1
≥ VBE(max) / ((IOUT2(min) −IC ) / hfe(min)
)
Figure 8. The Positive LDO Driver
Table 4. Pass Transistor Specifications
MMBT4401
0.65V
MMBT4403
VBE(max)
0.5V
90
h
130
fe(min)
DIODES Product, Package: SOT23
Example 5:
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:
R4
R7
≥ 0.5 / ((1mA − 30mA ) / 90) ≥ 750Ω
(min)
(min)
≥ 0.65 / ((1mA − 30mA ) / 130) ≥ 845
Ω
For system design
The minimum value can be used, however, the larger
value has the advantage of reducing quiescent current.
VOUT3 = 25V, R5 = 200kΩ, R6 = 10kΩ,
VOUT2
= −6V, R8 = 62kΩ, R9 = 10kΩ
So we choose6.8kΩ to be R4.
Flying Capacitors
Charge Pump Output Capacitor
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.
Using low ESR ceramic capacitor to reduce the output
voltage ripple is recommended and 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
–
–
–
Advanced Analog Technology, Inc. –
Version 1.00
Page 20 of 24
AAT [ ADVANCED ANALOG TECHNOLOGY, INC. ]
