Acutechnology Semiconductor Inc.
AQ1117
Application Notes
1. Typical Application
Notes:
1. Output voltage is 1.22V * (R2 +R1)/R1
2. Input and output capacitors should be located close to the device.
3. The AQ1117 will remain stable with C1 and C2 as low as 1.0µF. Overall transient
performance is improved with increased capacitance.
2. Stability
An input capacitor is recommended. A 1.0µF capacitor on the input is a suitable input
bypassing for almost all applications. A larger capacitor is also suitable. In the adjustable
version the “adjust” terminal can be bypassed to ground with a bypass capacitor (C
ADJ
) to
improve ripple rejection. This bypass capacitor prevents ripple from being amplified as the
output voltage is increased. At any ripple frequency, the impedance of the C
ADJ
should be
less than R1 (being R1 the resistor between the output and the adjust pin) to prevent the
ripple from being amplified:
Z = 1/(2π*f
RIPPLE
* C
ADJ
) < R1
R1 is normally in the range of 1KΩ.
The output capacitor is critical in maintaining regular stability. The AQ1117 is stable with an
output capacitor greater than. Of course any increase of the output capacitor will merely improve
the loop stability and the load transient response. In the case of the adjustable regulator, when
the C
ADJ
is used, a larger output capacitance may be required. Tantalum Capacitors exhibit the
best stability over a wide range of loads and are recommended.
3. Output Voltage
The AQ1117 adjustable version develops a 1.22V reference voltage between the output and
the adjust pin terminal. This voltage is applied across the resistor R1 to generate a constant
current (I1). The current from the adjust terminal could introduce error to the output, but since
it is very small (< 20µA) compared with the current I1 and very constant with line and load
changes, the error can be ignored. The constant current I1 then flows through resistor R2
and sets the output voltage to the desired level.
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Rev. 1.5 September 20, 2005
ACUTECH [ ACUTECHNOLOGY SEMICONDUCTOR ]
