Acutechnology Semiconductor Inc.
AQ112
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 AQ112 will remain stable with C1 and C2 as low as 1.0 µF.
Overall transient performance is improved with increased capacitance and the addition of C3.
4. The output is fully enabled when Enable is 950 mV above the expected VOUT. EN may
be driven by either a digital or analog signal to control either turn-on time or to give full
control of risetime.
5. Enable, tied to any separate source >0.95V + VOUT, will insure Ultra-low drop out voltage
(400mV@ 1A) from VIN to VOUT. Or, Enable, tied to Vin will support a low drop out
voltage (0.95V).
2. Stability
An Enable capacitor is recommended. A 1.0 µF capacitor on VIN is a suitable input bypass
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 (CADJ) 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*fRIPPLE* CADJ) < R1
R1 is normally in the range of 1KΩ.
The output capacitor is critical in maintaining regular stability. The AQ1541 is stable with an
output capacitor greater than 1.0 µF. 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 CADJ is used, a larger output capacitance may be required.
The capacitor C3 may also be necessary if large ripple is present on the VIN line. Tantalum
Capacitors exhibit the best stability over a wide range of loads and are recommended.
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Rev. 1.6 September 13, 2006
ACUTECH [ ACUTECHNOLOGY SEMICONDUCTOR ]
