A7801
AiT Semiconductor Inc.
DC-DC CONVERTER
www.ait-ic.com
LOW NOISE, REGULATED CHARGE PUMP
Where fOSC is the A7801 oscillator frequency (typically 0.4MHz) and COUT is the output charge storage
capacitor.
Both the style and value of the output capacitor can significantly affect the stability of the A7801. The A7801
use a linear control loop to adjust the strength of the charge pump to match the current required at the output.
The error signal of this loop is stored directly on the output charge storage capacitor.
The charge storage capacitor also serves to form the dominant pole for the control loop. To prevent ringing or
instability on the A7801 it is important for the output capacitor to maintain at least 2.2uF of capacitance over
all conditions.
Likewise excessive ESR on the output capacitor will tend to degrade the loop stability of the A7801.
Ceramic capacitors typically have exceptional ESR performance and combined with a tight board layout
should yield very good stability and load transient performance.
As the value of COUT controls the amount of output ripple, the value of CIN controls the amount of ripple
present at the input pin (VIN). The input current to the A7801 will be relatively constant while the charge pump
is on either the input charging phase or the output charging phase but will drop to zero during the clock non
overlap times. Since the non overlap time is small (~25ns), these missing “notches” will result in only a small
perturbation on the input power supply line. Note that a higher ESR capacitor such as tantalum will have
higher input noise due to the input current change times the ESR. Therefore ceramic capacitors are again
recommended for their exceptional ESR performance.
Flying Capacitor Selection
Warning: A polarized capacitor such as tantalum or aluminum should never be used for the flying capacitor
since its voltage can reverse upon start-up of the A7801. Low ESR ceramic capacitors should always be used
for the flying capacitor.
The flying capacitor controls the strength of the charge pump. In order to achieve the rated output current it is
necessary to have at least 2.2uF of capacitance for the flying capacitor.
For very light load applications the flying capacitor may be reduced to save space or cost. The theoretical
minimum output resistance of a voltage doubling charge pump is given by:
Where fOSC is the switching frequency (0.4MHz typ) and CFLY is the value of the flying capacitor. The charge
pump will typically be weaker than the theoretical limit due to additional switch resistance, however for very
REV1.0
- JAN 2015 RELEASED -
- 7 -
AITSEMI [ AiT Semiconductor ]
