A7801
AiT Semiconductor Inc.
DC-DC CONVERTER
www.ait-ic.com
LOW NOISE, REGULATED CHARGE PUMP
DETAILED INFORMATION
Operation
The A7801 use a switched capacitor charge pump to boost VIN to a regulated output voltage. Regulation is
achieved by sensing the output voltage through an internal resistor divider and modulating the charge pump
output current based on the error signal. A 2-phase non overlapping clock activates the charge pump
switches. The flying capacitor is charged from VIN on the first phase of the clock. On the second phase of the
clock it is stacked in series with VIN and connected to VOUT. This sequence of charging and discharging the
flying capacitor continues at a free running frequency of 0.4MHz (typ). In shutdown mode all circuitry is turned
off and the A7801 draw only leakage current from the VIN supply. Furthermore, VOUT is disconnected from VIN.
The EN pin is a CMOS input with a threshold voltage of approximately 0.8V. The A7801 is in shutdown when
a logic low is applied to the EN pin. Since the EN pin is a high impedance CMOS input it should never be
allowed to float. To ensure that its state is defined it must always be driven with a valid logic level.
Short-Circuit Protection
The A7801 have built-in short-circuit current limiting. During short-circuit conditions, they will automatically
limit their output current to approximately 300mA.
Soft-Start
The A7801 have built-in soft-start circuitry to prevent excessive current flow at VIN during start-up. The
soft-start time is preprogrammed to approximately 1ms, so the start-up current will be primarily dependent
upon the output capacitor.
VIN, VOUT Capacitor Selection
The style and value of capacitors used with the A7801 determine several important parameters such as
regulator control loop stability, output ripple, charge pump strength and minimum start-up time. To reduce
noise and ripple, it is recommended that low ESR (< 0.1Ω) ceramic capacitors be used for both CIN and COUT
.
These capacitors should be 2.2uF or greater. Tantalum and aluminum capacitors are not recommended
because of their high ESR. The value of COUT directly controls the amount of output ripple for a given load
current. Increasing the size of COUT will reduce the output ripple at the expense of higher minimum turn on
time and higher start-up current. The peak-to-peak output ripple is approximately given by the expression:
REV1.0
- JAN 2015 RELEASED -
- 6 -
AITSEMI [ AiT Semiconductor ]
