A7116
AiT Semiconductor Inc.
www.ait-ic.com
DC-DC CONVERTER/ BUCK (STEP-DOWN)
1.5MHz, 700mA SYNCHRONOUS WITH LOW QUIESCENT CURRENT
Dropout Operation
When the input voltage decreases toward the value of the output voltage, the A7116 allows the main switch to
(Note 6)
remain on for more than one switching cycle and increases the duty cycle
until it reaches 100% The
output voltage then is the input voltage minus the voltage drop across the main switch and the inductor. At
low input supply voltage, the RDS(ON) of the P-Channel MOSFET increases, and the efficiency of the converter
decreases. Caution must be exercised to ensure the heat dissipated not to exceed the maximum junction
temperature of the IC.
Note 6: The duty cycle D of a step-down converter is defined as:
V OUT
D = TON × fOSC ×100% ≈
×100%
VIN
Where TON is the main switch on time and fOSC is the oscillator frequency (1.5MHz).
Maximum Load Current
The A7116 will operate with input supply voltage as low as 2.5V, however, the maximum load current
decreases at lower input due to large IR drop on the main switch and synchronous rectifier. The slope
compensation signal reduces the peak inductor current as a function of the duty cycle to prevent
sub-harmonic oscillations at duty cycles greater than 50%. Conversely the current limit increases as the
duty cycle decreases.
Layout Guidance
When laying out the PCB board, the following suggestions should be taken to ensure proper operation of the
A7116. These items are also illustrated graphically in Figure 2 & Figure 3.
1. The power traces, including the GND trace, the SW trace and the VIN trace should be kept short, direct
and wide.
2. The VFB pin should be connected directly to the feedback resistor. The resistive divider R1/R2 must be
connected between the (+) plate of Cout and ground.
3. Connect the (+) plate of C1 to the VIN pin as closely as possible. This capacitor provides the AC current
to internal power MOSFET.
4. Keep the switching node, SW, away from the sensitive VFB node.
5. Keep the (-) plates of C1 and C3 as close as possible.
REV1.2
- JAN 2009 RELEASED, MAY 2016 UPDATED -
- 9 -
AITSEMI [ AiT Semiconductor ]
