A7121A
DC-DC CONVERTER BUCK (STEP-DOWN)
1MHz, 2.0A, SYNCHRONOUS
AiT Semiconductor Inc.
www.ait-ic.com
DETAILED INFORMATION
Operation
A7121A is a monolithic switching mode Step-Down DC-DC converter. It utilizes internal MOSFETs to achieve
high efficiency and can generate very low output voltage by using internal reference at 0.6V. It operates at a
fixed switching frequency, and uses the slope compensated current mode architecture. This Step-Down
DC-DC Converter supplies 2.0A output current at VOUT = 1.8V with input voltage range from 2.7V to 6.0V.
Current Mode PWM Control
Slope compensated current mode PWM control provides stable switching and cycle-by-cycle current limit for
excellent load and line responses and protection of the internal main switch (P-Ch MOSFET) and
synchronous rectifier (N-CH MOSFET). During normal operation, the internal P-Ch MOSFET is turned on for
a certain time to ramp the inductor current at each rising edge of the internal oscillator, and switched off when
the peak inductor current is above the error voltage. The current comparator, ICOMP, limits the peak inductor
current. When the main switch is off, the synchronous rectifier will be turned on immediately and stay on until
either the inductor current starts to reverse, as indicated by the current reversal comparator, IZERO, or the
beginning of the next clock cycle.
Idle Mode Operation
At very light loads, the A7121A automatically enters pulse skipping Mode. In the pulse skipping Mode, the
inductor current may reach zero or reverse on each pulse. The PWM control loop will automatically skip
pulses to maintain output regulation. The bottom MOSFET is turned off by the current reversal comparator,
I
ZERO, and the switch voltage will ring. This is discontinuous mode operation, and is normal behavior for the
switching regulator.
Power Good Output (PG)
PGOOD is an open-drain type output and requires a pullup resistor. PGOOD is actively held low in soft-start,
standby, and shutdown. It is released when the output voltage rises above 91.5% of nominal regulation point.
The PGOOD signal goes low if the output is turned off or falls below 91.5% of nominal regulation point.
Dropout Operation
When the input voltage decreases toward the value of the output voltage, the A7121A allows the main switch
to remain on for more than one switching cycle and increases the duty cycle NOTE5 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
REV1.3
- JUL 2013 RELEASED, NOV 2016 UPDATED -
- 8 -
AITSEMI [ AiT Semiconductor ]
