R16DS0199EU0101 Rev.1.01

Page 13

Feb 6, 2024

RAA808013 Datasheet

5.

Functional Description

The RAA808013 is the low voltage buck regulator capable of running at 100% duty cycle. It can output 3A current

continuously from a 2.7V to 5.5V input voltage with excellent load and line regulation. The Protection includes

input UVP, cycle-by-cycle peak current limit, hiccup mode OCP and OTP. The constant on-time control scheme

provides a fast response for dynamic load and high efficiency for a light load.

Comparied to the traditional COT, RAA808013 offers a nearly constant frequency by input voltage feed-forward

and output voltage feedback. It can keep the frequency at 1.5MHz (typical) across the full input and output voltage

range. It can run in DCM by using a zero-cross detector circuit to achieve high efficiency at light load. Also, in

sleep mode, it can achieve significantly low standby power loss.

The chip can run in constant off-time mode when VIN is close to VOUT. It can keep the VOUT in the regulation even

if VIN is close to VOUT. The 100% duty cycle can be achieved when VOUT equals VIN.

5.1

Soft-Start

When VIN reaches 2.65V and EN is high, the PWM starts up with an internal soft-start cap being charged by an

internal current source after a short time delay. The soft-start time is 800μs. The delay time of EN/Power-on to

soft-start is 750μs. Figure 32 shows the startup sequence diagram. RAA808013 supports pre-bias start-up.

5.2

Constant On-Time Operation

RAA808013 operates in a constant on-time mode in most light load to full load, regulating the output voltage with

the constant on-time operation. With VIN feed-forward and VOUT feedback, the switching frequency in this mode is

almost constant.

5.3

100% Duty Cycle

When the input voltage is lower than the output set point, the high-side MOSFET stays on to keep the output

voltage equal to the input voltage.

5.4

Light-Load Operation

When the load is light, with the valley of inductor current dropping to below zero, the RAA808013 switches to DCM

mode.

When the load is light enough, the RAA808013 enters into PFM Sleep mode. In the sleep mode, off-time

increases, and some unnecessary circuits are disabled for power saving. Figure 33 shows the DCM operation.

The IC enters the DCM mode of operation when eight consecutive cycles of inductor current crossing zero are

detected.

Figure 32. Startup Sequence

Vo

EN

tDELAY

tSS