AS3709 datasheet(10/74 Pages) AMSCO

10 / 74 page

AS3709 – 10

ams Datasheet, Confidential: 2013-Aug [1-02]

D e t a i l ed D e scr i pt io n - P o w e r M a na ge me nt F u n c ti o n s

Mode Settings

To allow optimized performance in different applications, there

are bit settings possible, to get the best compromise between

high efficiency and low input/output ripple.

Low-Ripple, Low-Noise Operation

Low-ripple, low-noise operation can be enabled by setting bit

sd_low_noise = 1.

In this mode there is no minimum coil current necessary before

switching OFF the PMOS. As long as the load current is superior

to the ripple current, the device operates in continuous mode.

When the load current gets lower, the discontinuous mode is

triggered. Resultant the auto-zero comparator stops the NMOS

conduction to avoid load discharger and the duty cycle is

reduced down to tMIN_ON to keep the regulation loop stable.

This results in a very low ripple and noise, but decreased

efficiency at light loads, especially at low input to output

voltage differences.

Only in the case the load current gets so small, that less than

the minimum on time of the PMOS would be needed to keep

the loop in regulation, the regulator will enter low power mode

operation.

The crossover point is about 15mA for VIN = 3V, VOUT = 1.2V,

1μH, 4MHz.

Figure 9:

DC/DC Buck Low Noise Mode

High-Efficiency Operation (Default Setting)

High-efficiency operation is enabled by setting bit

sd_low_noise = 0.

In this mode there is a minimum coil current necessary before

switching OFF the PMOS. Resultant there are less pulses

DC/DC buck burst mode: Shows the

DC/DC switching waveforms for low

noise operation.

VOUT (50mV/div)

LX (2V/div)

coil current (200mA/div)

time (5us/div)

AS3709 Datasheet(PDF) 10 Page - ams AG