AS3709 datasheet(11/74 Pages) AMSCO

11 / 74 page

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

AS3709 – 11

Detaile d Desc ri p t ion - P o w e r M a nagemen t Fu nc tions

necessary at low output loads, and therefore the efficiency

increases. As drawback, this mode increases the ripple up to a

higher output current.

The crossover point to low power mode is already reached at

reasonable high output currents. (e.g. @110mA for VIN = 3V,

VOUT = 1.2V, 1μH, 4MHz)

Figure 10:

DC/DC Buck High Efficiency Mode

Low Power Mode Operation (Automatically Controlled)

As soon as the output voltage stays above the desired target

value for a certain time, some internal blocks will be powered

down leaving the output floating to lower the power

consumption. Normal operation starts as soon as the output

drops below the target value for a similar amount of time. To

minimize the accuracy error some internal circuits are kept

powered to assure a minimized output voltage ripple.

Two addition guard bands, based on comparators, are set at

±5% of the target value to react quickly on large

over/undershoots by immediately turning on the output

drivers without the normal time delays. This ensures a

minimized ripple also in very extreme load conditions.

DVM (Dynamic Voltage Management)

To minimize the over-/undershoot during a change of the

output voltage, the DVM can be enabled. With DVM the output

voltage will ramp up/down with a selectable slope after the new

value was written to the registers. Without DVM the slew rate

of the output voltage is only determined by external

components like the coil and load capacitor as well as the load

current.

DVM can be selected for all step-down converters, but only for

one at a time. (see sd_dvm_select and dvm_time description)

DC/DC buck burst mode: Shows the

DC/DC switching waveforms for high

efficiency operation

VOUT (50mV/div)

LX (2V/div)

coil current (200mA/div)

time (20us/div)

AS3709 датащи(PDF) 11 Page - ams AG