LP3925 Datasheet, PDF (23/51 Pages) Texas Instruments – High Performance Power Management Unit for Handset Applications

In low load condition ‘comp’ is used and in high load condition
‘pwm’ is used. Once Vout voltage level gets too small, then in
low load condition (ECO mode) the “ECO Comparator” trig-
gers and “SW Control” opens PFET (wide pulse at SW pin;
see Figure 2). The wideness of that pulse is determined by
“ECO comparator” which has a built in hysteresis. Normal in
ECO mode after that wide pulse has passed no action should
be taken, until next “ECO Comparator” triggering. If some
other condition (PWM threshold/Current limit) is fulfilled, then
buck enters PWM mode.
If ‘peak current limit’ which is a current sensing signal for
PFET’s switch peak currents triggers, then buck enters PWM
mode from the next clock pulse. Peak current sensing is reset
at the beginning of every clock pulse. Once in PWM mode,
buck will stay there at least 32 clock pulses and if ‘PWM
threshold’ indicates that buck should be operating in ECO
mode, it will return into ECO mode. This is what actually hap-
pens on Figure 2 during wide ECO mode pulse. (The reason
behind that is in high peak current during the time PFET is
opened. Rough example:
30120409
FIGURE 12. Buck's Switches Controlling Diagram
where C_OUT = 10 µF, ripple V = 50 mV and PFET open time
= 350 ns).
The output voltage ripple is slightly higher in ECO mode (30
mV peak-peak ripple typ.)
30120457
FIGURE 14. Typical Ripple in ECO Mode
BUCK OUTPUT VOLTAGE SELECTION
The selection of the bucks’ output voltages can be done by
writing a specific code into the control registers (addr. 0x10
… 0x18). The required voltage can be calculated from the
following equation:
Where Y* denotes a number, which corresponds to the DVS
control code. If DVS is not in use, then VOUT0 is used. 1-bit
DVS uses VOUT0 and VOUT1 and 2-bit DVS uses V_OUTs 0
to 3.
30120458
FIGURE 13. Typical ECO Operation
23
www.ti.com