LP3905 datasheet(10/15 Pages) NSC

10 / 15 page

Functional Description (Continued)

the cycle repeats to restore the output voltage (average

voltage in pfm mode) to

∼1.15% above the nominal PWM

output voltage.

If the load current should increase during PFM mode caus-

ing the output voltage to fall below the ‘low2’ PFM threshold,

the part will automatically transition into fixed-frequency

PWM mode. When V

IN =2.8V the part transitions from PWM

to PFM mode at ~35mA output current and from PFM to

PWM mode at ~85mA , when V

IN=3.6V, PWM to PFM tran-

sition happens at ~50mA and PFM to PWM transition hap-

pens at ~100mA, when V

IN =4.5V, PWM to PFM transition

happens at ~65mA and PFM to PWM transition happens at

~115mA.

SOFT START

The LP3905 Buck Converters have a soft-start circuit that

limits in-rush current during start-up. Additionally, in case

EN1 and EN2 are enabled at the same time, a typical 500µs

delay between Buck1 and Buck2 Power Up prevents any

further Inrush current from the battery.

During start-up the switch current limit is increased in steps.

Soft start is activated only if EN goes from logic low to logic

high after Vin reaches 3V. Soft start is implemented by

increasing switch current limit in steps of 70mA, 140mA,

280mA and 1000mA (typ. switch current limit). The start-up

time thereby depends on the output capacitor and load

current demanded at start-up. Typical start-up times with

22µF output capacitor and 300mA load current is 400µs and

with 1mA load current its 275µs.

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FIGURE 10. Operation in PFM Mode and Transfer to PWM Mode

LP3905 bảng dữ liệu(PDF) 10 Page - National Semiconductor (TI)