Low2 PFM Threshold,

switch back to PWM mode

PFET on

until

lpfm limit

reached

Current load

increases,

draws Vout

towards

Low2 PFM

Threshold

Low PFM

Threshold,

turn on

PFET

High PFM

Voltage

Threshold

reached,

go into

sleep mode

NFET on

drains

conductor

current

until

I inductor = 0

Load current

increases

Low2 PFM Threshold

Vout

High PFM Threshold

~1.017*Vout

Low1 PFM Threshold

~1.006*Vout

PWM Mode at

Moderate to Heavy

Loads

PFM Mode at Light Load

LP3905

SNVS374D – JUNE 2006 – REVISED MAY 2013

www.ti.com

Figure 9. Operation in PFM Mode and Transfer to PWM Mode

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.

12

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