LP3913 datasheet(52/70 Pages) TI1

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PFM Mode at Light Load

PWM Mode at

Moderate to Heavy

Loads

Load current

increases

Low2 PFM Threshold

Vout

Low PFM

Threshold,

turn on

PFET

High PFM

Voltage

Threshold

reached,

go into

sleep mode

Nfet on

drains

conductor

until

I inductor=0

Pfet on

until

Ipfm limit

reached

Current load

increases,

draws Vout

towards

Low2 PFM

Threshold

Low2 PFM Threshold,

switch back to PWMmode

Low1 PFM Threshold

~1.008 * Vout

High PFM Threshold

~1.016 * Vout

LP3913

SNVS489I – NOVEMBER 2006 – REVISED SEPTEMBER 2013

www.ti.com

Once the PMOS power switch is turned off, the NMOS power switch is turned on until the inductor current ramps

to zero. When the NMOS zero-current condition is detected, the NMOS power switch is turned off. If the output

voltage is below the ‘high’ PFM comparator threshold (see Figure 66), the PMOS switch is again turned on and

the cycle is repeated until the output reaches the desired level. Once the output reaches the ‘high’ PFM

threshold, the NMOS switch is turned on briefly to ramp the inductor current to zero and then both output

switches are turned off and the part enters an extremely low power mode. Quiescent supply current during this

‘sleep’ mode is less than 30 µA, which allows the part to achieve high efficiencies under extremely light load

conditions. When the output drops below the ‘low’ PFM threshold, the cycle repeats to restore the output voltage

≈1.6% above the nominal PWM output voltage.

If the load current should increase during PFM mode (see Figure 66) causing the output voltage to fall below the

‘low2’ PFM threshold, the part will automatically transition into fixed-frequency PWM mode.

Figure 66.

BUCK1, BUCK2 Operation

BUCK1 is recommended to be used as the processor core supply and has I2C selectable output voltages ranging

from 0.8V to 2.0V (Typ.). BUCK2 is recommended for IO power, Memory power and logic power. Its voltage

range can be programmed using the I2C interface from 1.8V to 3.3V (Typ.). The default output voltage for each

buck converter is factory programmable (See APPLICATIONS INFORMATION).

The system designer can also determine the output voltage of either BUCK1 or BUCK2 through an external

feedback resistor ladder by clearing the output voltage selection field in the BUCK1 or BUCK2 control registers.

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