LP3906 datasheet(40/47 Pages) TI

40 / 47 page

3.5

4.0

4.5

VIN (V)

1.0

1.5

2.0

2.5

3.0

3.5

5.5

5.0

LP3906

SNVS456M – AUGUST 2006 – REVISED MAY 2013

www.ti.com

The I2C Chip ID address is offered as a metal mask option. The current value equals 0x60.

MODE BOUNCE

PFM-PWM transition at low load current.

To improve efficiency at lower load currents LP3906 buck converters employ an automatically invoked PFM

mode for the low load operation. The PFM mode operates with a much lower value quiescent current (IQ) than

the PWM mode of operation that is used in the higher load currents.

As shown in the datasheet section about SW operation, there is a DC voltage difference between the two modes

of operation, with Vout PFM being typically 1.2% higher than Vout PWM. So there is a DC voltage level transition

and some associated dynamic perturbation at the mode transition point.

The transition between the two modes of operation has an associated hysterisis in the transition current value,

That is, the transition point for increasing current (PFM to PWM) is at a higher value that the decreasing current

(PWM to PFM). This hysterisis is to ensure that in the event that the load current values equals the PFM PWM

transition value, the device will not make multiple transitions between modes; this reduces the noise at this load

by eliminating multiple transitions between modes (also known as mode bounce).

Under some conditions of high Vin and Low Vout the hystersis value is reduced and some amount of mode

bounce can occur. Under these conditions, the regulator still maintains DC regulation, however the output ripple

is more pronounced. Refer to the attached Vout vs Vin chart below that shows the operational area that may

exhibit this increased output ripple. If the application is expected to be operated in the area of concern AND have

a static load current of the transition current value, the user can avoid the possible noise increase by invoking the

components’ “Force PWM mode”.

Figure 40. LP3906 Buck Converter

VOUT vs VIN Operating Range

during PFM-PWM-PPM Transition

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LP3906 Datasheet(PDF) 40 Page - Texas Instruments