LP3906SQ-JXXI Datasheet, PDF (40/47 Pages) Texas Instruments – Dual High-Current Step-Down DC/DC and Dual Linear Regulator with I2C ompatible

LP3906
SNVS456M – AUGUST 2006 – REVISED MAY 2013
The I²C Chip ID address is offered as a metal mask option. The current value equals 0x60.
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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 (I_Q) 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”.
3.5
3.0
2.5
2.0
1.5
1.0
0
3.5
4.0
4.5
5.0
5.5
V_IN (V)
Figure 40. LP3906 Buck Converter
V_OUT vs V_IN Operating Range
during PFM-PWM-PPM Transition
40
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