LP3906SQ-JXXIEV National Semiconductor, LP3906SQ-JXXIEV Datasheet - Page 35

• Manufacturer Part Number: LP3906SQ-JXXIEV
• Description: BOARD EVALUATION LP3906SQ-JXXI
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LP3906SQ-JXXIEV

• Main Purpose: DC/DC, Step Down with LDO
• Outputs And Type: 4, Non-Isolated
• Current - Output: 1.5A, 1.5A, 300mA, 300mA
• Regulator Topology: Buck
• Frequency - Switching: 2MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LP3906
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Output: -
• Voltage - Input: -
• Power - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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
eration 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 opera-
tion, with Vout PFM being typically 1.2% higher than Vout
PWM. So there is a DC voltage level transition and some as-
sociated 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 cur-
rent values equals the PFM PWM transition value, the device
will not make multiple transitions between modes; this re-
duces 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 oc-
cur. 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
Q
) than the PWM mode of op-
35
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 cur-
rent value, the user can avoid the possible noise increase by
invoking the components’ “Force PWM mode”.
during PFM-PWM-PPM Transition
V
OUT
LP3906 Buck Converter
vs V
IN
Operating Range
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