LM46002PWPR datasheet(16/52 Pages) TI

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FBB

FBT

OUT

RFBT

RFBB

VOUT

1.E+03

1.E+04

1.E+05

1.E+06

0.001

0.010

0.100

1.000

LOAD CURRENT (A)

VIN = 12V

VIN = 24V

VIN = 36V

C007

LM46002

SNVSA13B – APRIL 2014 – REVISED SEPTEMBER 2014

www.ti.com

Feature Description (continued)

Figure 34. Switching Frequency Decreases with Lower Load Current in PFM Operation

VOUT = 5 V FS = 1 MHz

In PFM operation, a small positive DC offset is required at the output voltage to activate the PFM detector. The

lower the frequency in PFM, the more DC offset is needed at VOUT. Please refer to the Typical Characteristics for

typical DC offset at very light load. If the DC offset on VOUT is not acceptable for a given application, a static load

at output is recommended to reduce or eliminate the offset. Lowering values of the feedback divider RFBT and

RFBB can also serve as a static load. In conditions with low VIN and/or high frequency, the LM46002 may not

enter PFM mode if the output voltage cannot be charged up to provide the trigger to activate the PFM detector.

Once the LM46002 is operating in PFM mode at higher VIN, it will remain in PFM operation when VIN is reduced.

8.3.3 Adjustable Output Voltage

The voltage regulation loop in the LM46002 regulates output voltage by maintaining the voltage on FB pin ( VFB)

to be the same as the internal REF voltage (VREF). A resistor divider pair is needed to program the ratio from

output voltage VOUT to VFB. The resistor divider is connected from the VOUT of the LM46002 to ground with the

mid-point connecting to the FB pin.

Figure 35. Output Voltage Setting

The voltage reference system produces a precise voltage reference over temperature. The internal REF voltage

is 1.011 V typically. To program the output voltage of the LM46002 to be a certain value VOUT, RFBB can be

calculated with a selected RFBT by

(1)

The choice of the RFBT depends on the application. RFBT in the range from 10 kΩ to 100 kΩ is recommended for

most applications. A lower RFBT value can be used if static loading is desired to reduce VOUT offset in PFM

operation. Lower RFBT will reduce efficiency at very light load. Less static current goes through a larger RFBT and

might be more desirable when light load efficiency is critical. But RFBT larger than 1 MΩ is not recommended

because it makes the feedback path more susceptible to noise. Larger RFBT value requires more carefully

designed feedback path on the PCB. The tolerance and temperature variation of the resistor dividers affect the

output voltage regulation. It is recommended to use divider resistors with 1% tolerance or better and temperature

coefficient of 100 ppm or lower.

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