LM46001_15 Datasheet, PDF (16/52 Pages) Texas Instruments – LM46001 SIMPLE SWITCHER® 3.5 V to 60 V 1 A Synchronous Step-Down Voltage Converter

LM46001
SNVSA46A – JUNE 2014 – REVISED JULY 2014
Feature Description (continued)
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1000000
100000
10000
0.001
0.01
0.1
Load (A)
VIN = 8 V
VIN = 12 V
VIN = 18 V
VIN = 24 V
VIN = 36 V
1
C001
Figure 34. Switching Frequency Decreases with Lower Load Current in PFM Operation
V_OUT = 5 V, F_S = 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 V_OUT. Please refer to the Typical Characteristics for
typical DC offset at very light load. If the DC offset on V_OUT 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 R_FBT and
R_FBB can also serve as a static load. In conditions with low V_IN and/or high frequency, the LM46001 may not
enter PFM mode if the output voltage cannot be charged up to provide the trigger to activate the PFM detector.
Once the LM46001 is operating in PFM mode at higher V_IN, it will remain in PFM operation when V_IN is reduced.
7.3.3 Adjustable Output Voltage
The voltage regulation loop in the LM46001 regulates output voltage by maintaining the voltage on FB pin (V_FB)
to be the same as the internal REF voltage (V_REF). A resistor divider pair is needed to program the ratio from
output voltage V_OUT to V_FB. The resistor divider is connected from the V_OUT of the LM46001 to ground with the
mid-point connecting to the FB pin.
VOUT
R_FBT
FB
R_FBB
Figure 35. Output Voltage Setting
The voltage reference system produces a precise voltage reference over temperature. The internal REF voltage
is 1.016 V typically. To program the output voltage of the LM46001 to be a certain value V_OUT, R_FBB can be
calculated with a selected R_FBT by
R_FBB

V_FB
V_OUT V_FB
R_FBT
(1)
The choice of the R_FBT depends on the application. R_FBT in the range from 10 kΩ to 100 kΩ is recommended for
most applications. A lower R_FBT value can be used if static loading is desired to reduce V_OUT offset in PFM
operation. Lower R_FBT will reduce efficiency at very light load. Less static current goes through a larger R_FBT and
might be more desirable when light load efficiency is critical. But R_FBT larger than 1 MΩ is not recommended
because it makes the feedback path more susceptible to noise. Larger R_FBT 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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