Datasheet

2040 2060 2080 2100 2120 2140
-60
-50
-40
-30
-20
-10
0
dB
FREQUENCY (kHz)
LP3972
SNVS468K –SEPTEMBER 2006–REVISED MAY 2013 www.ti.com
LDO - LOW DROP OUT OPERATION
The LP3972 can operate at 100% duty cycle (no switching; PMOS switch completely on) for low drop out support
of the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage.
When the device operates near 100% duty cycle, output voltage ripple is approximately 25 mV. The minimum
input voltage needed to support the output voltage is
V
IN, MIN
= I
LOAD
* (R
DSON, PFET
+ R
INDUCTOR
) + V
OUT
where
• I
LOAD
= Load Current
• R
DSON, PFET
= Drain to source resistance of PFET switch in the triode region
• R
INDUCTOR
= Inductor resistance (1)
SPREAD SPECTRUM FEATURE
Periodic switching in the buck regulator is inherently a noisier function block compared to an LDO. It can be
challenging in some critical applications to comply with stringent regulatory standards or simply to minimize
interference to sensitive circuits in space limited portable systems. The regulator’s switching frequency and
harmonics can cause "noise" in the signal spectrum. The magnitude of this noise is measured by its power
spectral density. The power spectral density of the switching frequency, F
C
, is one parameter that system
designers want to be as low as practical to reduce interference to the environment and subsystems within their
products. The LP3972 has a user selectable function on chip, wherein a noise reduction technique known as
"spread spectrum" can be employed to ease customer’s design and production issues.
The principle behind spread spectrum is to modulate the switching frequency slightly and slowly, and spread the
signal frequency over a broader bandwidth. Thus, its power spectral density becomes attenuated, and the
associated interference electro-magnetic energy is reduced. The clock used to modulate the LP3972 buck
regulator can be used as a spread spectrum clock via 2 I
2
C control register (System Control Register 1 (SCR1)
8h’80) bits bk_ssen, and slomod. With this feature enabled, the intense energy of the clock frequency can be
spread across a small band of frequencies in the neighborhood of the center frequency. The results in a
reduction of the peak energy!
The LP3972 spread spectrum clock uses a triangular modulation profile with equal rise and fall slopes. The
modulation has the following characteristics:
• The center frequency: F
C
= 2 MHz, and
• The modulating frequency, f
M
= 6.8 kHz or 12 kHz.
• Peak frequency deviation: Δ_f = ±100 kHz (or ±5%)
• Modulation index β = Δ_f/f
M
= 14.7 or 8.3
Switching Energy RBW = 300 Hz
Figure 18.
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