Datasheet

C
dither
t
f
SW
x 0.12V
100 x 25 PA
Normal Operation Transition Region Low Dropout Mode
E
Dropout
reduce dropout
Forced
off-time
Forced
off-time
Regulation
Point
ON-TIM CLK
VOUT VIN
f
SW(minDFC)
Extended
T
ON(max)
T
ON(max)
T
ON(maxDFC)
increased to
LM25088
LM25088-Q1
SNVS609H –DECEMBER 2008–REVISED MARCH 2013 www.ti.com
Figure 18. Dropout Voltage Reduction using Dynamic Frequency Control
Frequency Dithering (LM25088-1 Only)
Electro-Magnetic Interference (EMI) emissions are fundamentally associated with switch-mode power supplies
due to sharp voltage transitions, diode reverse recovery currents and the ringing of parasitic L-C circuits. These
emissions will conduct back to the power source or radiate into the environment and potentially interfering with
nearby electronic systems. System designers typically use a combination of shielding, filtering and layout
techniques to reduce the EMI emissions sufficiently to satisfy EMI emission standards established by regulatory
bodies. In a typical fixed frequency switching converter, narrowband emissions typically peak at the switching
frequency with the successive harmonics having less energy. Dithering the oscillator frequency spreads the EMI
energy over a range of frequencies, thus reducing the peak levels. Dithering can also reduce the system cost by
reducing the size and quantity of EMI filtering components.
The LM25088-1 provides an optional frequency dithering function which is enabled by connecting a capacitor
from the dither pin (DITH) to GND. Connecting the DITH pin directly to GND disables frequency dithering causing
the oscillator to operate at the frequency established by the RT resistor. As shown in Figure 19, the C
dither
capacitor is used to generate a triangular wave centered at 1.2V. This triangular waveform is used to manipulate
the oscillator circuit such that the oscillator frequency modulates from -5% to +5% of the nominal operating
frequency set by the RT resistor. The C
dither
capacitor value sets the rate of the low frequency modulation i.e., a
lower value C
dither
capacitor will modulate the oscillator frequency from -5% to +5% at a faster rate than a higher
value capacitor. For the dither circuit to work effectively the modulation rate must be much less than the oscillator
frequency (f
SW
) , C
dither
should be selected such that;
(6)
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