lm5088mhx-2 National Semiconductor Corporation, lm5088mhx-2 Datasheet - Page 12

• Manufacturer Part Number: lm5088mhx-2
• Description: Wide Input Range Non-synchronous Buck Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
Where, T
ns typical, 365 ns maximum); f
frequency and oscillator period, respectively.
From the above equation, it can be seen that for a given out-
put voltage, reducing the dropout voltage requires either re-
ducing the forced off-time or oscillator frequency (1/T
The forced off-time is limited by the time required to replenish
the bootstrap capacitor and time required to sample the re-
circulating diode current. The 365 ns forced off-time of the
LM5088 controller is a good trade-off between these two re-
quirements. Thus the LM5088 reduces dropout voltage by
dynamically decreasing the operating frequency during
dropout. The Dynamic Frequency Control (DFC) is achieved
Frequency Dithering
(LM5088-1 Only)
Electro-Magnetic Interference (EMI) emissions are funda-
mentally 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 environ-
ment and potentially interfering with nearby electronic sys-
tems. System designers typically use a combination of
shielding, filtering and layout techniques to reduce the EMI
emissions sufficiently to satisfy EMI emission standards es-
tablished by regulatory bodies. In a typical fixed frequency
switching converter, narrowband emissions typically peak at
the switching frequency with the successive harmonics hav-
ing 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 re-
ducing the size and quantity of EMI filtering components.
The LM5088-1 provides an optional frequency dithering func-
tion which is enabled by connecting a capacitor from the dither
OSC
= 1/f
SW
and T
FIGURE 4. Dropout Voltage Reduction using Dynamic Frequency Control
OFF (max)
SW
and T
is the forced off-time (280
OSC
are the oscillator
OSC
).
12
using a dropout monitor, which detects a dropout condition
and reduces the operating frequency. The operating frequen-
cy will continue to decrease with decreasing input voltage until
the frequency falls to the minimum value set by the DFC cir-
cuitry.
If the VIN voltage continues to fall below this point, output
regulation can no longer be maintained. The oscillator fre-
quency will revert back to the nominal operating frequency set
by the RT resistor when the input voltage increases above the
dropout range. DFC circuitry does not affect the PWM during
normal operating conditions.
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 5, the C
wave centered at 1.2V. This triangular waveform is used to
manipulate the oscillator circuit such that the oscillator fre-
quency modulates from -5% to +5% of the nominal operating
frequency set by the RT resistor. The C
sets the rate of the low frequency modulation i.e., a lower val-
ue C
-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
be selected such that;
dither
capacitor will modulate the oscillator frequency from
dither
f
SW(minDFC)
capacitor is used to generate a triangular
≊
1/3 x f
SW(nominal)
30083910
dither
SW
capacitor value
) , C
dither
should