lm5088mhx-2 National Semiconductor Corporation, lm5088mhx-2 Datasheet - Page 13

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

Figure 6 shows the conducted emissions on the LM5088
evaluation board input power line. It can be seen from the
above picture that, the peak emissions with non-dithering op-
eration are centered narrowly at the operating frequency of
the converter. With dithering operation, the conducted emis-
sions are spread around the operating frequency and the
maximum amplitude is reduced by approximately 10dB. (Fig-
ure 6 was captured using a Chroma DC power supply model
number 62006P and an Agilent network analyzer model num-
ber 4395A).
FIGURE 6. Conducted Emissions Measured at the Input
of a LM5088 Based Buck Converter
FIGURE 5. Frequency Dithering Scheme
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Cycle-by-Cycle Current Limit
The LM5088 contains a current limit feature that protects the
circuit from extended over current conditions. The emulated
current signal is directly proportional to the buck switch cur-
rent and is applied to the current limit comparator. If the
emulated current exceeds 1.2V, the PWM cycle is terminated.
The peak inductor current required to trigger the current limit
comparator is given by:
Where, A = 10V/V is the current sense amplifier gain, C
is the Ramp capacitor, R
is the voltage ramp added for slope compensation and 1.2V
is the reference of the current limit comparator.
Since the current that charges the RAMP capacitor is propor-
tional to VIN-VOUT, if the output is suddenly shorted, the
VOUT term is zero and the RAMP charging current increases.
The increased RAMP charging current will immediately re-
duce the PWM duty cycle.The LM5088 also includes a buck
switch protection scheme. A dedicated comparator monitors
S
is the sense resistor,
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RAMP