Datasheet

V
OUT
VIN x f
SW
x C
RAMP
25 PA x
I
PEAK
=
A x R
S
V
OUT
VIN x f
SW
x C
RAMP
1.2V - 25 PA x
0.12V
R
S
or
I
PEAK
#
LM5088, LM5088-Q1
www.ti.com SNVS600H –DECEMBER 2008–REVISED MARCH 2013
Figure 20. Conducted Emissions Measured at the Input of a LM5088 Based Buck Converter
Figure 20 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 operation are centered narrowly at the operating
frequency of the converter. With dithering operation, the conducted emissions are spread around the operating
frequency and the maximum amplitude is reduced by approximately 10dB. (Figure 20 was captured using a
Chroma DC power supply model number 62006P and an Agilent network analyzer model number 4395A).
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 current 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
RAMP
is the ramp capacitor
• R
S
is the sense resistor (7)
(8)
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 proportional 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 reduce the PWM duty cycle. The LM5088 also includes a buck switch protection
scheme. A dedicated comparator monitors the drain to source voltage of the buck FET when it is turned ON, if
the V
DS
exceeds 1.5V, the comparator turns off the buck FET immediately. This feature will help protect the buck
FET in catastrophic conditions such as a sudden saturation of the inductor.
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