lm5088mhx-2 National Semiconductor Corporation, lm5088mhx-2 Datasheet - Page 16

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

www.national.com
The maximum ripple current occurs at the maximum input
voltage. Typically, I
the full load current. Higher ripple current will result in a small-
er inductor. However, it places more burden on the output
capacitor to smooth out the ripple current to achieve low out-
put ripple voltage. For this example 40% ripple was chosen
for a smaller sized inductor.
The nearest standard value of 6.8 µH will be used. To prevent
saturation, the inductor must be rated for the peak current.
During normal operation, the peak current occurs at maxi-
mum load current (plus maximum ripple). With properly
scaled component values, the peak current is limited to V
(TH)
voltage with a shorted output, the chosen inductor must be
evaluated at elevated temperature. It should be noted that the
saturation current rating of inductors drops significantly at el-
evated temperatures.
CURRENT SENSE RESISTOR
The current limit value (I
tor (R
R
Some ‘margin’ beyond the maximum load current is recom-
mended for the current limit threshold. In this design example,
S
/R
can be calculated by
S
S
).
During overload conditions. At the maximum input
PP
is selected between 20% and 40% of
LIM
) is set by the current sense resis-
FIGURE 9. Inductor Current
CS
16
the current limit is set at 10% above the maximum load cur-
rent, resulting in a R
should be Kelvin connected to the current sense resistor.
RAMP CAPACITOR
With the inductor and sense resistor value selected, the value
of the ramp capacitor (C
ramp circuit is given by:
Where, L is the value of the output inductor, gm is the ramp
generator transconductance (5 µA/V) and A is the current
sense amplifier gain (10V/V). For the current design example,
the ramp capacitor is calculated as:
The next lowest standard value 270 pF was selected for
C
ommended. It should be noted that selecting a capacitor value
lower than the calculated value will increase the slope com-
pensation. Furthermore, selecting a ramp capacitor substan-
tially lower or higher than the calculated value will also result
in incorrect PWM operation.
For VOUT > 5V, internal slope compensation provided by the
LM5088 may not be adequate for certain operating conditions
especially at low input voltages. A pull-up resistor may be
added from VCC to RAMP the pin to increase the slope com-
pensation. Optimal slope compensation current may be cal-
culated from
and R
RAMP
RAMP
. An NPO capacitor with 5% or better tolerance is rec-
is given by
S
I
OS
value of 10 mΩ. The CS and CSG pins
= V
RAMP
OUT
) necessary for the emulation
x 5 µA/V
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