lm5022mme National Semiconductor Corporation, lm5022mme Datasheet - Page 14

• Manufacturer Part Number: lm5022mme
• Description: 60v Low Side Controller For Boost And Sepic
• Manufacturer: National Semiconductor Corporation

www.national.com
The next highest standard 20% capacitor value is 6.8 µF, but
because the actual input source impedance and resistance
are not known, two 4.7 µF capacitors will be used. In general,
doubling the calculated value of input capacitance provides a
good safety margin. The final calculation is for the RMS cur-
rent. For boost converters operating in CCM this can be
estimated as:
From the inductor section, maximum inductor ripple current is
0.58A, hence the input capacitor(s) must be rated to handle
0.29 x 0.58 = 170 mA
The input capacitors can be ceramic, tantalum, aluminum, or
almost any type, however the low capacitance requirement
makes ceramic capacitors particularly attractive. As with the
output capacitors, the minimum quality dielectric used should
X5R, with X7R or better preferred. The voltage rating for input
capacitors need not be as conservative as the output capac-
itors, as the need for capacitance decreases as input voltage
increases. For this example, the capacitor selected will be 4.7
µF ±20%, rated to 50V, in the 1812 case size. The RMS cur-
rent rating of these capacitors is over 2A each, more than
enough for this application.
CURRENT SENSE FILTER
Parasitic circuit capacitance, inductance and gate drive cur-
rent create a spike in the current sense voltage at the point
where Q1 turns on. In order to prevent this spike from termi-
nating the on-time prematurely, every circuit should have a
low-pass filter that consists of C
1. The time constant of this filter should be long enough to
reduce the parasitic spike without significantly affecting the
shape of the actual current sense voltage. The recommended
range for R
mended range for C
example, the values of R
respectively.
R
The current sensing resistor R
regulation of the inductor current and to sense over-current
conditions. The slope compensation resistor is used to ensure
control loop stability, and both resistors affect the current limit
threshold. The R
keep the power dissipation to a minimum, yet high enough to
provide good signal-to-noise ratio for the current sensing cir-
cuitry. R
comparator, with a threshold of 0.5V, trips before the sensed
current exceeds the peak current rating of the inductor, with-
out limiting the output power in steady state.
For this example the peak current, at V
the inductor itself is rated to 3.2A. The threshold for current
limit, I
for tolerance of the circuit components, at a level of 3.0A. The
required resistor calculation must take into account both the
switch current through R
rent flowing through the internal 2 kΩ, R
R
with more limited selection. The following equation should be
evaluated at V
SNS
SNS
, R
should be selected first because it is a power resistor
LIM
S2
SNS
, is set slightly between these two values to account
AND CURRENT LIMIT
, and R
S1
IN(MIN)
is between 10Ω and 500Ω, and the recom-
SNS
S2
I
CS
RMS
, when duty cycle is highest:
value selected must be low enough to
RMS
should be set so that the current limit
is between 100 pF and 2.2 nF. For this
SNS
= 0.29 x Δi
.
S1
and C
and the compensation ramp cur-
SNS
CS
CS
and R
L(MAX)
is used for steady state
will be 100Ω and 1 nF,
IN(MIN)
S1
S1
and R
, shown in Figure
, is 2.5A, while
S2
resistors.
14
The closest 5% value is 100 mΩ. Power dissipation in R
can be estimated by calculating the average current. The
worst-case average current through R
input voltage/maximum duty cycle and can be calculated as:
For this example a 0.1Ω ±1%, thick-film chip resistor in a 1210
case size rated to 0.5W will be used.
With R
ing expression:
The closest 1% tolerance value is 3.57 kΩ.
CONTROL LOOP COMPENSATION
The LM5022 uses peak current-mode PWM control to correct
changes in output voltage due to line and load transients.
Peak current-mode provides inherent cycle-by-cycle current
limiting, improved line transient response, and easier control
loop compensation.
The control loop is comprised of two parts. The first is the
power stage, which consists of the pulse width modulator,
output filter, and the load. The second part is the error ampli-
fier, which is an op-amp configured as an inverting amplifier.
Figure 7 shows the regulator control loop components.
SNS
P
selected, R
CS
= [(0.5 / 0.22)
L in µH, f
S2
can be determined using the follow-
2
SW
x 0.1] x 0.78 = 0.4W
in MHz
SNS
occurs at minimum
SNS