Datasheet

R
S2
=
0.5 - 3 x 0.1
45P x 0.78
- 2000 - 100 = 3598:
R
S2
=
V
CL
- I
ILIM
x R
SNS
45P x D
- 2000 - R
S1
I
O
1-D
2
P
CS
=
x R
SNS
x D
R
SNS
=
33 x 0.5 x 0.5
(40 - 9) x 3 x 0.78 + 33 x 0.5 x 3
= 0.068:
R
SNS
=
L x f
SW
x V
CL
(V
O
± V
IN
) x 3 x D + L x f
SW
x I
LIM
LM5022
www.ti.com SNVS480G –JANUARY 2007–REVISED DECEMBER 2013
R
SNS
, R
S2
AND CURRENT LIMIT
The current sensing resistor R
SNS
is used for steady state 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
SNS
value selected must be low enough to keep the power dissipation to a
minimum, yet high enough to provide good signal-to-noise ratio for the current sensing circuitry. R
SNS
, and R
S2
should be set so that the current limit comparator, with a threshold of 0.5V, trips before the sensed current
exceeds the peak current rating of the inductor, without limiting the output power in steady state.
For this example the peak current, at V
IN(MIN)
, is 2.5A, while the inductor itself is rated to 3.2A. The threshold for
current limit, I
LIM
, is set slightly between these two values to account 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
SNS
and
the compensation ramp current flowing through the internal 2 kΩ, R
S1
and R
S2
resistors. R
SNS
should be selected
first because it is a power resistor with more limited selection. The following equation should be evaluated at
V
IN(MIN)
, when duty cycle is highest:
(36)
where
• L is in µH
• f
SW
in MHz (37)
The closest 5% value is 100 mΩ. Power dissipation in R
SNS
can be estimated by calculating the average current.
The worst-case average current through R
SNS
occurs at minimum input voltage/maximum duty cycle and can be
calculated as:
(38)
P
CS
= [(0.5 / 0.22)
2
x 0.1] x 0.78 = 0.4W (39)
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
SNS
selected, R
S2
can be determined using the following expression:
(40)
(41)
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 amplifier, which is an op-amp configured as an
inverting amplifier. Figure 18 shows the regulator control loop components.
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