Datasheet

DCGain
(MOD)
=
R
LOAD
x V
IN
10R
S
(V
IN
+ 2V
OUT
)
OFF
IN
C21 R1 R3 R1 R3
t Ln 1 .98
R1 R3 V R3
ª º
   
  
« »
 
¬ ¼
R3 = 1.23 x
V
IN(MIN)
+ 5 PA x R1 - 1.23
R1
LM5118, LM5118-Q1
www.ti.com SNVS566H –APRIL 2008–REVISED JANUARY 2014
R1, R3, C21
A voltage divider can be connected to the UVLO pin to set a minimum operating voltage VIN
(UVLO)
for the
regulator. If this feature is required, the easiest approach to select the divider resistor values is to choose a value
for R1 between 10 kΩ and 100 kΩ, while observing the minimum value of R1 necessary to allow the UVLO
switch to pull the UVLO pin low. This value is:
R1 ≥ 1000 x V
IN(MAX)
R1 ≥ 75 k in our example
R3 is then calculated from
(28)
Since VIN
(MIN)
for our example is 5 V, set VIN
(UVLO)
to 4.0 V for some margin in component tolerances and input
ripple.
R1 = 75 k is chosen since it is a standard value
R3 = 29.332 k is calculated from Equation 28. 29.4 k was used since it is a standard value.
Capacitor C21 provides filtering for the divider and the off time of the “hiccup” duty cycle during current limit. The
voltage at the UVLO pin should never exceed 15 V when using an external set-point divider. It may be necessary
to clamp the UVLO pin at high input voltages.
Knowing the desired off time during “hiccup” current limit, the value of C21 is given by:
(29)
Notice that t
OFF
varies with V
IN
In this example, C21 was chosen to be 0.1 µF. This will set the t
OFF
time to 723 µs with VIN = 12 V.
R2
A 1M pull-up resistor connected from the EN pin to the VIN pin is sufficient to keep enable in a high state if on-off
control is not used.
Snubber
A snubber network across the buck recirculating diode reduces ringing and spikes at the switching node.
Excessive ringing and spikes can cause erratic operation and increase noise at the regulator output. In the limit,
spikes beyond the maximum voltage rating of the LM5118 or the recirculating diode can damage these devices.
Selecting the values for the snubber is best accomplished through empirical methods. First, make sure the lead
lengths for the snubber connections are very short. Start with a resistor value between 5 and 20 Ω. Increasing
the value of the snubber capacitor results in more damping, however the snubber losses increase. Select a
minimum value of the capacitor that provides adequate clamping of the diode waveform at maximum load. A
snubber may be required for the boost diode as well. The same empirical procedure applies. Snubbers were not
necessary in this example.
ERROR AMPLIFIER CONFIGURATION
R4, C18, C17
These components configure the error amplifier gain characteristics to accomplish a stable overall loop gain. One
advantage of current mode control is the ability to close the loop with only three feedback components, R4, C18
and C17. The overall loop gain is the product of the modulator gain and the error amplifier gain. The DC
modulator gain of the LM5118 is as follows:
(30)
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