Datasheet

-60
-40
-20
0
20
40
60
1.E+01
1.E+02
1.E+03 1.E+04
1.E+05
FREQUENCY (Hz)
GAIN (dB)
-200
-160
-120
-80
-40
0
40
80
PHASE (°)
120
160
200
LM5088, LM5088-Q1
SNVS600H –DECEMBER 2008–REVISED MARCH 2013 www.ti.com
SNUBBER COMPONENTS SELECTION
Excessive ringing and spikes can cause erratic operation and couple spikes and noise to the output. Voltage
spikes beyond the rating of the LM5088 or the re-circulating diode can damage these devices. A snubber
network across the power diode reduces ringing and spikes at the switching node. Selecting the values for the
snubber is best accomplished through empirical methods. First, make sure that the lead lengths for the snubber
connections are very short. For the current levels typical for the LM5088, a resistor value between 3 and 10Ω
should be adequate. As a rule of thumb, a snubber capacitor which is 4~5 times the Schottky diode’s junction
capacitance will reduce spikes adequately. Increasing the value of the snubber capacitor will result in more
damping but also results in higher losses. The resistor’s power dissipation is independent of the resistance value
as the resistor dissipates the energy stored by the snubber capacitor. The resistor’s power dissipation can be
approximated as:
P
R_SNUB
= C
SNUB
x VIN
max
2
x f
SW
(28)
ERROR AMPLIFIER COMPENSATION
R
COMP
, C
COMP
and C
HF
configure the error amplifier gain characteristics to accomplish a stable voltage loop gain.
One advantage of current mode control is the ability of to close the loop with only two feedback components
R
COMP
and C
COMP
. The voltage loop gain is the product of the modulator gain and the error amplifier gain. For
this example, the modulator can be treated as an ideal voltage-to-current (transconductance) converter, The DC
modulator gain of the LM5088 can be modeled as:
DC Gain
(MOD)
= R
LOAD
/ (A x R
S
)
The dominant low frequency pole of the modulator is determined by the load resistance (R
LOAD
) and the output
capacitance (C
OUT
). The corner frequency of this pole is:
For, R
LOAD
= 5V/7A = 0.714Ω and C
OUT
= 500 µF (effective), then FP
(MOD)
= 550 Hz.
DC Gain
(MOD)
= 0.714/ (10 x 10 mΩ) = 7.14 = 17dB
For the 5V design example the modulator gain vs. frequency characteristic was measured as shown in Figure 25.
Figure 25. Modular Gain Phase
Components R
COMP
and C
COMP
configure the error amplifier as a type II compensation configuration. The DC
gain of the amplifier is 80dB which has a pole at low frequency and a zero at F
Zero
= 1/(2π x R
COMP
x C
COMP
).
The error amplifier zero is set such that it cancels the modulator pole leaving a single pole response at the
crossover frequency of the voltage loop. A single pole response at the crossover frequency yields a very stable
loop with 90° of phase margin. For the design example, a target loop bandwidth (crossover frequency) of 15 kHz
was selected. The compensation network zero (F
Zero
) should be at least an order of magnitude lower than the
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