Datasheet

1
(2S x R
LOAD
x C
OUT
)
f
P(MOD)
=
R
LOAD
(A x R
S
)
DC_GAIN
(MOD)
=
LM5119/LM5119Q
www.ti.com SNVS676F –AUGUST 2010–REVISED FEBRUARY 2013
MOSFET SNUBBER
A resistor-capacitor snubber network across the low-side MOSFET reduces ringing and spikes at the switching
node. Excessive ringing and spikes can cause erratic operation and couple noise to the output. 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 50Ω. Increasing the value of the
snubber capacitor results in more damping, but higher snubber losses. Select a minimum value for the snubber
capacitor that provides adequate damping of the spikes on the switch waveform at high load. A snubber may not
be necessary with an optimized layout.
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 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 the 5V
output design example, the modulator is treated as an ideal voltage-to-current converter. The DC modulator gain
of the LM5119 can be modeled as:
(40)
Note that A is the gain of the current sense amplifier which is 10 in the LM5119. The dominant low frequency
pole of the modulator is determined by the load resistance (R
LOAD
) and output capacitance (C
OUT
). The corner
frequency of this pole is:
(41)
For R
LOAD
= 5V / 8A = 0.625Ω and C
OUT
= 514μF (effective) then f
P(MOD)
= 496Hz
DC Gain
(MOD)
= 0.625Ω / (10 x 10mΩ) = 6.25 = 15.9dB
For the 5.0V design example, the modulator gain vs. frequency characteristic is shown in Figure 10.
Figure 10. Modulator Gain and Phase
Components R
COMP
and C
COMP
configure the error amplifier as a Type II configuration. The DC gain of the
amplifier is 80dB with a pole at 0Hz and a zero at f
ZEA
= 1 / (2π x R
COMP
x C
COMP
). The error amplifier zero
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 degrees of phase margin. For
the design example, a conservative target loop bandwidth (crossover frequency) of 11kHz was selected. The
compensation network zero (f
ZEA
) should be selected at least an order of magnitude less than the target
crossover frequency. This constrains the product of R
COMP
and C
COMP
for a desired compensation network zero 1
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