Datasheet

LTC3813
23
3813fb
Figure 11. Type 2 Schematic and Transfer Function
Figure 12. Type 3 Schematic and Transfer Function
The two types of compensation networks, Type 2 and Type
3 are shown in Figures 11 and 12. When component values
are chosen properly, these networks provide a “phase
bump” at the crossover frequency. Type 2 uses a single
pole-zero pair to provide up to about 60° of phase boost
while Type 3 uses two poles and two zeros to provide up
to 150° of phase boost.
The compensation of boost converters are complicated
by two factors: the RHP zero and the dependence of the
loop gain on the duty cycle. The RHP zero adds additional
phase lag and gain. The phase lag degrades phase margin
and the added gain keeps the gain high typically in the
frequency region where the user is trying the roll off the
gain below 0dB. This often forces the user to choose a
crossover frequency at a lower frequency than originally
desired. The duty cycle effect of gain (see above transfer
function) causes the phase margin and crossover frequency
to be dependent on the input supply voltage which may
cause problems if the input voltage varies over a wide range
since the compensation network can only be optimized
for a specifi c crossover frequency. These two factors
usually can be overcome if the crossover frequency is
chosen low enough.
Feedback Component Selection
Selecting the R and C values for a typical Type 2 or
Type 3 loop is a nontrivial task. The applications shown
in this data sheet show typical values, optimized for the
power components shown. They should give acceptable
performance with similar power components, but can be
way off if even one major power component is changed
signifi cantly. Applications that require optimized transient
response will require recalculation of the compensation
values specifi cally for the circuit in question. The underly-
ing mathematics are complex, but the component values
can be calculated in a straightforward manner if we know
the gain and phase of the modulator at the crossover
frequency.
Modulator gain and phase can be obtained in one of
three ways: measured directly from a breadboard, or if
APPLICATIONS INFORMATION
GAIN (dB)
3813 F11
0
PHASE
–6dB/OCT
–6dB/OCT
GAIN
PHASE (DEG)
FREQ
–90
–180
–270
–360
R
B
V
REF
R1
R2
FB
C2
IN
OUT
+
–
C1
GAIN (dB)
3813 F12
0
PHASE
–6dB/OCT
+6dB/OCT –6dB/OCT
GAIN
PHASE (DEG)
FREQ
–90
–180
–270
–360
R
B
V
REF
R1
R2
FB
C2
IN
OUT
+
–
C1
C3
R3