LTC3814-5_15 Datasheet, PDF (20/30 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

LTC3814-5
APPLICATIONS INFORMATION
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 specific 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
significantly. Applications that require optimized transient
response will require recalculation of the compensation
values specifically 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
the appropriate parasitic values are known, simulated or
generated from the modulator transfer function. Mea-
surement will give more accurate results, but simulation
or transfer function can often get close enough to give
a working system. To measure the modulator gain and
phase directly, wire up a breadboard with an LTC3814-5
and the actual MOSFETs, inductor and input and output
capacitors that the final design will use. This breadboard
should use appropriate construction techniques for high
speed analog circuitry: bypass capacitors located close
to the LTC3814-5, no long wires connecting components,
appropriately sized ground returns, etc. Wire the feedback
amplifier with a 0.1μF feedback capacitor from I_TH to FB
and a 10k to 100k resistor from V_OUT to FB. Choose the
bias resistor (R_B) as required to set the desired output
voltage. Disconnect R_B from ground and connect it to
a signal generator or to the source output of a network
analyzer to inject a test signal into the loop. Measure the
gain and phase from the I_TH pin to the output node at the
positive terminal of the output capacitor. Make sure the
analyzer’s input is AC coupled so that the DC voltages
present at both the I_TH and V_OUT nodes don’t corrupt the
measurements or damage the analyzer.
C2
IN
R2
C1
R1
FB _–
R_B
V_REF +
–6dB/OCT
GAIN
OUT 0
PHASE
–6dB/OCT
FREQ
–90
–180
–270
–360
38145 F11
IN
C2
C3 _R2
C1
R1 R3
FB _–
R_B
V_REF +
OUT 0
–6dB/OCT
GAIN
+6dB/OCT
–6dB/OCT
PHASE
FREQ
–90
–180
–270
–360
38145 F12
Figure 11. Type 2 Schematic and Transfer Function
20
Figure 12. Type 3 Schematic and Transfer Function
38145fc