LTC3812-5_15 Datasheet, PDF (23/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
APPLICATIONS INFORMATION
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
38125 F11
Figure 11. Type 3 Schematic and Transfer Function
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.
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.
Measurement 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 LTC3812-
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 LTC3812-5, 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.
If breadboard measurement is not practical, a SPICE
simulation can be used to generate approximate gain/phase
curves. Plug the expected capacitor, inductor and MOSFET
values into the following SPICE deck and generate an AC
plot of V_OUT/V_ITH with gain in dB and phase in degrees.
Refer to your SPICE manual for details of how to generate
this plot.
*3810 modulator gain/phase
*2006 Linear Technology
*this file simulates a simplified model of
*the LTC3810 for generating a v(out)/v(ith)
*bode plot
.param rdson=.0135 ;MOSFET rdson
.param Vrng=2
;use 1.4 for INTVCC and
0.7 for ground
.param vsnsmax={0.173*Vrng-0.026}
.param Imax={vsnsmax/rdson}
.param DL=4
;inductor ripple current
*inductor current
gl out 0 value={(v(ith)-1.2)*Imax/1.2+DL/2}
*output cap
cout out out2 270u ;capacitor value
resr out2 0 0.018 ;capacitor ESR
*load
Rout out 0 2 ; load resistor
vstim ith 0 0 ac 1 ;ac stimulus
.ac dec 100 100 10meg
.probe
.end
38125fc
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