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LTC3810-5 Datasheet, PDF (25/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller | |||
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LTC3810-5
APPLICATIONS INFORMATION
Refer to your SPICE manual for details of how to generate
this plot.
*3810-5 modulator gain/phase
*2006 Linear Technology
*this ï¬le simulates a simpliï¬ed model of
*the LTC3810-5 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
Mathematical software such as MATHCAD or MATLAB
can also be used to generate plots using the following
transfer function of the modulator:
H(s)
=
VSENSE(MAX)
1.2 ⢠RDS(ON)
â¢
1+ s â¢
1+ s
RESR
⢠RL â¢
⢠COUT
COUT
⢠RL
(2)
s = j2f
With the gain/phase plot in hand, a loop crossover frequency
can be chosen. Usually the curves look something like
Figure 13. Choose the crossover frequency about 25%
of the switching frequency for maximum bandwidth. Al-
though it may be tempting to go beyond fSW/4, remember
that signiï¬cant phase shift occurs at half the switching
frequency that isnât modeled in the above H(s) equation
and PSPICE code. Note the gain (GAIN, in dB) and phase
GAIN
0
0
PHASE
â90
FREQUENCY (Hz)
â180
38105 F13
Figure 13. Transfer Function of Buck Modulator
(PHASE, in degrees) at this point. The desired feedback
ampliï¬er gain will be âGAIN to make the loop gain at 0dB
at this frequency. Now calculate the needed phase boost,
assuming 60° as a target phase margin:
BOOST = â (PHASE + 30°)
If the required BOOST is less than 60°, a Type 2 loop can
be used successfully, saving two external components.
BOOST values greater than 60° usually require Type 3
loops for satisfactory performance.
Finally, choose a convenient resistor value for RFB1 (10k is
usually a good value). Now calculate the remaining values:
(K is a constant used in the calculations)
f = chosen crossover frequency
G = 10(GAIN/20) (this converts GAIN in dB to G in
absolute gain)
TYPE 2 Loop:
K
=
tan
BOOST
2
+
45°
C2
=
2
â¢
f
â¢
1
Gâ¢
K
â¢
RFB1
( ) C1= C2 K2 1
R2
=
2
K
â¢f
â¢
C1
RFB2
=
VREF (RFB1)
VOUT VREF
38105fc
25
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