LTC3815_15 Datasheet, PDF (24/42 Pages) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

LTC3815
Applications Information
very different than the one specified in the application
circuits or if a Type 3 compensation network is required.
A Type 3 compensation network includes the additional
components R_C3 and C_C3 shown in Figure 9.
1) Choose the crossover frequency. For best performance,
the crossover frequency should be as high as possible
but not greater than about 20% of the switching fre-
quency.
2) Plot Gain and Phase of the modulator and output filter.
The modulator and output filter is the portion of the
loop from the error amp output (I_TH) to the regulator
output (V_OUT). To do this, insert a 10Ω to 50Ω resistor
between the output capacitor and the V_CCSEN pin (this
is R7 on the DC2065 demo board). Then use a network
analyzer to inject an AC signal across this resistor and
plot the Gain and Phase. If a network analyzer is not
available, a close approximation can obtained with a
PSPICE simulator using the LTC3815 power supply
model shown in Figure 9. The error amplifier EA can
be modeled as an ideal op amp (for crossover frequen-
cies < 200kHz) and the inductor as a voltage controlled
current source. The gain/phase plot should look similar
to Figure 10.
3) Calculate the component values. Once the gain and
phase are known, note the gain (GAIN, in dB) and phase
(PHASE, in degrees) at the crossover frequency. The
compensation components can then be calculated to
make the loop gain (V_CCSEN to V_OUT) equal to 0dB and
the phase margin equal to 60° at this frequency. Use
the equations below to calculate the component values.
Normally the Type 2 network will provide the required
phase margin. If not, use the Type 3 equations.
4) Add the calculated components to the LTC3815 circuit
and check the load step response. If not adequate, the
components values may need to be tweaked further or
recalculated with a lower crossover frequency until the
desired response is obtained.
R_C3* ^CC3*
C_C1
R_C2
C_C2
V_CCSEN
R_C1
V_REF
–
EA
+
*TYPE 3 ONLY
I_TH
g_m = 12 • V_ITH
R_ESR
+
R_LOAD
3815 F09
Figure 9. PSPICE Model of a LTC3815 Current Mode Regulator
GAIN
0
0
PHASE
–90
FREQUENCY (Hz)
–180
3815 F10
Figure 10. Transfer Function of Buck Modulator
3815p
24
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