LTC3425_15 Datasheet, PDF (20/24 Pages) Linear Technology – 5A, 8MHz, 4-Phase Synchronous Step-Up DC/DC Converter

LTC3425
APPLICATIO S I FOR ATIO
modulator control to output DC gain, and the error amp
open-loop gain gives the DC gain of the system:
G_DC
=
G_{CONTROLOUTPUT}
• G _EA
•
V_REF
V_OUT
G_CONTROL
=
8 • V_{IN ,}
I_OUT
G_EA
≈ 5,000
The output filter pole is given by:
F_FILTERPOLE
=
π
•
I_OUT
V_OUT • C_OUT
where C_OUT is the output filter capacitor.
The output filter zero is given by:
F_FILTERZERO
=
2
•
π
•
1
R_ESR
•
C_OUT
where R_ESR is the output capacitor equivalent series
resistance.
A troublesome feature of the boost regulator topology is
the right half plane zero (RHP), and is given by:
F_RHPZ
=
2
•
V_IN2
π • I_OUT
•
L
At heavy loads this gain increase with phase lag can occur
at a relatively low frequency. The loop gain is typically
rolled off before the RHP zero frequency.
The typical error amp compensation is shown in Figure 9.
The equations for the loop dynamics are as follows:
F_POLE1
≈
2•
π
1
•100e⁶
• C_C1
which is extremely close to DC
F_ZERO1
=
2
•
π
1
• R_Z
•
C_C1
F_POLE2
=
2
•
π
1
• R_Z
•
C_C2
ERROR
AMP
1.25V
FB
V_OUT
R1
V_C
C_C1
R_Z
Figure 9
R2
C_C2
3425 F09
3425f
20