MAX15041_10 Datasheet, PDF (14/18 Pages) Maxim Integrated Products – Low-Cost, 3A, 4.5V to 28V Input, 350kHz, PWM Step-Down DC-DC Regulator with Internal Switches

Low-Cost, 3A, 4.5V to 28V Input, 350kHz, PWM
Step-Down DC-DC Regulator with Internal Switches
GAIN
1ST ASYMPTOTE
V_FB x V_OUT ^-1 x 10^AVEA[dB]/20 x G_MOD x R_LOAD
2ND ASYMPTOTE
V_FB x V_OUT ^-1 x g_MV x (C_C)^-1 x G_MOD x R_LOAD
UNITY
1ST POLE
g_MV x (10^AVEA[dB]/20 C_C)^-1
3RD ASYMPTOTE
V_FB x V_OUT ^-1 x g_MV x (C_C)^-1 x G_MOD x R_LOAD x ( C_OUT(ESR + R_LOAD))^-1
1ST ZERO
(C_CR_C)^-1
2ND POLE
(C_OUT(ESR + R_LOAD))^-1
4TH ASYMPTOTE
V_FB x V_OUT ^-1 x g_MV x R_C x G_MOD x R_LOAD x (C_OUT(ESR + R_LOAD))^-1
2ND ZERO
(C_OUTESR)^-1
3RD POLE
(C_CCR_C)^-1
RAD/S
CO
5TH ASYMPTOTE
V_FB x V_OUT ^-1 x g_MV x R_C x G_MOD x (ESR || R_LOAD)
6TH ASYMPTOTE
V_FB x V_OUT ^-1 x g_MV x ( C_CC)^-1 x G_MOD x (ESR || R_LOAD)
Figure 2. Asymptotic Loop Response of Peak Current-Mode Regulator
If C_OUT is large, or exhibits a lossy equivalent series
resistance (large ESR), the circuit’s second zero may
come into play around the crossover frequency (f_CO =
ω_CO/2π). In this case, a third pole may be induced by a
second (optional) small compensation capacitor (C_CC),
connected from COMP to SGND.
The loop response’s fourth asymptote (in bold, Figure
2) is the one of interest in establishing the desired
crossover frequency (and determining the compensa-
tion component values). A lower crossover frequency
provides for stable closed-loop operation at the
expense of a slower load and line transient response.
Increasing the crossover frequency improves the tran-
sient response at the (potential) cost of system instabili-
ty. A standard rule of thumb sets the crossover
frequency ≤ 1/10 of the switching frequency (for the
MAX15041, this is approximately 35kHz for the 350kHz
fixed switching frequency).
First, select the passive and active power components
that meet the application’s requirements. Then, choose
the small-signal compensation components to achieve
the desired closed-loop frequency response and phase
margin as outlined in the Closing the Loop: Designing
the Compensation Circuitry section.
Closing the Loop: Designing the
Compensation Circuitry
1) Select the desired crossover frequency. Choose f_CO
equal to 1/10^th of f_SW, or f_CO ≈ 35kHz.
2) Select R_C using the transfer-loop’s fourth asymptote
gain (assuming f_CO > f_P1, f_P2, and f_Z1 and setting
the overall loop gain to unity) as follows:
1=
V_FB
V_OUT
× g_MV
× R_C
× G_MOD
× R_LOAD
×
2π
×
f_CO
×
1
C_OUT ×
(ESR
+
R_LOAD
)
therefore:
( ) _RC
=
V_OUT
V_FB
×
2π
×
f_CO × C_OUT ×
g_MV × G_MOD
ESR + R_LOAD
× R_LOAD
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