MAX15035 Datasheet, PDF (22/27 Pages) Maxim Integrated Products – 15A Step-Down Regulator with Internal Switches

15A Step-Down Regulator with Internal Switches
With most chemistries (polymer, tantalum, aluminum
electrolytic), the actual capacitance value required
relates to the physical size needed to achieve low ESR
and the chemistry limits of the selected capacitor tech-
nology. Ceramic capacitors provide low ESR, but the
capacitance and voltage rating (after derating) are
determined by the capacity needed to prevent V_SAG
and V_SOAR from causing problems during load tran-
sients. Generally, once enough capacitance is added to
meet the overshoot requirement, undershoot at the ris-
ing load edge is no longer a problem (see the V_SAG and
V_SOAR equations in the Transient Response section).
Thus, the output capacitor selection requires carefully
balancing capacitor chemistry limitations (capacitance
vs. ESR vs. voltage rating) and cost. See Figure 9.
Output Capacitor Stability Considerations
For Quick-PWM controllers, stability is determined by the
in-phase feedback ripple relative to the switching frequen-
cy, which is typically dominated by the output ESR. The
boundary of instability is given by the following equation:
f_{SW ≥}
1
π 2πR_EFFC_OUT
R_EFF = R_ESR + R_PCB + R_COMP
where C_OUT is the total output capacitance, R_ESR is the
total ESR of the output capacitors, R_PCB is the parasitic
board resistance between the output capacitors and
feedback sense point, and R_COMP is the effective resis-
tance of the DC- or AC-coupled current-sense compen-
sation (see Figure 11).
For a standard 300kHz application, the effective zero
frequency must be well below 95kHz, preferably below
50kHz. With these frequency requirements, standard
tantalum and polymer capacitors already commonly
used have typical ESR zero frequencies below 50kHz,
allowing the stability requirements to be achieved with-
out any additional current-sense compensation. In the
standard application circuit (Figure 1), the ESR needed
to support a 15mV_P-P ripple is 15mV/(10A x 0.3) =
5mΩ. Two 330µF, 9mΩ polymer capacitors in parallel
provide 4.5mΩ (max) ESR and 1/(2π x 330µF x 9mΩ) =
53kHz ESR zero frequency. See Figure 10.
IN
BST
C_IN
PWR
INPUT
LX
PGND
MAX15035
FB
AGND
AGND
L1
PWR
C_OUT
PWR
OUTPUT
STABILITY REQUIREMENT
R_ESRC_OUT ≥ ¹
2f_SW
Figure 9. Standard Application with Output Polymer or Tantalum
IN
BST
DH
LX
PGND
MAX15035
PWR
FB
GND
AGND
PWR
C_IN
PWR
L1
C_COMP
0.1µF
INPUT
PCB PARASITIC RESISTANCE-SENSE
RESISTANCE FOR EVALUATION
C_OUT
PWR
R_COMP
100Ω
C_LOAD
PWR
OUTPUT
OUTPUT VOLTAGE REMOTELY
SENSED NEAR POINT OF LOAD
STABILITY REQUIREMENT
R_ESRC_OUT ≥ ¹ AND R_COMPC_COMP ≥ ¹
2f_SW
f_SW
FEEDBACK RIPPLE IN PHASE WITH INDUCTOR CURRENT
Figure 10. Remote-Sense Compensation for Stability and Noise Immunity
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