Datasheet
MAX15035
15A Step-Down Regulator with Internal Switches
22 ______________________________________________________________________________________
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:
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.
f
RC
RRRR
SW
EFF OUT
EFF E
S
RP
C
B
CO
MP
ππ
≥
=++
1
2
C
IN
L1
C
OUT
PWR
PWR
PWR
BST
LX
IN
PGND
FB
AGND
AGND
2f
SW
R
ESR
C
OUT
1
≥
STABILITY REQUIREMENT
MAX15035
OUTPUT
INPUT
Figure 9. Standard Application with Output Polymer or Tantalum
PCB PARASITIC RESISTANCE-SENSE
RESISTANCE FOR EVALUATION
OUTPUT VOLTAGE REMOTELY
SENSED NEAR POINT OF LOAD
FEEDBACK RIPPLE IN PHASE WITH INDUCTOR CURRENT
C
IN
L1
C
OUT
PWR
PWR
PWR
R
COMP
100Ω
PWR
BST
LX
IN
DH
PGND
FB
GND
AGND
2f
SW
R
ESR
C
OUT
1
≥
f
SW
AND R
COMP
C
COMP
1
≥
STABILITY REQUIREMENT
MAX15035
C
LOAD
PWR
OUTPUT
INPUT
C
COMP
0.1μF
Figure 10. Remote-Sense Compensation for Stability and Noise Immunity