Datasheet

MAX8792
Single Quick-PWM Step-Down
Controller with Dynamic REFIN
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
rising load edge is no longer a problem (see the V
SAG
and V
SOAR
equations in the Transient Response sec-
tion). Thus, the output capacitor selection requires
carefully balancing capacitor chemistry limitations
(capacitance vs. ESR vs. voltage rating) and cost.
Output Capacitor Stability Considerations
For Quick-PWM controllers, stability is determined by
the in-phase feedback ripple relative to the switching
frequency, 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 equivalent-series resistance of the output capaci-
tors, R
PCB
is the parasitic board resistance between
the output capacitors and feedback sense point, and
R
COMP
is the effective resistance of the DC- or AC-cou-
pled current-sense compensation (see Figure 10).
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.
f
RC
RRR R
SW
EFF OUT
EFF ESR PCB COMP
ππ
≥
=++
1
2
C
IN
L1
C
OUT
PWR
PWR
PWR
PWR
BST
LX
TON
DH
DL
FB
GND
AGND
2f
SW
R
ESR
C
OUT
1
≥
STABILITY REQUIREMENT
MAX8792
OUTPUT
INPUT
Figure 8. 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
TON
DH
DL
FB
GND
AGND
2f
SW
R
ESR
C
OUT
1
≥
f
SW
AND R
COMP
C
COMP
1
≥
STABILITY REQUIREMENT
MAX8792
C
LOAD
PWR
OUTPUT
INPUT
Figure 9. Remote-Sense Compensation for Stability and Noise Immunity