MAX17085BETL+ Maxim Integrated Products, MAX17085BETL+ Datasheet - Page 31

• Manufacturer Part Number: MAX17085BETL+
• Description: Battery Management Dual Main Step-Down Controller
• Manufacturer: Maxim Integrated Products

Specifications of MAX17085BETL+

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The output filter capacitor must have low enough equivalent
series resistance (ESR) to meet output ripple and load-
transient requirements, yet have high enough ESR to satisfy
stability requirements.
For processor core voltage converters and other applica-
tions where the output is subject to violent load transients,
the output capacitor’s size depends on how much ESR is
needed to prevent the output from dipping too low under
a load transient. Ignoring the sag due to finite capacitance:
In applications without large and fast load transients, the
output capacitor’s size often depends on how much ESR
is needed to maintain an acceptable level of output voltage
ripple. The output voltage ripple of a step-down controller
equals the total inductor ripple current multiplied by the out-
put capacitor’s ESR. Therefore, the maximum ESR required
to meet ripple specifications is:
The actual capacitance value required relates to the physi-
cal size needed to achieve low ESR, as well as to the
chemistry of the capacitor technology. Thus, the capacitor
is usually selected by ESR and voltage rating rather than
by capacitance value (this is true of tantalums, OS-CONs,
polymers, and other electrolytics).
When using low-capacity filter capacitors, such as ceramic
capacitors, size is usually determined by the capacity need-
ed to prevent V
during load transients. Generally, once enough capacitance
is added to meet the overshoot requirement, undershoot
at the rising load edge is no longer a problem (see the
Transient Response section). However, low-capacity filter
capacitors typically have high ESR zeros that may affect
the overall stability (see the Output Capacitor Stability
Considerations section).
For Quick-PWM controllers, stability is determined by the
value of the ESR zero relative to the switching frequency.
The boundary of instability is given by the following equation:
where:
Output Capacitor Stability Considerations
Integrated Charger, Dual Main Step-Down
SAG
R
R
______________________________________________________________________________________
f
ESR
ESR
ESR
and V
f
≤
=
ESR
≤
I
2 R
Controllers, and Dual LDO Regulators
LOAD(MAX)
D
G
I
SOAR
LOAD(MAX)
≤
V
ESR OUT
V
RIPPLE
f
SW
STEP
G
1
C
Output Capacitor ESR
from causing problems
LIR
For a typical 600kHz application, the ESR zero frequency
must be well below 200kHz, preferably below 100kHz.
Tantalum and OS-CON capacitors in widespread use at
the time of publication have typical ESR zero frequen-
cies of 25kHz. In the design example used for inductor
selection, the ESR needed to support 25mV
25mV/1.2A = 20.8mI. One 220FF/4V SANYO polymer
(TPE) capacitor provides 15mI (max) ESR. This results
in a zero at 48kHz, well within the bounds of stability.
Do not put high-value ceramic capacitors directly across
the feedback sense point without taking precautions to
ensure stability. Large ceramic capacitors can have a
high ESR zero frequency and cause erratic, unstable
operation. Unstable operation manifests itself in two
related but distinctly different ways: double-pulsing and
fast-feedback loop instability. Double-pulsing occurs
due to noise on the output or because the ESR is so
low that there is not enough voltage ramp in the output
voltage signal. This “fools” the error comparator into trig-
gering a new cycle immediately after the 400ns minimum
off-time period has expired. Double-pulsing is more
annoying than harmful, resulting in nothing worse than
increased output ripple. However, it can indicate the
possible presence of loop instability due to insufficient
ESR. Loop instability results in oscillations at the output
after line or load steps. Such perturbations are usually
damped, but can cause the output voltage to rise above
or fall below the tolerance limits.
The easiest method for checking stability is to apply
a very fast zero-to-max load transient and carefully
observe the output voltage ripple envelope for overshoot
and ringing. It can help to simultaneously monitor the
inductor current with an AC current probe. Do not allow
more than one cycle of ringing after the initial step-
response under/overshoot.
The inductor ripple current also impacts transient-
response performance, especially at low V
differentials. Low inductor values allow the inductor cur-
rent to slew faster, replenishing charge removed from
the output filter capacitors by a sudden load step. This
favors higher switching-frequency operation.
The SMPSs include an extended on-time feature that
reduces the output capacitor requirements due to heavy
load transients. The capacitance required is also a func-
tion of the maximum duty factor and can be calculated
from the following equation:
C
OUT
≥
L I
(
D
2V
LOAD(MAX)
SAG OUT
V
Transient Response
)
2
K
P-P
SYS
ripple is
- V
OUT
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