MAX1845 Maxim, MAX1845 Datasheet - Page 19

• Manufacturer Part Number: MAX1845
• Description: Dual / High-Efficiency / Step-Down Controller with Accurate Current Limit
• Manufacturer: Maxim

Table 4. Frequency Selection Guidelines
The actual microfarad capacitance value required
relates to the physical 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 volt-
age rating rather than by capacitance value (this is true
of tantalums, OS-CONs™, and other electrolytics).
When using low-capacity filter capacitors such as
ceramic or polymer types, capacitor size is usually
determined by the capacity needed to prevent V
and V
sients. Also, the capacitance must be great enough to
prevent the inductor’s stored energy from launching the
output above the overvoltage protection threshold.
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
V
Stability is determined by the value of the ESR zero rel-
ative to the switching frequency. The point of instability
is given by the following equation:
where:
For a typical 300kHz application, the ESR zero frequen-
cy must be well below 95kHz, preferably below 50kHz.
Tantalum and OS-CON capacitors in widespread use
at the time of publication have typical ESR zero fre-
quencies of 15kHz. In the design example used for
inductor selection, the ESR needed to support 20mV
ripple is 20mV/2A = 10mΩ. Three 470µF/6V Kemet
T510 low-ESR tantalum capacitors in parallel provide
OS-CON is a trademark of Sanyo.
SOAR
TON SETTING
SOAR
FLOAT
AGND
equations in the Transient Response section).
V
REF
CC
f
ESR
from causing problems during load tran-
=
______________________________________________________________________________________
2
Output Capacitor Stability
f
Controller with Accurate Current Limit
ESR
×
FREQUENCY
π
SIDE 1
(kHz)
×
235
345
485
620
≤
R
1
f
SW
ESR
π
Considerations
×
Dual, High-Efficiency, Step-Down
C
F
K-FACTOR
SIDE 1
SAG
4.24
2.96
2.08
1.63
(µs)
SAG
and
P-P
10mΩ (max) ESR. Their typical combined ESR results in
a zero at 11.3kHz, well within the bounds of stability.
Do not put high-value ceramic capacitors directly
across the outputs without taking precautions to ensure
stability. Large ceramic capacitors can have a high-
ESR zero frequency and cause erratic, unstable opera-
tion. However, it is easy to add enough series
resistance by placing the capacitors a couple of inches
downstream from the inductor and connecting OUT_ or
the FB_ divider close to the inductor.
Unstable operation manifests itself in two related but
distinctly different ways: double-pulsing and feedback-
loop instability.
Double-pulsing occurs due to noise on the output or
because the ESR is so low that there is not enough volt-
age ramp in the output voltage signal. This “fools” the
error comparator into triggering a new cycle immedi-
ately 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, which is caused by insufficient ESR.
Loop instability can result in oscillations at the output
after line or load perturbations that can trip the overvolt-
age protection latch or cause the output voltage to fall
below the tolerance limit.
The easiest method for checking stability is to apply a
very fast zero-to-max load transient (refer to the
MAX1845 EV kit manual) and carefully observe the out-
put voltage ripple envelope for overshoot and ringing. It
helps 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- or
overshoot.
FREQUENCY
SIDE 2
(kHz)
170
255
355
460
K-FACTOR
SIDE 2
5.81
4.03
2.81
2.18
(µs)
APPROXIMATE
ERROR (%)
K-FACTOR
±12.5
±12.5
±10
±10
19