MAX1714BEEE Maxim Integrated Products, MAX1714BEEE Datasheet - Page 17

• Manufacturer Part Number: MAX1714BEEE
• Description: Current Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1714BEEE

• Number Of Outputs: 1
• Output Voltage: 1 V to 5.5 V
• Output Current: 3000 mA
• Mounting Style: SMD/SMT
• Package / Case: QSOP-16
• Switching Frequency: 600 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Buck
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

the current-limit tolerance. A 1µA minimum divider current
is recommended.
The output filter capacitor must have low enough effective
series resistance (ESR) to meet output ripple and load-
transient requirements, yet have high enough ESR to sat-
isfy stability requirements. Also, the capacitance value
must be high enough to absorb the inductor energy
going from a full-load to no-load condition without tripping
the overvoltage protection circuit.
In CPU V
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 non-CPU applications, the output capacitor’s size
depends on how much ESR is needed to maintain an
acceptable level of output voltage ripple:
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 voltage rating
rather than by capacitance value (this is true of tantalums,
OS-CONs, and other electrolytics).
When using low-capacity filter capacitors such as ceram-
ic or polymer types, capacitor size is usually determined
by the capacity needed to prevent V
causing problems 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 (also, see the V
tion in the Transient Response section).
Stability is determined by the value of the ESR zero rela-
tive to the switching frequency. The point of instability is
given by the following equation:
For a typical 300kHz application, the ESR zero frequency
must be well below 95kHz, preferably below 50kHz.
Output Capacitor Stability Considerations
CORE
where f
R
converters and other applications where
ESR
ESR
______________________________________________________________________________________
R
ESR
≤
Output Capacitor Selection
=
LIR I
f
2
ESR
≤
⋅ ⋅
⋅
I
π
LOAD(MAX)
Vp - p
LOAD(MAX)
=
R
V
ESR
1
π
f
DIP
SAG
SAG
High-Speed Step-Down Controller
⋅
C
and V
F
and V
SOAR
SOAR
equa-
from
for Notebook Computers
Tantalum and OS-CON capacitors in widespread use at
the time of publication have typical ESR zero frequencies
of 25kHz. In the design example used for inductor selec-
tion, the ESR needed to support 50mVp-p ripple is
60mV/2.7A = 22mΩ. Two 470µF/4V Kemet T510 low-ESR
tantalum capacitors in parallel provide 22mΩ max ESR.
Their typical combined ESR results in a zero at 27kHz,
well within the bounds of stability.
Don’t 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. However, it’s easy to add enough series resis-
tance by placing the capacitors a couple of inches
downstream from the feedback sense point, which
should be as close as possible to the inductor (see the
All-Ceramic-Capacitor Application section).
Unstable operation manifests itself in two related but dis-
tinctly 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 isn’t enough volt-
age ramp in the output voltage signal. This “fools” the
error comparator into triggering 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,
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 and carefully
observe the output voltage ripple envelope for over-
shoot and ringing. It can help to simultaneously monitor
the inductor current with an AC current probe. Don’t
allow more than one cycle of ringing after the initial
step-response under- or overshoot.
The input capacitor must meet the ripple current
requirement (I
Nontantalum chemistries (ceramic, aluminum, or OS-
CON) are preferred due to their resistance to power-up
surge currents.
I
RMS
RMS
=
I
LOAD
) imposed by the switching currents.
Input Capacitor Selection



 

V
OUT
(
V - V
V
IN
IN
OUT
)



 

17