MAX8744ETJ+ Maxim Integrated Products, MAX8744ETJ+ Datasheet - Page 29

• Manufacturer Part Number: MAX8744ETJ+
• Description: IC CNTRLR PWR SUP QUAD 32TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8744ETJ+

• Applications: Controller, Notebook Computers
• Voltage - Input: 6 ~ 26 V
• Number Of Outputs: 4
• Voltage - Output: 3.3V, 5V, 1 ~ 26 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Duty Cycle (max): 99 %
• Output Voltage: 3.315 V, 5.015 V, 2 V to 5.5 V
• Mounting Style: SMD/SMT
• Switching Frequency: 200 KHz, 300 KHz, 500 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Boost, Flyback, Forward
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

(see the Output-Capacitor Stability Consideration section),
the filter capacitor’s ESR dominates the output voltage rip-
ple. So the output capacitor’s size depends on the maxi-
mum ESR required to meet the output voltage ripple
(V
In idle mode, the inductor current becomes discontinu-
ous, with peak currents set by the idle mode current-
sense threshold (V
no-load output ripple may be determined as follows:
The actual 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 tanta-
lums, OS-CONs, polymers, and other electrolytics).
When using low-capacity filter capacitors, such as
ceramic capacitors, 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 over-
shoot requirement, undershoot at the rising load edge
is no longer a problem (see the V
tions in 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).
Stability is determined by the value of the ESR zero rel-
ative to the switching frequency. The boundary of insta-
bility 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 25kHz. In the design example used for
inductor selection, the ESR needed to support 25mV
ripple is 25mV / 1.5A = 16.7mΩ. One 220µF/4V Sanyo
polymer (TPE) capacitor provides 15mΩ (max) ESR.
This results in a zero at 48kHz, well within the bounds
of stability.
RIPPLE(P-P)
Supply Controllers for Notebook Computers
V
Output-Capacitor Stability Considerations
RIPPLE(P-P)
) specifications:
High-Efficiency, Quad-Output, Main Power-
V
RIPPLE P P
f
ESR
______________________________________________________________________________________
IDLE
f
( – )
ESR
=
= R
= 0.2V
2π
≤
R
ESR
=
f
ESR OUT
OSC
V
π
I
IDLE ESR
R
1
LOAD(MAX)
LIMIT
SENSE
C
SAG
SAG
R
). In idle mode, the
and V
and V
LIR
SOAR
SOAR
equa-
from
P-P
For low-input voltage applications where the duty cycle
exceeds 50% (V
age should not be greater than twice the internal slope-
compensation voltage:
where V
case ESR limit occurs when V
above equation may be simplified to provide the follow-
ing boundary condition:
Do not put high-value ceramic capacitors directly
across the feedback sense point without taking precau-
tions to ensure stability. Large ceramic capacitors can
have a high ESR zero frequency and cause erratic,
unstable operation. However, it is easy to add enough
series resistance by placing the capacitors a couple of
inches downstream from the feedback sense point,
which should be as close as possible to the inductor.
Unstable operation manifests itself in two related but
distinctly different ways: short/long pulses and cycle
skipping resulting in lower frequency operation.
Instability 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 com-
parator into triggering too early or into skipping a cycle.
Cycle skipping 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 can
result 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 over-
shoot and ringing. It may help to simultaneously moni-
tor the inductor current with an AC current probe. Do
not allow more than three cycles of ringing after the ini-
tial step-response under/overshoot.
The input capacitor must meet the ripple current
requirement (I
For an out-of-phase regulator, the total RMS current in
the input capacitor is a function of the load currents,
the input currents, the duty cycles, and the amount of
overlap as defined in Figure 8.
RIPPLE
RMS
equals ΔI
OUT
V
R
RIPPLE
) imposed by the switching currents.
ESR
/V
Input Capacitor Selection
IN
≤ 0.04 x L x f
≤ 0.02 x V
≥ 50%), the output ripple volt-
INDUCTOR
IN
OUT
= 2 x V
SW
x R
ESR
. The worst-
OUT
, so the
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