MAX1717 Maxim, MAX1717 Datasheet - Page 26

• Manufacturer Part Number: MAX1717
• Description: Dynamically Adjustable / Synchronous Step-Down Controller for Notebook CPUs
• Manufacturer: Maxim

Dynamically Adjustable, Synchronous
Step-Down Controller for Notebook CPUs
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. 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
equation in the Design Procedure). The amount of over-
shoot due to stored inductor energy can be calculated
as:
where I
Stability is determined by the value of the ESR zero rela-
tive to the switching frequency. The voltage-positioned
circuits in this data sheet have their ESR zero frequencies
lowered due to the external resistor in series with the
output capacitor ESR, guaranteeing stability. For voltage-
positioned circuits, the minimum ESR requirement of the
output capacitor is reduced by the voltage-positioning
resistor value.
For nonvoltage-positioned circuits, the following criteria
must be satisfied. The boundary of instability is given
by the following equation:
For a standard 300kHz application, the ESR zero fre-
quency must be well below 95kHz, preferably below
50kHz. Tantalum and OS-CON capacitors in wide-
spread use at the time of publication have typical ESR
zero frequencies of 15kHz. In the standard application
used for inductor selection, the ESR needed to support
50mVp-p ripple is 50mV/4.2A = 11.9mΩ. Six 470µF/4V
Kemet T510 low-ESR tantalum capacitors in parallel pro-
vide 5mΩ max ESR. Their typical combined ESR results
in a zero at 17kHz, well within the bounds of stability.
Don’t put high-value ceramic capacitors directly across
the fast-feedback inputs (FB to GND) without taking
precautions to ensure stability. Ceramic capacitors
have a high ESR zero frequency and may cause erratic,
26
______________________________________________________________________________________
where
SOAR
PEAK
:
from causing problems during load tran-
is the peak inductor current.
ƒ
V
ESR
ƒ
SOAR
ESR
Output Capacitor Stability
≤ ƒ
≈
=
2
SW
L I
2
⋅ ⋅
⋅
C V
⋅
PEAK
/
π
π
OUT
⋅
2
R
Considerations
ESR
1
⋅
C
OUT
SAG
SAG
unstable operation. However, it’s easy to add enough
series resistance by placing the capacitors a couple of
inches downstream from the junction of the inductor
and FB pin, or use a voltage-positioned circuit (see
Voltage Positioning and Effective Efficiency section).
Unstable operation manifests itself in two related but
distinctly different ways: double-pulsing and fast-feed-
back loop instability.
Double-pulsing occurs due to noise on the output or
because the ESR is so low that there isn’t enough voltage
ramp in the output voltage signal. This “fools” the error
comparator into triggering a new cycle immediately
after the minimum off-time period has expired. Double-
pulsing is more annoying than harmful, resulting in noth-
ing 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 cause the output
voltage to rise above or 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/overshoot.
The input capacitor must meet the ripple current
requirement (I
defined by the following equation:
For most applications, nontantalum chemistries (ceramic,
aluminum, or OS-CON) are preferred due to their resis-
tance to inrush surge currents typical of systems with a
mechanical switch or a connector in series with the bat-
tery. If the MAX1717 is operated as the second stage of
a two-stage power-conversion system, tantalum input
capacitors are acceptable. In either configuration,
choose an input capacitor that exhibits less than +10°C
temperature rise at the RMS input current for optimal
circuit longevity.
Most of the following MOSFET guidelines focus on the
challenge of obtaining high load-current capability
(>12A) when using high-voltage (>20V) AC adapters.
Low-current applications usually require less attention.
I
RMS
RMS
=
I
LOAD
) imposed by the switching currents
Input Capacitor Selection
Power MOSFET Selection
V
OUT
(
V
V
IN
IN
−
V
OUT
)