Rainbow Electronics MAX1717 - Manual (Page 30)

MAX1717
Dynamically Adjustable, Synchronous
Step-Down Controller for Notebook CPUs
30 ______________________________________________________________________________________
One-Stage (Battery Input) vs. Two-Stage
(5V Input) Applications
The MAX1717 can be used with a direct battery connec-
tion (one stage) or can obtain power from a regulated 5V
supply (two stage). Each approach has advantages,
and careful consideration should go into the selection of
the final design.
The one-stage approach offers smaller total inductor
size and fewer capacitors overall due to the reduced
demands on the 5V supply. The transient response of
the single stage is better due to the ability to ramp the
inductor current faster. The total efficiency of a single
stage is better than the two-stage approach.
The two-stage approach allows flexible placement due
to smaller circuit size and reduced local power dissipa-
tion. The power supply can be placed closer to the
CPU for better regulation and lower I
2 R losses from PC
board traces. Although the two-stage design has worse
transient response than the single stage, this can be
offset by the use of a voltage-positioned converter.
Ceramic Output Capacitor
Applications
Ceramic capacitors have advantages and disadvan-
tages. They have ultra-low ESR and are noncombustible,
relatively small, and nonpolarized. They are also expen-
sive and brittle, and their ultra-low ESR characteristic can
result in excessively high ESR zero frequencies (affecting
stability in nonvoltage-positioned circuits). In addition,
their relatively low capacitance value can cause output
overshoot when going abruptly from full-load to no-load
conditions, unless the inductor value can be made small
(high switching frequency), or there are some bulk tanta-
lum or electrolytic capacitors in parallel to absorb the
stored energy in the inductor. In some cases, there may
be no room for electrolytics, creating a need for a DC-DC
design that uses nothing but ceramics.
The MAX1717 can take full advantage of the small size
and low ESR of ceramic output capacitors in a voltage-
positioned circuit. The addition of the positioning resistor
increases the ripple at FB, lowering the effective ESR
zero frequency of the ceramic output capacitor.
Output overshoot (V SOAR ) determines the minimum
output capacitance requirement (see Output Capacitor
Selection section). Often the switching frequency is
increased to 550kHz or 1000kHz, and the inductor value
is reduced to minimize the energy transferred from induc-
tor to capacitor during load-step recovery. The efficiency
penalty for operating at 550kHz is about 2% to 3% and
about 5% at 1000kHz when compared to the 300kHz
voltage-positioned circuit, primarily due to the high-side
MOSFET switching losses.
Table 1 and the Typical Operating Characteristics
include two circuits using ceramic capacitors with
1000kHz switching frequencies. The efficiency of the
+5V input circuit (circuit 4) is substantially higher than
circuit 5, which accommodates the full battery voltage
range. Circuit 4 is an excellent choice for two-stage
conversion applications if the goal is to minimize size
and power dissipation near the CPU.
PC Board Layout Guidelines
Careful PC board layout is critical to achieve low
switching losses and clean, stable operation. The
switching power stage requires particular attention
(Figure 12). If possible, mount all of the power compo-
nents on the top side of the board with their ground ter-
minals flush against one another. Follow these
guidelines for good PC board layout:
1) Keep the high-current paths short, especially at the
ground terminals. This is essential for stable, jitter-
free operation.
2) All analog grounding is done to a separate solid cop-
per plane, which connects to the MAX1717 at the
GND pin. This includes the V
CC , REF, and CC
capacitors, the TIME resistor, as well as any other
resistor-dividers.
3) Keep the power traces and load connections short.
This is essential for high efficiency. The use of thick
copper PC boards (2oz vs. 1oz) can enhance full-
load efficiency by 1% or more. Correctly routing PC
board traces is a difficult task that must be
approached in terms of fractions of centimeters,
where a single m
Ω of excess trace resistance causes
a measurable efficiency penalty.
4) LX and GND connections to Q2 for current limiting
must be made using Kelvin sense connections to
guarantee the current-limit accuracy. With SO-8
MOSFETs, this is best done by routing power to the
MOSFETs from outside using the top copper layer,
while connecting GND and LX inside (underneath)
the SO-8 package.
5) When trade-offs in trace lengths must be made, it’s
preferable to allow the inductor charging path to be
made longer than the discharge path. For example,
it’s better to allow some extra distance between the
input capacitors and the high-side MOSFET than to
allow distance between the inductor and the low-side
MOSFET or between the inductor and the output filter
capacitor.
6) Ensure the FB connection to the output is short and
direct. In voltage-positioned circuits, the FB connection