MAX1981A Maxim Integrated Products, MAX1981A Datasheet - Page 38

• Manufacturer Part Number: MAX1981A
• Description: (MAX1907A / MAX1981A) Quick-PWM Master Controllers
• Manufacturer: Maxim Integrated Products

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Quick-PWM Master Controllers for Voltage-
Positioned CPU Core Power Supplies (IMVP-IV)
Figure 10. Transient Response Regions
Calculate effective efficiency as follows:
1) Start with the efficiency data for the positioned cir-
2) Model the load resistance for each data point:
3) Calculate the output current that would exist for
where V
4) Calculate effective efficiency as:
5) Plot the efficiency data point at the non-positioned
The MAX1907A/MAX1981A can be used with a direct
battery connection (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.
38
One-Stage (Battery Input) vs. Two-Stage
I
cuit (V
each R
tion:
Effective efficiency = (V
culated non-positioned power output divided by the
measured voltage-positioned power input.
current, I
The effective efficiency of voltage-positioned circuits
is shown in the Typical Operating Characteristics .
V
LOAD
______________________________________________________________________________________
OUT
(dV/dt = I
CAPACITIVE SAG
NP
IN
OUT
LOAD
= 1.6V (in this example).
, I
/C
NP
IN
OUT
.
, V
)
data point in a non-positioned applica-
R
ESR VOLTAGE STEP
OUT
I
LOAD
(I
NP
STEP
= V
, I
x R
OUT
= V
ESR
NP
(5V Input) Applications
)
NP
OUT
).
/ R
✕
RECOVERY
LOAD
(dV/dt = I
CAPACITIVE SOAR
I
/ I
NP
OUT
) / (V
OUT
/C
IN
OUT
✕
)
I
IN
) = cal-
The one-stage approach offers smaller total inductor
size and fewer capacitors overall due to the reduced
demands on the 5V supply. Due to the high input volt-
age, the one-stage approach requires lower DC input
currents, reducing input connection/bus requirements
and power dissipation due to input resistance. 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
board traces. Although the two-stage design has slow-
er transient response than the single stage, this can be
offset by the use of a voltage-positioned converter.
Ceramic capacitors have advantages and disadvan-
tages. They have ultra-low ESR and are noncom-
bustible, relatively small, and nonpolarized. However,
they are also expensive and brittle, and their ultra-low
ESR characteristic can result in excessively high ESR
zero frequencies. In addition, their relatively low capac-
itance value can cause output overshoot when step-
ping from full-load to no-load conditions, unless a small
inductor value is used (high switching frequency), or
there are some bulk tantalum or electrolytic capacitors
in parallel to absorb the stored inductor energy. In
some cases, there may be no room for electrolytics,
creating a need for a DC-DC design that uses nothing
but ceramics.
The MAX1907A/MAX1981A can take full advantage of
the small size and low ESR of ceramic output capaci-
tors 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
output capacitance requirement (see the Output
Capacitor Selection section). Often the switching fre-
quency is increased to 550kHz, and the inductor value
is reduced to minimize the energy transferred from
inductor to capacitor during load-step recovery. The
efficiency penalty for operating at 550kHz is about 2%
when compared to the 300kHz circuit, primarily due to
the high-side MOSFET switching losses.
Ceramic Output Capacitor Applications
SOAR
) determines the minimum
2
R losses from PC