MAX1887EEE Maxim Integrated Products, MAX1887EEE Datasheet - Page 26

• Manufacturer Part Number: MAX1887EEE
• Description: Current Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1887EEE

• Number Of Outputs: 1
• Mounting Style: SMD/SMT
• Package / Case: QSOP-16
• Switching Frequency: 550 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Boost

which results in an overall power savings of:
In effect, 2.2W of CPU dissipation is saved and the
power supply dissipates much of the savings, but both
the net savings and the transfer of dissipation away
from the hot CPU are beneficial. Effective efficiency is
defined as the efficiency required of a nonvoltage-posi-
tioned circuit to equal the total dissipation of a voltage-
positioned circuit for a given CPU operating condition.
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 each
where V
4) Calculate effective efficiency as:
5) Plot the efficiency data point at the nonpositioned
Quick-PWM Slave Controllers for
Multiphase, Step-Down Supplies
Figure 7. Voltage Positioning the Output
26
cuit (V
R
Effective efficiency = (V
culated nonpositioned power output divided by the
measured voltage-positioned power input.
______________________________________________________________________________________
LOAD
NP
1.4V
1.4V
35.2W - (33.03W + 1.06W) = 1.10W.
IN
data point in a nonpositioned application:
= 1.6V (in this example).
, I
IN
VOLTAGE POSITIONING THE OUTPUT
A. CONVENTIONAL CONVERTER (50mV/div)
B. VOLTAGE-POSITIONED OUTPUT (50mV/div)
, V
50mV x 21.3A = 1.06W,
R
OUT
I
LOAD
NP
= V
, I
OUT
= V
NP
NP
OUT
).
/ R
✕
LOAD
/ I
I
NP
OUT
) / (V
IN
✕
A
B
I
IN
) = cal-
The effective efficiency of voltage-positioned circuits is
shown in the Typical Operating Characteristics.
The MAX1887/MAX1897 can be used with a direct bat-
tery 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.
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.
Figure 8. Transient Response Regions
current, I
I
V
LOAD
OUT
(dV/dt = I
CAPACITIVE SAG
Two-Stage (5V Input) Applications
OUT
NP
One-Stage (Battery Input) Versus
/C
OUT
.
)
ESR VOLTAGE STEP
(I
STEP
x R
ESR
)
RECOVERY
(dV/dt = I
CAPACITIVE SOAR
OUT
2
R losses from PC
/C
OUT
)