MAX1717 Maxim, MAX1717 Datasheet - Page 28

• 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
The no-load output voltage is raised by adding a fixed
offset to GNDS through a resistor divider from REF. A
27mV nominal value is appropriate for 1.6V applications.
This 27mV corresponds to a 0.9 · 27mV = 24mV = 1.5%
increase with a V
circuit (Figure 3), this is realized with resistors R4 and
R5. Use a 10µA resistor divider current.
Adding a series output resistor positions the full-load out-
put voltage below the actual DAC programmed voltage.
Connect FB and FBS directly to the inductor side of the
voltage-positioning resistor (R6, 5mΩ). The other side of
the voltage-positioning resistor should be tied directly to
the output filter capacitor with a short, wide PC board
trace. With a 14A full-load current, R6 causes a 70mV
drop. This 70mV is a -4.4% error, but it is compensated
by the +1.5% error from the GNDS offset, resulting in a
net error of -2.9%. This is well within the typical specifica-
tion for voltage accuracy.
An additional benefit of voltage positioning is reduced
power consumption at high load currents. Because the
output voltage is lower under load, the CPU draws less
current. The result is lower power dissipation in the
CPU, though some extra power is dissipated in R6. For
a nominal 1.6V, 12A output, reducing the output volt-
age 2.9% gives an output voltage of 1.55V and an out-
put current of 11.65A. Given these values, CPU power
consumption is reduced from 19.2W to 18.1W. The
additional power consumption of R6 is:
and the overall power savings is as follows:
In effect, 1W 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-positioned circuit to equal the total dis-
sipation 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 circuit
2) Model the load resistance for each data point:
3) Calculate the output current that would exist for each
28
(V
R
LOAD
IN
______________________________________________________________________________________
, I
IN
data point in a nonpositioned application:
, V
OUT
19.2 - (18.1 + 0.68) = 0.42W
R
I
NP
5mΩ · 11.65A
LOAD
, I
OUT
OUT
= V
of 1.6V. In the voltage-positioned
NP
= V
).
/ R
OUT
LOAD
2
/ I
= 0.68W
OUT
4) Calculate effective efficiency as:
5) Plot the efficiency data point at the nonpositioned
The effective efficiency of voltage-positioned circuits is
shown in the Typical Operating Characteristics section.
The output voltage adjust range for continuous-conduc-
tion operation is restricted by the nonadjustable 500ns
(max) minimum off-time one-shot (375ns max at
1000kHz). For best dropout performance, use the slower
(200kHz) on-time settings. When working with low input
voltages, the duty-factor limit must be calculated using
worst-case values for on- and off-times. Manufacturing
tolerances and internal propagation delays introduce
an error to the TON K-factor. This error is greater at
higher frequencies (Table 3). Also, keep in mind that
transient response performance of buck regulators
operated close to dropout is poor, and bulk output
capacitance must often be added (see the VSAG equa-
tion in the Design Procedure section).
The absolute point of dropout is when the inductor cur-
rent ramps down during the minimum off-time (∆I
Figure 11. Adjusting V
where V
Effective efficiency = (V
calculated nonpositioned power output divided by
the measured voltage-positioned power input.
current, I
FB
MAX1717
180k
NP
NP
GNDS
GND
FBS
DH
DL
= 1.6V (in this example).
.
V
OUT
OUT
= V
FB
with a Resistor-Divider
•
V
R2
(
BATT
1 +
Dropout Performance
1k
R2 || 180k
NP
R1
· I
)
NP
R1
R2
) / (V
IN
· I
DOWN
IN
V
OUT
) =
)