FAN5250 Fairchild Semiconductor, FAN5250 Datasheet - Page 11

• Manufacturer Part Number: FAN5250
• Description: Mobile Processor Core-Voltage Regulator
• Manufacturer: Fairchild Semiconductor

Additionally, the CPU power dissipation is also slightly
reduced as it is proportional to the applied voltage squared
and even slight voltage decrease translates to a measurable
reduction in power dissipated.
The Crusoe processor regulation window including
transients is specified as +5%…–2%. To accommodate the
droop, the output voltage of the converter is raised by about
3.25% at no load as shown below (R24 = 1K and
R25 = 30.1K):
The converter response to the load step is shown in Figure
11. At zero load current, the output voltage is raised ~50mV
above nominal value of 1.35V. When the load current
increases, the output voltage droops down approximately
55mV. Due to use of Active Droop, the converter’s output
voltage adaptively changes with the load current allowing
better utilization of the regulation window.
REV. 1.1.6 3/12/03
droop » ESR
Figure 10. Setting the No-Load Output Voltage Rise
(no droop)
1.2
ILOAD
Figure 9. Effect of Active Droop on ESR
Vout
Vout
16
VCORE+
lower lim
Figure 8. Active Droop
V
upper lim
ESR
V
I
LOAD
ESR
R25
R24
upper lim
lower lim
V CORE
C
OUT
V
DROOP
I
MAX
The current through R
shortly after Q2 is turned on. That current is held, and then
injected (with a 1/48 gain) into the inverting path of the error
amp to produce an offset to the sensed output voltage at
V
Setting the Current Limit
A ratio of ISNS is also compared to the current established
when a 1.2 V internal reference drives the ILIM pin. The
threshold is determined at the point when the
Since
therefore,
Since the tolerance on the current limit is largely dependent
on the ratio of the external resistors it is fairly accurate if the
voltage drop on the Switching Node side of R
accurate representation of the load current. When using the
MOSFET as the sensing element, the variation of R
causes proportional variation in the ISNS. This value not
only varies from device to device, but also has a typical
junction temperature coefficient of about 0.4%/°C
(consult the MOSFET datasheet for actual values), so the
actual current limit set point will decrease proportional to
increasing MOSFET die temperature. The same discussion
applies to the V
initial error of ±20% due to its value being determined by
a ratio between R
CORE
Figure 11. Converter Response to 5A Load Step
2
1
+ proportional to the load current.
I
Ch1 50mV
V
V
LIMIT
DROOP
DROOP
DROOP
=
ISNS
SENSE
V
------------ -
R
1.2V
CPU
=
=
LIM
calculation, which has an additional
ISNS
-------------- -
SENSE
Ch2 2.0A
2083
and the internal 100K resistor.
100K
= 1.35V
I
=
8
CPU
×
I
--------------------------------------------
4
-- -
3
LOAD
= 0A...5.0A
resistor (ISNS) is sampled
>
×
×
×
ILIM 4
--------------------- -
I
--------------------------------------------
8
--------------------------------------------------- -
I
--------------------------------------------
R
LOAD
LOAD
×
SENSE
48 R
×
3
(
R
R
100
×
M50µs
×
R
SENSE
DS ON
×
×
DS ON
R
R
(
SENSE
+
lower limit
DS ON
(
upper limit
DS ON
R
)
(
SENSE
(
)
SENSE
)
)
)
is an
DS(ON)
FAN5250
(10)
(9a)
(9b)
11