FAN5250 Fairchild Semiconductor, FAN5250 Datasheet - Page 10

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

FAN5250
The switching frequency is primarily a function of:
1. Spread between the two hysteretic thresholds
2. I
3. Output Inductor and Capacitor ESR
A transition back to PWM (Continuous Conduction Mode
or CCM) mode occurs when the inductor current has risen
sufficient as to be positive for 8 consecutive cycles. This
occurs when:
where ∆V
series resistance of C
Because of the different control mechanisms, the value of the
load current where transition into CCM operation takes place
is typically higher compared to the load level at which transi-
tion into hysteretic mode had occurred. Hysteretic mode can
be disabled by setting the FPWM pin low. The presence of
this pin enhances applicability of the controller. Figure 6
shows an application circuit where hysteretic mode is only
allowed in a Deep Sleep Extension (DSX) mode. In this
mode the CPU has stopped and its current is significantly
lower compared to other modes of operation. Using the
FPWM pin simplifies control over converter modes of opera-
tion and increases efficiency.
10
LOAD
HYSTERESIS
C
SS
I
VCORE+
LOAD CCM
SS
(
OUT
16
14
= 15mV and ESR is the equivalent
)
=
.
EA Out
∆V
---------------------------------------- -
HYSTERESIS
2 ESR
1.5M
300K
Soft Start
DAC and
Figure 7. Current Limit and Active Droop Circuits
17pf
100K
200K
R
DROOP
ILIM
det.
(7)
2.5V
ISNS
I1A =
Current Processing Section
The following discussion refers to Figure 7.
Active Droop
“Active Droop” or voltage positioning is now widely used in
the computer power applications. The technique is based on
raising the converter voltage at light load in anticipation of a
step increase in load current, and conversely, lowering
V
With Active Droop, the output voltage varies with the load as
if a resistor were connected in series with the converter’s out-
put, in other words, it's effect is to raise the output resistance
of the converter. To get the most from the Active Droop, its
magnitude should be scaled to match the output capacitor’s
ESR voltage drop.
Active Droop allows the size and cost of the output capaci-
tors required to handle CPU current transients to be reduced.
The reduction may be almost a factor of 2 when compared to
a system without Active Droop.
48
CORE
I2 =
Figure 6. Allowing Hysteretic Mode in Deep Sleep
V to I
START
in anticipation of a step decrease in load current.
1.2V
S/H
ISNS
I1B =
R7
4 * ILIM
8
ILIM mirror
V
3
DROOP
in +
in –
DSX
=
21
22
15
I
MAX
R8
LDRV
PGND
ISNS
ILIM
×
FPWM
ALTV
ESR
R
R
SENSE
ILIM
6
5
REV. 1.1.6 3/12/03
Q2
(8)