FAN5250 View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
Manufacturer
FAN5250 Datasheet PDF : 17 Pages
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FAN5250
The switching frequency is primarily a function of:
1. Spread between the two hysteretic thresholds
2. ILOAD
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:
ILOAD(CCM)
=
-∆----V----H----Y---S---T----E---R----E---S----I--S-
2 ESR
(7)
where ∆VHYSTERESIS = 15mV and ESR is the equivalent
series resistance of COUT.
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.
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
VCORE in anticipation of a step decrease in load current.
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.
VDROOP = IMAX × ESR
(8)
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.
ALTV 6
R7
R8
START
DSX
FPWM 5
Figure 6. Allowing Hysteretic Mode in Deep Sleep
VCORE+ 16
1.5M 17pf
100K
RDROOP
200K
EA Out
CSS
SS 14
300K
ILIM
det.
DAC and
Soft Start
S/H
V to I
in +
21 ISNS RSENSE
I1A =
ISNS
48
I1B =
ISNS
8
Q2
LDRV
in –
22 PGND
2.5V
1.2V
15 ILIM RILIM
I2 =
4 * ILIM
3
ILIM mirror
Figure 7. Current Limit and Active Droop Circuits
10
REV. 1.1.6 3/12/03
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