FAN5068 Datasheet, PDF (11/18 Pages) Fairchild Semiconductor – DDR-1/DDR-2 plus ACPI Regulator Combo

FAN5068
COMP
FB
SS/EN
C_SS
COMP
FB
SS
1µA
I_SS
–
+ E/A
+
VREF
ISNS
RAMP
–
+ PWM
+
PRODUCT SPECIFICATION
Figure 7. SS Clamp and FB Open Protection
4.41K
Reference and
Soft Start
TO
PWM
COMP
S/H
V to I
ISNS
in +
ISNS
in –
ILIM det.
2.5V
I2 =
ILIM*9.6
0.9V
ILIM
mirror
R_SENSE
ISNS
LDRV
PGND
ILIM
R_ILIM
Figure 8. Current Limit / Summing Circuits
Current Processing Section
The following discussion refers to Figure 8.
The current through R_SENSE resistor (ISNS) is sampled
shortly after Q2 is turned on. That current is held, and
summed with the output of the error amplifier. This effec-
tively creates a current mode control loop. R_SENSE sets the
gain in the current feedback loop. For stable operation, the
voltage induced by the current feedback at the PWM com-
parator input should be set to 30% of the ramp amplitude at
maximum load currrent and line voltage. The following
expression estimates the recommended value of R_SENSE as a
function of the maximum load current (I_LOAD(MAX)) and the
value of the MOSFET’s R_DS(ON).:
R_SENSE = ^I---L---^O--3--^A-0--^D-%---⁽--^M--_•---^A-0--^X-.-1--⁾-2--^•-5---R--_•---^D-V--^S---_I⁽--_NO---₍-^N-_M---⁾--_A-^•--_X--4--₎-.-1----k- – 100
(5)
RSENSE must, however, be kept higher than:
R_SENSE(MIN) = -I--^L---^O----^A---^D----⁽--^M---1--^A-5--^X-0---_µ⁾---^•-A---R-----^D---^S---⁽--^O----^N----⁾ – 100
(6)
Setting the Current Limit
An ISNS is compared to the current established when a 0.9 V
internal reference drives the ILIM pin. R_ILIM, the R_DS(ON) of
Q2, and R_SENSE determine the current limit:
R_ILIM
=
-----9---.--6------
I_LIMIT
×
-⁽--1---0---0----+-----R-----^S---^E---^N---^S---^E----⁾
R_DS(ON)
(7)
Where ILIMIT is the peak inductor current. Since the toler-
ance 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_SENSE is an accurate repre-
sentation of the load current. When using the MOSFET as
the sensing element, the variation of R_DS(ON) causes propor-
tional 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 propotional to increasing MOSFET die tempera-
ture. A factor of 1.6 in the current limit setpoint should
compensate for all MOSFET R_DS(ON) variations, assuming
the MOSFET’s heat sinking will keep its operating die
temperature below 125°C.
REV. 1.0.1 9/9/04
11