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FAN5234QSCX データシート(PDF) 7 Page - Fairchild Semiconductor

部品番号 FAN5234QSCX
部品情報  Mobile-Friendly PWM/PFM Controller
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メーカー  FAIRCHILD [Fairchild Semiconductor]
ホームページ  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

FAN5234QSCX データシート(HTML) 7 Page - Fairchild Semiconductor

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FAN5234
PRODUCT SPECIFICATION
REV. 1.0.10 5/3/04
7
Figure 4. Current Limit / Summing Circuits
LDRV
PGND
ISNS
in +
in –
2.5V
ILIM det.
R
SENSE
SS
1.5M
C
SS
VSEN
V to I
Reference and
Soft Start
17pf
ISNS
S/H
TO PWM COMP
4.14K
300K
ISNS
0.17pf
I2 =
ILIM*11.2
ILIM
0.9V
R
ILIM
ILIM
The hysteretic comparator causes HDRV turn-on when the
output voltage (at VSEN) falls below the lower threshold
(10mV below VREF) and terminates the PFM signal when
VSEN rises over the higher threshold (5mV above VREF).
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 rises suffi-
ciently to stay positive for 8 consecutive cycles. This occurs
when:
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 PWM operation takes
place is typically higher compared to the load level at which
transition into hysteretic mode occurs. Hysteretic mode
can be disabled by setting the FPWM pin HIGH.
Current Processing Section
The following discussion refers to Figure 4.
The current through RSENSE 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. The resistor con-
nected to ISNS pin (RSENSE) sets the gain in the current
feedback loop. For stable operation, the voltage induced by
the current feedback at the PWM comparator input should be
set to 30% of the ramp amplitude at maximum load currrent
and line voltage. The following expression estimates the rec-
ommended value of RSENSE as a function of the maximum
load current (ILOAD(MAX)) and the value of the MOSFET’s
RDS(ON):
RSENSE must, however, be kept higher than:
Setting the Current Limit
A ratio of ISNS is also compared to the current established
when a 0.9 V internal reference drives the ILIM pin:
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 RSENSE is an
accurate representation of the load current. When using the
MOSFET as the sensing element, the variation of RDS(ON)
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 propotional to increasing
MOSFET die temperature. A factor of 1.6 in the current
limit setpoint should compensate for all MOSFET RDS(ON)
variations, assuming the MOSFET’s heat sinking will keep
its operating die temperature below 125°C.
I
LOAD CCM
()
∆V
HYSTERESIS
2 ESR
---------------------------------------
=
(3)
R
SENSE
I
LOAD MAX
()
R
DS ON
()
×
4.1K
×
30%
0.125
×
V
IN MAX
()
×
------------------------------------------------------------------------------
100
=
(4a)
R
SENSE
I
LOAD MAX
()
R
DS ON
()
×
150
µA
------------------------------------------------------------
100
=
(4b)
(5)
R
ILIM
11.2
I
LIMIT
----------------
100
R
SENSE
+
()
R
DS ON
()
----------------------------------------
×
=


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