FAN5236 Fairchild Semiconductor, FAN5236 Datasheet - Page 12

• Manufacturer Part Number: FAN5236
• Manufacturer: Fairchild Semiconductor

© 2002 Fairchild Semiconductor Corporation
FAN5236 • Rev. 1.3.2
A transition back to PWM continuous conduction mode
(CCM) mode occurs when the inductor current rises
sufficiently to stay positive for eight consecutive cycles.
This occurs when:
where ΔV
series resistance of C
Current Processing Section
The current through the R
sampled (typically 400ns) after Q2 is turned on, as
shown in Figure 12. That current is held and summed
with the output of the error amplifier. This effectively
creates a current-mode control loop. The resistor
connected to ISNSx pin (R
current feedback loop. The following expression
estimates the recommended value of R
function of the maximum load current (I
the value of the MOSFET R
R
the number calculated comes out to be less than 700Ω.
Setting the Current Limit
A ratio of I
when a 0.9V internal reference drives the ILIM pin:
I
R
R
LOAD
SENSE
SENSE
LIM
(
=
CCM
I
LOAD
=
must, however, be kept higher than 700Ω even if
11
)
⎛
⎜
⎜
⎝
I
HYSTERESIS
=
LOAD
x
SNS
⎛ Δ
⎜
⎝
⎛
⎜
⎜ ⎜
⎝
(
100
(
V
MAX
R
is compared to the current established
HYSTERESIS
75
2
DS
+
)
µA
R
ESR
•
(
ON
SENSE
R
= 15mV and ESR is the equivalent
DS
)
OUT
(
ON
)
.
⎞
⎟
⎟ ⎟
⎠
)
−
⎞
⎟
⎠
DS(ON)
100
SENSE
SENSE
⎞
⎟
⎟
⎠
:
) sets the gain in the
Figure 12.
resistor (I
LOAD(MAX)
SENSE
Current Limit / Summing Circuits
SNS
) and
as a
(3)
(4)
(5)
) is
12
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 transition into Hysteretic Mode occurs.
Hysteretic Mode can be disabled by setting the FPWM
pin LOW.
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
current. When using the MOSFET as the sensing
element, the variation of R
variation in the I
device
coefficient of about 0.4%/°C (consult the MOSFET
datasheet for actual values), so the actual current limit
set point decreases proportional to increasing MOSFET
die temperature. A factor of 1.6 in the current limit set
point
variations, assuming the MOSFET heat sinking keeps
its operating die temperature below 125°C.
Figure 13.
should
and
SENSE
is an accurate representation of the load
Improving Current-Sensing Accuracy
has
compensate
SNS
. This value varies from device to
a
ISNS
LDRV
PGND
typical
R
SENS E
DS(ON)
for
junction
causes proportional
Q2
MOSFET
temperature
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R
DS(ON)