FAN5234QSC_Q Fairchild Semiconductor, FAN5234QSC_Q Datasheet - Page 8

• Manufacturer Part Number: FAN5234QSC_Q
• Description: Switching Converters, Regulators & Controllers PWM Controller
• Manufacturer: Fairchild Semiconductor

Specifications of FAN5234QSC_Q

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

© 2004 Fairchild Semiconductor Corporation
FAN5234 • Rev. 2.0.0
The hysteretic comparator causes HDRV turn-on when
the output voltage (at V
threshold (10mV below V
signal when V
above V
The switching frequency is primarily a function of:
A transition back to PWM Continuous Conduction Mode
or (CCM) occurs when the inductor current rises
sufficiently to stay positive for eight consecutive cycles.
This occurs when:
where ΔV
series resistance of C
Due to 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
The following discussion refers to Figure 6.
The current through R
shortly after Q2 is turned on. 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 the ISNS pin (R
gain in the current feedback loop. Equation 4 estimates
the recommended value of R
maximum load current (I
MOSFET R
700Ω even if the number calculated comes out less
than 700Ω:
R
I
LOAD
V
V
SENSE
CORE
CORE
I
Spread between the two hysteretic thresholds
I
Output inductor and capacitor ESR
(
I
L
LOAD
CCM
L
=
0
0
REF
⎛
⎜
⎜
⎝
)
I
LOAD
HYSTERESIS
=
).
⎛ Δ
⎜
⎝
DS(ON)
(
MAX
V
150
SEN
HYSTERESIS
2
)
μ
×
. R
A
rises over the higher threshold (5mV
ESR
R
DS
= 15mV and ESR is the equivalent
SENSE
OUT
(
ON
SENSE
)
.
LOAD(MAX)
−
REF
Figure 4. Transitioning between PWM and Hysteretic Mode
100
SEN
⎞
⎟
⎠
must be kept higher than
) and terminates the PFM
⎞
⎟
⎟
⎠
resistor (I
SENSE
) falls below the lower
1
) and the value of the
as a function of the
PWMMode
SNS
SENSE
2
) is sampled
1
) sets the
2
HystereticMode
3
(3)
(4)
3
8
4
4
Setting the Current Limit
A ratio of I
established when a 0.9V 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 R
current. When using the MOSFET as the sensing
element, the variation of R
variation in I
and has a typical junction temperature coefficient of
about 0.4%/°C (consult the MOSFET datasheet for
actual values), the actual current limit set point
decreases proportional to increasing MOSFET die
temperature. A factor of 1.6 in the current limit set point
should compensate for all MOSFET R
assuming the MOSFET’s heat sinking keep its
operating die temperature below 125°C.
R
LIM
5
Figure 5. Improving Current-Sensing Accuracy
=
I
6
LOAD
11
SENSE
5
x
7
⎛
⎜
⎜
⎜
⎝
SNS
(
SNS
100
is an accurate representation of the load
. This value varies from device to device
R
8
DS
+
R
6
is also compared to the current
(
ON
SENSE
ISNS
LDRV
PGND
HystereticMode
)
)
⎞
⎟
⎟
⎟
⎠
R
SENSE
DS(ON)
7
causes proportional
Q2
8
DS(ON)
PWMMode
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variations,
(5)