FAN5234QSCX_NL Fairchild Semiconductor, FAN5234QSCX_NL Datasheet - Page 7

• Manufacturer Part Number: FAN5234QSCX_NL
• Manufacturer: Fairchild Semiconductor

Specifications of FAN5234QSCX_NL

• Dc To Dc Converter Type: Synchronous Buck Controller/PWM DC to DC Controller
• Number Of Outputs: 1
• Pin Count: 16
• Input Voltage: 5 to 24V
• Output Voltage: 0.9 to 5.5V
• Package Type: QSOP
• Mounting: Surface Mount
• Operating Temperature Classification: Commercial
• Operating Temperature (min): -10C
• Operating Temperature (max): 85C
• Lead Free Status / Rohs Status: Compliant

FAN5234
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. I
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 ∆V
series resistance of C
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 R
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 (R
REV. 1.0.10 5/3/04
LOAD
I
LOAD CCM
HYSTERESIS
(
C
SS
)
VSEN
=
SS
OUT
∆V
-------------------------------------- -
SENSE
SENSE
= 15mV and ESR is the equivalent
HYSTERESIS
.
2 ESR
) sets the gain in the current
resistor (ISNS) is sampled
300K
Reference and
Soft Start
ILIM det.
1.5M
Figure 4. Current Limit / Summing Circuits
TO PWM COMP
0.17pf
17pf
4.14K
2.5V
(3)
ILIM*11.2
I2 =
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 R
load current (I
R
R
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 R
accurate representation of the load current. When using the
MOSFET as the sensing element, the variation of R
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 R
variations, assuming the MOSFET’s heat sinking will keep
its operating die temperature below 125°C.
DS(ON)
SENSE
R
ISNS
SENSE
R
R
ILIM
SENSE
:
must, however, be kept higher than:
V to I
=
0.9V
ILIM
=
I
----------------------------------------------------------------------------- - 100
S/H
LOAD(MAX)
LOAD MAX
=
ISNS
--------------- -
I
LIMIT
11.2
30% 0.125
I
----------------------------------------------------------- - 100
LOAD MAX
(
×
in +
in –
SENSE
×
(
(
--------------------------------------- -
100
)
150µA
) and the value of the MOSFET’s
×
R
R
as a function of the maximum
)
+
×
DS ON
DS ON
×
V
R
LDRV
PGND
ISNS
ILIM
R
(
(
SENSE
IN MAX
DS ON
(
PRODUCT SPECIFICATION
)
)
(
×
R
R
)
SENSE
4.1K
ILIM
)
)
–
–
SENSE
DS(ON)
DS(ON)
is an
(4a)
(4b)
(5)
7