FAN2108EMPX Fairchild Semiconductor, FAN2108EMPX Datasheet - Page 11

• Manufacturer Part Number: FAN2108EMPX
• Description: IC REG SYNC BUCK 3-24V 8A 25MLP
• Manufacturer: Fairchild Semiconductor
• Series: TinyBuck™r
• Type: Step-Down (Buck)r

Specifications of FAN2108EMPX

• Internal Switch(s): Yes
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 19 V
• Current - Output: 8A
• Frequency - Switching: 200kHz ~ 600kHz
• Voltage - Input: 3 ~ 24 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 25-MLP
• Power - Output: 2.8W
• Mounting Style: SMD/SMT
• Efficiency: 95 %
• Input / Supply Voltage (max): 24 V
• Input / Supply Voltage (min): 3 V
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Operating Temperature Range: - 40 C to + 85 C
• Output Current: 8 A
• Output Power: 2.8 W
• Output Voltage: 3.2 V
• Supply Current: 8 mA
• Switching Frequency: 600 KHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: FAN2108EMPXTR

© 2008 Fairchild Semiconductor Corporation
FAN2108 • Rev. 1.0.1
Calculating the Inductor Value
Typically the inductor is set for a ripple current (ΔI
10% to 35% of the maximum DC load. Regulators
requiring fast transient response use a value on the
high side of this range; while regulators that require very
low output ripple and/or use high-ESR capacitors
restrict allowable ripple current.
where f is the oscillator frequency.
Setting the Ramp Resistor Value
The internal ramp voltage excursion (ΔV
should be set to 0.6V at nominal operating point. R
is approximately:
where frequency (f) is expressed in KHz.
Setting the Current Limit
There are two levels of current-limit thresholds in
FAN2108. The first level of protection is through an
internal default limit set at the factory to limit output
current beyond normal usage levels. The second level
of protection is externally settable at the ILIM pin.
Current-limit protection is enabled whenever the lower
of the two thresholds is reached. FAN2108 uses its
internal low-side MOSFET for current-sensing. The
current-limit threshold voltage (V
scaled version of voltage drop across the low-side
MOSFET, sampled at the end of each PWM off-
time/cycle. The internal default threshold (with I
is temperature compensated.
The ILIM pin can source a 10µA current that can be
used to establish a lower, temperature–dependent,
current-limit threshold by connecting a resistor (R
between ILIM and AGND. R
with the equation:
where:
I
K
K1
V
R
f
After 16 consecutive, pulse-by-pulse, current-limit
cycles, the fault latch is set and the regulator shuts
down. Cycling V
normal soft-start cycle (refer to Auto-Restart section).
L
R
R
sw
T
OUT
ramp
ILIM
RAMP
=
V
(
OUT
K
= Desired current limit setpoint in Amps;
= Normalized temperature coefficient of the
= Overload co-efficient (use 1.2 to 1.4);
= Set output voltage;
= Ramp resistor used in KΩ; and
= Selected switching frequency in KHz.
Ω
(
K
)
Ω
low-side MOSFET (Q2 from Figure 8);
=
•
Δ
)
(1
I
95
=
L
•
-
18
(
V
+
f
V
IN
V
3
x
OUT
IN
10
CC
•
−
I
out
. 1
−
)
or EN restores operation after a
6
) 8
•
•
V
•
K
IN
V
T
OUT
•
•
K
f
ILIM
1
−
+
2
ILIM
V
can be approximated
out
) is compared to a
R
ramp
•
. 3
RAMP
33
•
f
•
sw
) during t
10
LIM
6
open)
L
RAMP
ILIM
) of
(4)
(5)
ON
(6)
)
11
The over-current protection fault latch is active during
the soft-start cycle. Use a 1% resistor for R
In case R
default current-limit threshold.
Loop Compensation
The loop is compensated using a feedback network
around the error amplifier. Figure 22 shows a complete
type-3
compensation, eliminate R3 and C3.
Since the FAN2108 employs summing current-mode
architecture, type-2 compensation can be used for
many applications. For applications that require wide
loop bandwidth and/or use very low-ESR output
capacitors, type-3 compensation may be required.
Protection
The converter output is monitored and protected
against extreme overload, short-circuit, over-voltage,
under-voltage, and over-temperature conditions.
An internal fault latch is set for any fault intended to
shut down the IC. When the fault latch is set, the IC
discharges V
until FB<0.25V. The MOSFET is not turned on again
unless FB>0.5V. This behavior discharges the output
without causing undershoot (negative output voltage).
Under-Voltage Shutdown
If voltage on the FB pin remains below the under-
voltage threshold for 16 consecutive clock cycles, the
fault latch is set and the converter shuts down. This
protection is not active until the internal SS ramp
reaches 1.0V during soft-start.
Over-Voltage Protection / Shutdown
If voltage on the FB pin exceeds the over-voltage
threshold for two consecutive clock cycles, the fault
latch is set and shutdown occurs.
A shorted high-side MOSFET condition is detected
when SW voltage exceeds ~0.7V while the low-side
MOSFET is fully enhanced. The fault latch is set
immediately upon detection.
The two fault protection circuits above are active all the
time, including during soft-start.
ILIM
compensation
Figure 22. Compensation Network
OUT
is not connected, the IC uses an internal
by enhancing the low-side MOSFET
network.
For
ILIM
www.fairchildsemi.com
.
type-2