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PDF FAN2108 Data sheet ( Hoja de datos )
| Número de pieza | FAN2108 | |
| Descripción | Integrated Synchronous Buck Regulator | |
| Fabricantes | Fairchild Semiconductor | |
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September 2008
FAN2108www.datasheet4u.com — TinyBuck™
3-24V Input, 8A, High-Efficiency, Integrated Synchronous
Buck Regulator
Features
Wide Input Voltage Range: 3V-24V
Wide Output Voltage Range: 0.8V to 80% VIN
8A Output Current
Programmable Frequency Operation: 200KHz to
600KHz
Over 95% Peak Efficiency
Integrated Schottky Diode on Low-side MOSFET
Boosts Efficiency
Internal Bootstrap diode
Power-Good Signal
Pre-Bias Startup
Accepts Ceramic Capacitors on Output
External Compensation for Flexible Design
Input Under-Voltage Lockout
Programmable Current Limit
Under-Voltage, Over-Voltage, and Thermal
Shutdown Protections
Internal Soft-Start
5x6mm, 25-Pin, 3-Pad MLP Package
Applications
Servers
Point-of-Load Regulation
High-End Computing Systems
Graphics Cards
Battery-Powered Equipment
Set-Top Boxes
Description
The FAN2108 TinyBuck™ is a highly efficient, small
footprint, 8A, synchronous buck regulator.
The FAN2108 contains both synchronous MOSFETs
and a controller/driver with optimized interconnects in
one package, which enables designers to solve high-
current requirements in a small area with minimal
external components.
External compensation, programmable switching
frequency, and current limit features allow design
optimization and flexibility.
The summing current mode modulator uses lossless
current sensing for current feedback and over-current
protection. Voltage feedforward helps operation over a
wide input voltage range.
Fairchild’s advanced BiCMOS power process,
combined with low-RDS(ON) internal MOSFETs and a
thermally efficient MLP package, provide the ability to
dissipate high power in a small package.
Output over-voltage, under-voltage, and thermal
shutdown protections help protect the device from
damage during fault conditions. FAN2108 prevents pre-
biased output discharge during startup in point-of-load
applications.
Ordering Information
Part Number
FAN2108MPX
FAN2108EMPX
Operating
Temperature Range
-10°C to 85°C
-40°C to 85°C
Package
Molded Leadless Package (MLP) 5x6mm
Molded Leadless Package (MLP) 5x6mm
Eco
Status
Green
Green
Packing
Method
Tape and Reel
Tape and Reel
For Fairchild’s definition of “green” Eco Status, please visit: http://www.fairchildsemi.com/company/green/rohs_green.html.
© 2008 Fairchild Semiconductor Corporation
FAN2108 • Rev. 1.0.0
www.fairchildsemi.com
1 page  
Electrical Specifications
Electrical specifications are the result of using the circuit shown in Figure 1 unless otherwise noted.
Symbol
Parameter
Conditions
www.datashPeeotw4ue.croSmupplies
ICC VCC Current
VUVLO VCC UVLO Threshold
SW=Open, FB=0.7V, VCC=5V,
fSW=600KHz
Shutdown: EN=0, VCC=5V
Rising VCC
Hysteresis
Oscillator
f Frequency
tON Minimum On-Time(2)
VRAMP Ramp Amplitude, peak-to–peak
tOFF Minimum Off-Time(2)
Reference
VFB
Reference Voltage (see Figure 4 for
Temperature Coefficient)
RT=50KΩ
RT=24KΩ
16VIN, 1.8VOUT, RT=30KΩ,
RRAMP=200KΩ
FAN2108MPX, 25°C
FAN2108EMPX, 25°C
Error Amplifier
G DC Gain(2)
BW Gain Bandwidth Product(2)
VCOMP
ISINK
ISOURCE
IBIAS
Output Voltage
Output Current, Sourcing
Output Current, Sinking
FB Bias Current
VCC=5V
VCC=5V, VCOMP=2.2V
VCC=5V, VCOMP=1.2V
VFB=0.8V, 25°C
Protection and Shutdown
ILIM Current Limit
RILIM Open at 25°C (see Circuit
Description)
IILIM
TTSD
THYS
VOVP
VUVLO
VFLT
VFLT_HYS
ILIM Current
Over-Temperature Shutdown
Over-Temperature Hysteresis
Over-Voltage Threshold
Under-Voltage Shutdown
Fault Discharge Threshold
Fault Discharge Hysteresis
Internal IC Temperature
Two Consecutive Clock Cycles
16 Consecutive Clock Cycles
Measured at FB Pin
Measured at FB Pin (VFB ~500mV)
Soft-Start
tSS VOUT to Regulation (T0.8)
tEN Fault Enable/SSOK (T1.0)
Frequency=600KHz
Note:
2. Specifications guaranteed by design and characterization; not production tested.
Min. Typ. Max. Unit
8 12 mA
7 10 µA
4.1 4.3 4.5 V
300 mV
255 300 345 KHz
540 600 660 KHz
50 65 ns
0.53 V
100 150 ns
794 800 806 mV
795 800 805 mV
80
12
0.4
1.5
0.8
-850
85
15
2.2
1.2
-650
3.2
-450
dB
MHz
V
mA
mA
nA
12 15 18
A
-11 -10 -9 µA
+155
°C
+30 °C
110 115 121 %VOUT
68 73 78 %VOUT
250 mV
250 mV
5.3 ms
6.7 ms
© 2008 Fairchild Semiconductor Corporation
FAN2108 • Rev. 1.0.0
5
www.fairchildsemi.com
5 Page 
Calculating the Inductor Value
Typically the inductor is set for a ripple current (ΔIL) of
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
www.datashlreoeewst4truic.octuoatmplluotwarbipleplreippalendc/uorrrenuts.e high-ESR capacitors
L
=
VOUT
• (1-
ΔIL •
VOUT
VIN
f
)
(4)
where f is the oscillator frequency.
The over-current protection fault latch is active during
the soft-start cycle. Use a 1% resistor for RILIM.
In case RILIM is not connected, the IC uses an internal
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 network. For type-2
compensation, eliminate R3 and C3.
Setting the Ramp Resistor Value
The internal ramp voltage excursion (ΔVRAMP) during tON
should be set to 0.6V at nominal operating point. RRAMP
is approximately:
RRAMP(KΩ)
=
(VIN − 1.8) •VOUT
18x10−6 •VIN • f
−2
(5)
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 (VILIM) is compared to a
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 ILIM open)
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 (RILIM)
between ILIM and AGND. RILIM can be approximated
with the equation:
RILIM(KΩ)
=
95 + 3 • Iout
• KT
• K1+
Vout • 3.33 • 106
Rramp • fsw
(6)
where:
I = Desired current limit setpoint in Amps;
KT = Normalized temperature coefficient of the
low-side MOSFET (Q2 from Figure 8);
K1 = Overload co-efficient (use 1.2 to 1.4);
VOUT = Set output voltage;
Rramp = Ramp resistor used in KΩ; and
fsw = Selected switching frequency in KHz.
After 16 consecutive, pulse-by-pulse, current-limit
cycles, the fault latch is set and the regulator shuts
down. Cycling VCC or EN restores operation after a
normal soft-start cycle (refer to Auto-Restart section).
Figure 22. Compensation Network
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 VOUT by enhancing the low-side MOSFET
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.
© 2008 Fairchild Semiconductor Corporation
FAN2108 • Rev. 1.0.0
11
www.fairchildsemi.com
11 Page | ||
| Páginas | Total 14 Páginas | |
| PDF Descargar | [ Datasheet FAN2108.PDF ] | |
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