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FAN2108MPX 数据表(PDF) 11 Page - Fairchild Semiconductor

部件名 FAN2108MPX
功能描述  TinyBuck??3-24 V Input, 8 A, High-Efficiency, Integrated Synchronous Buck Regulator
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制造商  FAIRCHILD [Fairchild Semiconductor]
网页  http://www.fairchildsemi.com
标志 FAIRCHILD - Fairchild Semiconductor

FAN2108MPX 数据表(HTML) 11 Page - Fairchild Semiconductor

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© 2008 Fairchild Semiconductor Corporation
www.fairchildsemi.com
FAN2108 • Rev. 1.0.2
11
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 low
output ripple and/or use high-ESR capacitors restrict
allowable ripple current.
f
)
V
-
(1
V
OUT
OUT
Δ
=
L
IN
I
V
L
(4)
where f is the oscillator frequency.
Setting the Ramp Resistor Value
The internal ramp voltage excursion (∆VRAMP) during tON
should be set to 0.6 V at nominal operating point. RRAMP
is approximately:
2
10
18
)
8
.
1
(
6
)
(
=
Ω
f
V
x
V
V
R
IN
OUT
IN
K
RAMP
(5)
where frequency (f) is expressed in KHz.
Setting the Current Limit
The current limit system involves two comparators. The
MAX ILIMIT comparator is used with a VILIM fixed-voltage
reference and represents the maximum current limit
allowable. This reference voltage is temperature
compensated to reflect the RDSON variation of the low-
side MOSFET. The ADJUST ILIMIT comparator is used
where the current limit needs to be set lower than the
VILIM fixed reference. The 10 µA current source does not
track the RDSON changes over temperature, so change is
added into the equations for calculating the ADJUST
ILIMIT comparator reference voltage, as is shown below.
Figure 22 shows a simplified schematic of the over-
current system.
Figure 22. Current-Limit System Schematic
Since the ILIM voltage is set by a 10 µA current source
into the RILIM resistor, the basic equation for setting the
reference voltage is:
VRILIM = 10µA*RILIM
(6)
To calculate RILIM:
RILIM = VRILIM/ 10µA
(7)
The voltage VRILIM is made up of two components, VBOT
(which relates to the current through the low-side
MOSFET) and VRMPEAK (which relates to the peak
current through the inductor). Combining those two
voltage terms results in:
RILIM = (VBOT + VRMPEAK)/ 10µA
(8)
RILIM = {0.96 + (ILOAD * RDSON *KT*8)} +
{D*(VIN – 1.8)/(fSW*0.03*RRAMP)}/10µA
(9)
where:
VBOT = 0.96 + (ILOAD * RDSON *KT*8);
VRMPEAK = D*(VIN – 1.8)/(fSW*0.03*RRAMP);
ILOAD = the desired maximum load current;
RDSON = the nominal RDSON of the low-side MOSFET;
KT = the normalized temperature coefficient for the
low-side MOSFET (on datasheet graph);
D = VOUT/VIN duty cycle;
fSW = Clock frequency in kHz; and
RRAMP = chosen ramp resistor value in k
Ω.
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 the Auto-Restart
section)
.
The over-current protection fault latch is active during
the soft-start cycle. Use 1% resistor for RILIM.
Loop Compensation
The loop is compensated using a feedback network
around the error amplifier. Figure 23 shows a complete
type-3 compensation network. For type-2 compensation,
eliminate R3 and C3.
Figure 23. 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
+
_
VCC
10µA
ILIMIT
ILIM
RILIM
+
_
ILIMIT
ADJUST
MAX
+
_
COMP
PWM
VERR
PWM
ILIMTRIP
VILIM
RAMP


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