FAN2103 Fairchild Semiconductor, FAN2103 Datasheet - Page 11

• Manufacturer Part Number: FAN2103
• Description: This part is covered in Fairchild's href="http://www
• Manufacturer: Fairchild Semiconductor

© 2007 Fairchild Semiconductor Corporation
FAN2103 • Rev. 1.0.7
Calculating the Inductor Value
Typically the inductor is set for a ripple current (I
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 and:
Setting the Ramp Resistor Value
The internal ramp voltage excursion (∆V
should be set to 0.6V. R
where frequency (f) is expressed in KHz.
Setting the Current Limit
There are two levels of current-limit thresholds in
FAN2103. 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 a flexible one to be set externally by the
user. Current-limit protection is enabled whenever the
lower of the two thresholds is reached. The FAN2103
uses its internal low-side MOSFET for current-sensing.
The current-limit threshold voltage (V
the voltage drop across the low-side MOSFET, sampled
at the end of each PWM off-time/cycle. The internal
default threshold (with I
compensated.
The 10µA current sourced from the ILIM pin can be
used to establish a lower, temperature–dependent,
current-limit threshold by connecting an external resistor
(R
where:
After
cycles, the fault latch is set and the regulator shuts
down. Cycling V
normal soft-start cycle (refer to Auto-Restart section).
The over-current protection fault latch is active during
the soft-start cycle. Use a 1% resistor for R
L
R
R

I
RAMP
ILIM(KΩ)

ILIM
L
V

I
K
low-side MOSFET (Q2) from Figure 8.
OUT
OUT
) to AGND:
V
T
16
OUT
K (
= the normalized temperature coefficient of the

= desired current limit set point in Amps,
I

L


L
)

(1
10
consecutive,



f
(1
f
-
4 .
(
18
D)
V
-
IN

x
CC
D)
10
K

T
or EN restores operation after a

. 1

6
) 8
I (
RAMP

OUT

V
pulse-by-pulse,
IN
V
LIM
OUT
is approximately:


f

open) is temperature
2
I
L

2
)
ILIM

142
) is compared to
RAMP
ILIM
5 .
current-limit
) during t
.
L
) of
(5)
(6)
(7)
(8)
ON
11
Loop Compensation
The loop is compensated using a feedback network
around the error amplifier. Figure 22 shows a complete
Type-3 compensation network. Type-2 compensation
eliminates R3 and C3.
Because the FAN2103 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.
R
in V
increases as V
to compensate the loop. For designs with low input
voltages (3V to 6.5V), it is recommended that a separate
R
used as compared to designs with V
24V.
Protection
The converter output is monitored and protected against
extreme overload, short-circuit, over-voltage, and under-
voltage 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).
0.25/0.5V
RAMP
RAMP
FB
IN
. With a fixed R
provides feedforward compensation for changes
and the compensation component values are
Figure 23. Latched Fault Response
Figure 22. Compensation Network
OUT
IN
FAULT
is reduced, which could make it difficult
by enhancing the low-side MOSFET
PWM LATCH
RAMP
value, the modulator gain
IN
between 6.5V and
PWM
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