FAN2106 Fairchild Semiconductor, FAN2106 Datasheet - Page 12

• Manufacturer Part Number: FAN2106
• Description: This part is covered in Fairchild's href="http://www
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN2106 • Rev. 1.1.1
where f is the switching frequency.
Setting the Ramp Resistor Value
R
providing charging current to the internal ramp capacitor
and also serving as a means to provide input voltage
feedforward.
R
where frequency (f) is expressed in KHz.
For wide input operation, first calculate R
minimum and maximum input voltage conditions and
use larger of the two values calculated.
In all applications, current through the R
be greater than 10µA from the equation below for proper
operation:
If the calculated R
current less than 10µA, use the R
satisfies Equation (6). In applications with large input
ripple voltage, the R
decoupled from the input voltage to minimize ripple on
the RAMP pin
There are two levels of current-limit thresholds. 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 set
externally at the ILIM pin by connecting a resistor (R
between ILIM and AGND. Current-limit protection is
enabled whenever the lower of the two thresholds is
reached ( see Figure 24 ). FAN2106 uses its internal low-
side MOSFET for current-sensing. The current-limit
threshold voltage (V
of voltage drop across the low-side MOSFET sampled
at the end of each PWM off-time/cycle. The internal
default threshold (with I
compensated.
For a given R
point varies slightly in an inverse relationship with
respect to input voltage (V
R
Setting the Current Limit

RAMP
RAMP
R
RAMP
V
I
RAMP
L
IN

(
is calculated by the following formula:
K

V
resistor plays a critical role in the design by

1
OUT
)
8 .


L
2
(
18

(

V

.
(1
IN
)
f
ILIM
10

-

V
D)
IN

RAMP
. 1
and R
A
ILIM
) 8

RAMP
f


) is compared to a scaled version
V
values in Equation (5) result in a
10
OUT
RAMP
resistor should be adequately
IN

6
).
LIM

setting, the current limit
2
open) is temperature
RAMP
RAMP
RAMP
value that
pin must
for the
ILIM
(4)
(5)
(6)
)
12
The ILIM pin can source a 10µA current that can be
used to establish a lower, temperature-dependent,
current-limit threshold by connecting an external resistor
(R
equation:
where:
I is desired current limit set point in Amps;
R
K
side MOSFET (Q2) from Figure 8. Use 0.35 in equation.
After
cycles, the fault latch is set and the regulator shuts
down. Cycling V
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 R
Loop Compensation
The loop is compensated using a feedback network
around the error amplifier. Figure 25 shows a complete
Type-3
compensation, eliminate R3 and C3.
Since the FAN2106 employs a 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
T
DS
ILIM
ILIM
is the normalized temperature coefficient of the low-
is expressed in m; and
(
) to AGND. R
ILIM
K

16
)

10 µA
VCOMP
RAMP
Signal
R
. 0
compensation
VILIM
ILIM
consecutive,
Figure 25. Compensation Network
45

Figure 24. ILIM Network
R
CC
DS
ILIM
or EN restores operation after a

1 (
can be approximated with the

K
pulse-by-pulse,
T
)
network.

(
I
OUT


2
I
L
ILIM
For
)

.
www.fairchildsemi.com
current-limit
142
To
Counter
PW M
5 .
Type-2
(7)