FAN2106_09 FAIRCHILD [Fairchild Semiconductor], FAN2106_09 Datasheet - Page 12

• Manufacturer Part Number: FAN2106_09
• Description: 3-24V Input, 6A, High-Efficiency, Integrated Synchronous Buck Regulator
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

© 2009 Fairchild Semiconductor Corporation
FAN2106 • Rev. 1.1.0
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
Setting the Current Limit
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
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
compensated.
For a given R
point varies slightly in an inverse relationship with
respect to input voltage (V
R
Δ
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
Ω
1
OUT
)
8 .
+
=
L
2
(
18
•
(
•
V
≥
.
(1
IN
)
f
ILIM
10
•
-
−
V
D)
IN
μ
. 1
RAMP
and R
A
) 8
•
RAMP
f
•
ILIM
•
V
values in Equation (5) result in a
10
OUT
) is compared to a scaled
RAMP
resistor should be adequately
IN
−
6
).
−
setting, the current limit
2
LIM
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
the 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 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 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
is the normalized temperature coefficient of the low-
is expressed in mΩ; and
(
ILIM
K
Ω
)
=
10 µA
VCOMP
RAMP
Signal
ILIM
R
. 0
compensation
VILIM
ILIM
Figure 25. Compensation Network
45
) to AGND. R
•
Figure 24. ILIM Network
R
CC
DS
or EN restores operation after a
•
1 (
+
K
ILIM
T
)
network.
•
can be approximated with
(
I
OUT
−
Δ
2
I
L
ILIM
For
)
+
.
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142
To
Counter
PW M
5 .
Type-2
(7)