FAN21SV06 Fairchild Semiconductor, FAN21SV06 Datasheet - Page 14
FAN21SV06
Manufacturer Part Number
FAN21SV06
Description
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Manufacturer
Fairchild Semiconductor
Datasheet
1.FAN21SV06.pdf
(17 pages)
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© 2006 Fairchild Semiconductor Corporation
FAN21SV06 Rev. 1.0.1
Calculating the Inductor Value
Typically the inductor value is chosen based on ripple
current (I
maximum DC load. Regulator designs that require fast
transient response use a higher ripple-current setting
while regulator designs that require higher efficiency
keep ripple current on the low side and operate at a
lower switching frequency.
where f is the oscillator frequency, and
Setting the Ramp-Resistor Value
As a starting point, set the internal ramp amplitude
(∆V
where frequency (f) is expressed in KHz.
Refer to
determine the optimal R
Setting the Current Limit
There are two levels of current-limit thresholds in
FAN21SV06. The first level of protection is through an
internal default limit set at the factory to provide cycle–
by-cycle current limit and prevent 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 FAN21SV06 uses its internal
low-side MOSFET as the current-sensing element. The
current-limit threshold voltage (V
voltage drop across the low-side MOSFET, sampled at
the end of each PWM off-time/cycle. The internal default
threshold (I
L
R
I
RAMP
L
RAMP
V
OUT
V
(
OUT
K
) to 0.5V. R
I
L
)
L
AN-6033 — FAN21SV06 Design Guide
L
LIM
) which is chosen between 10 to 35% of the
(1
f
(
18
(1
f
-
V
open) is temperature compensated.
D)
IN
-
Figure 32. ILIM Network
x
D)
10
. 1
RAMP
6
) 8
V
RAMP
V
IN
is approximately:
OUT
f
value.
2
ILIM
) is compared to the
(4)
(5)
(6)
to
14
R
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 16 consecutive pulse-by-pulse current-limit cycles,
the fault latch is set and the regulator shuts down.
Cycling VIN_Reg or EN restores operation after a
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
given R
varies slightly in an inverse relationship to VIN. In case
R
current-limit threshold.
Loop Compensation
The control loop is compensated using a feedback
network around the error amplifier. Figure 33 shows a
complete
compensation eliminates R3 and C3.
Since the FAN21SV06 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 low-input-voltage-range
designs (3V to 8V), R
component values are going to be different as compared
to designs with V
ILIM
ILIM
RAMP
ILIM
(
K
I=desired current-limit set point in Amps,
K
low-side MOSFET (Q2) from Figure 8.
K1=Overload co-efficient (use 1.2 to 1.4)
V
R
f
IN
SW
) to AGND:
is not connected, the IC uses the internal default
)
T
OUT
. With a fixed R
RAMP
=the normalized temperature coefficient of the
provides feedforward compensation for changes
=Selected switching frequency, in KHz.
95
RAMP
=Set output voltage
Figure 33. Compensation Network
=Ramp resistor used, in k
5
Type-3
I
OUT
and R
IN
IN
is reduced, which could make it difficult
K
between 8V and 24V.
T
ILIM
compensation
K
1
RAMP
setting, the current-limit point
V
RAMP
OUT
R
RAMP
value, the modulator gain
. 3
and the compensation
33
f
SW
10
6
network.
www.fairchildsemi.com
ILIM
. For a
Type-2
(7)