FAN21SV06 Fairchild Semiconductor, FAN21SV06 Datasheet - Page 14

• Manufacturer Part Number: FAN21SV06
• Description: This part is covered in Fairchild's href="http://www
• Manufacturer: Fairchild Semiconductor

© 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.
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ILIM
. For a
Type-2
(7)