FAN21SV04 Fairchild Semiconductor, FAN21SV04 Datasheet - Page 14

• Manufacturer Part Number: FAN21SV04
• Description: This part is covered in Fairchild's href="http://www
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FAN21SV04 • Rev. 1.0.2
Application Information
5V_Reg Output
The 5V_Reg pin is the output of the internal regulator
that supplies all power to the control circuit. It is
important to keep this pin decoupled to AGND with a
>2.2µf X5R or X7R decoupling capacitor. In addition, for
operation with V
with the boot capacitor to reduce the switching noise into
the regulator.
Setting the Output Voltage
The output voltage of the regulator can be set from 0.8V
to ~80% of V
R
current rating may need to be de-rated depending on the
ambient temperature, power dissipated in the package,
and the PCB layout (refer to Thermal Information table
on page 4, Figure 20, Figure 21, and Figure 23).
The internal reference is set to 0.8V with 650nA sourced
from the FB pin to ensure that the regulator does not
start if the pin is left open.
The external resistor divider is calculated using:
Connect R
If R1 is open (see Figure 1), the output voltage is not
regulated and a latched fault occurs after the SS is
complete (T1.0).
If the parallel combination of R1 and R
internal SS ramp is not released and the regulator does
not start.
Setting the Switching Frequency
Switching frequency is determined by a resistor, R
connected between the R
Mode) or 5V_Reg (Slave Mode):
where R
where frequency (f) is expressed in KHz.
In Slave Mode, the switching frequency is about 10%
slower for the same R
R
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
R
R
0
BIAS
T
T
BIAS
8 .
(
is open in Master Mode.
K
V

in Figure 1). For output voltages >3.3V, output
)


T
(
V
is expressed in k:
10
L
BIAS
OUT
), which is chosen between 10 to 35% of the
6
IN
/
between FB and AGND.
R
65
f

1
by an external resistor divider (R1 and
)
IN
0

>20V, add a 3.3 resistor in series
8 .
135
V
T

. The regulator does not start if
650
T
nA
pin and AGND (Master
BIAS
is  1K, the
(1)
(2)
T
,
14
while regulator designs that require higher efficiency
keep ripple current on the low side and operate at a
lower switching frequency. The inductor value is
calculated by the following formula:
where f is the switching frequency.
Setting the Ramp Resistor Value
R
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 (5). 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 in
FAN21SV04. 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 levels. The second level of protection
is set 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 33). The FAN21SV04 uses its
internal low-side MOSFET as the current-sensing
element. The current-limit threshold voltage (V
compared to the voltage drop across the low-side
MOSFET sampled at the end of each PWM off-time
cycle. The internal default threshold (I
temperature compensated.
L
R
R
RAMP
RAMP
RAMP
V

RAMP
IN
V
OUT
K (

is calculated by the following formula:
resistor plays a critical role by providing charging

1
)
8 .




2
(
I
(1
L
30


-
5 .
10
f
V

V
OUT

4
IN
(
RAMP
5 .
V
A
IN

RAMP
)
I

OUT
. 1
values in Equation (4) result in a
) 8
)
resistor should be adequately


V
V
IN
OUT

f

10

6
RAMP

2
RAMP
LIM
RAMP
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value that
open) is
pin must
for the
ILIM
ILIM
) is
(3)
(4)
(5)
)