FAN2110 Fairchild Semiconductor, FAN2110 Datasheet - Page 14

• Manufacturer Part Number: FAN2110
• Description: This part is covered in Fairchild's href="http://www
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN2110 • Rev. 1.0.3
Application Information
Bias Supply
The FAN2110 requires a 5V supply rail to bias the IC
and provide gate-drive energy. Connect a  2.2µf X5R
or X7R decoupling capacitor between VCC and AGND.
Since V
supply current is frequency and voltage dependent.
Approximate V
where frequency (f) is expressed in KHz.
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 22, and Figure 23.)
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 Clock Frequency
Oscillator frequency is determined by an external resistor,
R
is calculated by:
where R
The regulator cannot start if 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,
while regulator designs that require higher efficiency
keep ripple current on the low side and operate at a
I
R
R
CC
0
BIAS
T
T
BIAS
8 .
, connected between the R
(
(
K
mA
V

in Figure 1). For output voltages >3.3V, output
)
)

CC


T
V
is in K and frequency (f) is in KHz.
(
. 4
L
BIAS
10
OUT
is used to drive the internal MOSFET gates,
), which is chosen between 10 to 35% of the
58
IN
6
R
CC
between FB and AGND.
/


by an external resistor divider (R1 and
65
f
1
[(
)
0
current (I
V

8 .
CC
135
V
227


5
650

CC
. 0
nA
) is calculated by:
T
T
013
pin and AGND. Resistance
is left open.
)

(
f

BIAS
128
is  1K, the
)]
(2)
(3)
(1)
14
lower switching frequency. The inductor value is
calculated by the following formula:
where f is the oscillator 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. For example, see Figure 11.
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 33). FAN2110 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.
R
L
R
RAMP
RAMP
RAMP
V

RAMP
IN
V
OUT
(

K
is calculated by the following formula:

resistor plays a critical role in the design by
1
)
8 .




2
(1
I
(
L
31

-


f
V
10
. 2
Vin
O
05

UT
(
V
RAMP
A
IN

ILIM
)
I
RAMP
OUT

. 1
) is compared to a scaled version
values in Equation (5) result in a
) 8
)


resistor should be adequately
V
V
IN
LIM
OUT

f

open) is temperature
10

6

RAMP
2
RAMP
RAMP
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value that
pin must
for the
ILIM
(4)
(6)
(5)
)