FAN2110 Datasheet, PDF (14/17 Pages) Fairchild Semiconductor – TinyBuck™, 3-24V Input, 10A, High-Efficiency, Integrated Synchronous Buck Regulator

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 VCC is used to drive the internal MOSFET gates,
supply current is frequency and voltage dependent.
Approximate VCC current (ICC) is calculated by:
I_{CC ( mA )}
=
4.58 + [(^{VCC − 5}
227
+ 0.013) • (f
− 128)]
(1)
where frequency (f) is expressed in KHz.
L
=
V_OUT
• (1-
V_{O UT}
Vin
)
(4)
ΔIL • f
where f is the oscillator frequency.
Setting the Ramp Resistor Value
RRAMP resistor plays a critical role in the design by
providing charging current to the internal ramp capacitor
and also serving as a means to provide input voltage
feedforward.
RRAMP is calculated by the following formula:
Setting the Output Voltage
The output voltage of the regulator can be set from 0.8V
to 80% of VIN by an external resistor divider (R1 and
RBIAS in Figure 1). For output voltages >3.3V, output
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:
0.8V ₌ V_OUT − 0.8V _{+ 650nA}
R_BIAS
R1
(2)
Connect RBIAS between FB and AGND.
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 RBIAS is ≤ 1KΩ, the
internal SS ramp is not released and the regulator does
not start.
Setting the Clock Frequency
Oscillator frequency is determined by an external resistor,
RT, connected between the RT pin and AGND.
Resistance is calculated by:
R_{T (KΩ)}
=
(10⁶
/ f ) − 135
65
(3)
where RT is in KΩ and frequency (f) is in KHz.
The regulator cannot start if RT is left open.
Calculating the Inductor Value
Typically the inductor value is chosen based on ripple
current (ΔIL), which is chosen between 10 to 35% of the
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. The inductor value is
calculated by the following formula:
R_{RAMP (KΩ)}
=
(31 −
(V_IN − 1.8) •V_OUT
2.05 • I_OUT ) •V_IN • f
• 10 ^−6
−2
(5)
where frequency (f) is expressed in KHz.
For wide input operation, first calculate RRAMP for the
minimum and maximum input voltage conditions and
use larger of the two values calculated.
In all applications, current through the RRAMP pin must
be greater than 10µA from the equation below for
proper operation:
V_IN − 1.8 _{≥ 10μA}
R_RAMP + 2
(6)
If the calculated RRAMP values in Equation (5) result in a
current less than 10µA, use the RRAMP value that
satisfies Equation (6). In applications with large Input
ripple voltage, the RRAMP resistor should be adequately
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 (RILIM)
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 (VILIM) is compared to a scaled
version of voltage drop across the low-side MOSFET
sampled at the end of each PWM off-time/cycle. The
internal default threshold (with ILIM open) is temperature
compensated.
© 2008 Fairchild Semiconductor Corporation
FAN2110 • Rev. 1.0.2
14
www.fairchildsemi.com