FAN2106_09 Datasheet, PDF (12/15 Pages) Fairchild Semiconductor – 3-24V Input, 6A, High-Efficiency, Integrated Synchronous Buck Regulator

ΔIL = ^{VOUT • (1- D)}
(4)
L•f
where f is the switching 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:
R_{RAMP (KΩ)}
=
(V_IN − 1.8) •V_OUT
(18) •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}
(6)
R_RAMP + 2
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.
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 24). FAN2106 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.
For a given RILIM and RRAMP setting, the current limit
point varies slightly in an inverse relationship with
respect to input voltage (VIN).
RAMP
Signal
VCOMP
PW M
VILIM
10 µA
ILIM
RILIM
To
Counter
Figure 24.ILIM Network
The ILIM pin can source a 10µA current that can be
used to establish a lower, temperature-dependent,
current-limit threshold by connecting an external
resistor (RILIM) to AGND. RILIM can be approximated with
the equation:
R_ILIM(KΩ)
=
0.45 • R_DS
• (1+ K_T ) • (I_OUT
−
ΔIL
2
) + 142.5
(7)
where:
I is desired current limit set point in Amps;
RDS is expressed in mΩ; and
KT is the normalized temperature coefficient of the low-
side MOSFET (Q2) from Figure 8. Use 0.35 in equation.
After 16 consecutive, pulse-by-pulse, current-limit
cycles, the fault latch is set and the regulator shuts
down. Cycling VCC or EN restores operation after a
normal soft-start cycle (refer to the Auto-Restart
section).
The over-current protection fault latch is active during
the soft-start cycle. Use 1% resistor for RILIM.
Loop Compensation
The loop is compensated using a feedback network
around the error amplifier. Figure 25 shows a complete
Type-3 compensation network. For Type-2
compensation, eliminate R3 and C3.
Figure 25. Compensation Network
Since the FAN2106 employs a 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.
© 2009 Fairchild Semiconductor Corporation
FAN2106 • Rev. 1.1.0
12
www.fairchildsemi.com