FAN2103 Datasheet, PDF (11/14 Pages) Fairchild Semiconductor – 3A, 24V Input Integrated Synchronous Buck Regulator

Setting the Ramp Resistor Value
The internal ramp voltage excursion (ΔVRAMP) during tON
should be set to 0.6V. RRAMP is approximately:
R_{RAMP(KΩ )}
=
(V_IN − 1.8) • V_OUT
18x10^−6 • V_IN • F
−2
(7)
where frequency (F) is expressed in KHz.
Setting the Current Limit
There are two levels of current limit thresholds in
FAN2103. 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 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 FAN2103
uses its internal low-side MOSFET as the current-
sensing element. The current-limit threshold voltage
(VILIM) is compared to the 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.
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 (RILIM) to AGND:
R_{ILIM(KΩ)}
= 10.4
• K_T
•
(I_OUT
−
ΔIL
2
)
+ 142.5
(8)
where:
IOUT = desired current limit set point in Amps,
KT = the normalized temperature coefficient of the
low-side MOSFET (Q2) from Figure 8.
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 Auto-Restart section).
The over-current protection fault latch is active during
the soft-start cycle. Use a 1% resistor for RILIM.
Loop Compensation
The loop is compensated using a feedback network
around the error amplifier. Figure 22 shows a complete
Type-3 compensation network. Type-2 compensation
eliminates R3 and C3.
Figure 22. Compensation Network
Because the FAN2103 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.
RRAMP provides feedforward compensation for changes
in VIN. With a fixed RRAMP value, the modulator gain
increases as VIN is reduced, which could make it difficult
to compensate the loop. For designs with low input
voltages (3V to 6.5V), it is recommended that separate
RRAMP and the compensation component values are
used as compared to designs with VIN between 6.5V
and 24V.
Protection
The converter output is monitored and protected
against extreme overload, short-circuit, over-voltage,
and under-voltage conditions.
An internal “Fault Latch” is set for any fault intended to
shut down the IC. When the fault latch is set, the IC
discharges VOUT by enhancing the low-side MOSFET
until FB<0.25V. The MOSFET is not turned on again
unless FB>0.5V. This behavior discharges the output
without causing undershoot (negative output voltage).
0.25/0.5V
FB
FAULT
PWM LATCH
PWM GATE
DRIVE
Figure 23. Latched Fault Response
Under-Voltage Shutdown
If FB remains below the under-voltage threshold for 16
consecutive clock cycles, the fault latch is set and the
converter shuts down. This fault is prevented from
setting the fault latch during soft-start.
Over-Voltage Protection / Shutdown
If FB exceeds 115% • VREF for two consecutive clock
cycles, the fault latch is set and shutdown occurs.
A shorted high-side MOSFET condition is detected
when SW voltage exceeds ~0.7V while the low-side
MOSFET is fully enhanced. The fault latch is set
immediately upon detection.
These two fault conditions are allowed to set the fault
latch at any time, including during soft-start.
Auto-Restart
After a fault, EN is discharged with 1µA to a 1.1V
threshold before the 800KΩ pull-up is restored. A new
soft-start cycle begins when EN charges above 1.35V.
Depending on the external circuit, the FAN2103 can be
provisioned to remain latched-off or automatically
restart after a fault.
© 2007 Fairchild Semiconductor Corporation
FAN2103 Rev. 1.0.3
11
www.fairchildsemi.com