FAN2103_12 Datasheet, PDF (11/14 Pages) Fairchild Semiconductor – 3 A, 24 V Input, Integrated Synchronous Buck Regulator

Calculating the Inductor Value
Typically the inductor is set for a ripple current (ΔI_L) of
10% to 35% of the maximum DC load. Regulators
requiring fast transient response use a value on the high
side of this range, while regulators that require very low
output ripple and/or use high-ESR capacitors restrict
allowable ripple current:
ΔIL = ^{VOUT • (1 - D)}
(5)
L•f
where f is the oscillator frequency and:
L
=
V_OUT • (1
ΔIL • f
-
D)
(6)
Setting the Ramp Resistor Value
The internal ramp voltage excursion (∆V_RAMP) during t_ON
should be set to 0.6 V. R_RAMP 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
The current limit system involves two comparators. The
MAX I_LIMIT comparator is used with a V_ILIM fixed-voltage
reference and represents the maximum current limit
allowable. This reference voltage is temperature
compensated to reflect the R_DSON variation of the low-
side MOSFET. The ADJUST I_LIMIT comparator is used
where the current limit needs to be set lower than the
V_ILIM fixed reference. The 10 µA current source does not
track the R_DSON changes over temperature, so change is
added into the equations for calculating the ADJUST
I_LIMIT comparator reference voltage, as is shown below.
Figure 22 shows a simplified schematic of the over-
current system.
RAMP
VERR
PWM
+ ^COMP
_
PWM
VCC
10µA
ILIM
RILIM
VILIM
MAX
+ ^ILIMIT
_
ADJUST
+ ^ILIMIT
_
ILIMTRIP
V_RILIM = 10µA*R_ILIM
(8)
To calculate R_ILIM:
R_ILIM = V_RILIM/ 10µA
(9)
The voltage V_RILIM is made up of two components, V_BOT
(which relates to the current through the low-side
MOSFET) and V_RMPEAK (which relates to the peak
current through the inductor). Combining those two
voltage terms results in:
R_ILIM = (V_BOT + V_RMPEAK)/ 10µA
(10)
R_ILIM = {0.96 + (I_LOAD * R_DSON *K_T*8)} +
{D*(V_IN – 1.8)/(f_SW*0.03*R_RAMP)}/10µA
(11)
where:
V_BOT = 0.96 + (I_LOAD * R_DSON *K_T*8);
V_RMPEAK = D*(V_IN – 1.8)/(f_SW*0.03*R_RAMP);
I_LOAD = the desired maximum load current;
R_DSON = the nominal R_DSON of the low-side MOSFET;
K_T = the normalized temperature coefficient for the
low-side MOSFET (on datasheet graph);
D = V_OUT/V_IN duty cycle;
f_SW = Clock frequency in kHz; and
R_RAMP = chosen ramp resistor value in kΩ.
After 16 consecutive, pulse-by-pulse, current-limit
cycles, the fault latch is set and the regulator shuts
down. Cycling V_CC 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 R_ILIM.
Loop Compensation
The loop is compensated using a feedback network
around the error amplifier. Figure 23 shows a complete
Type-3 compensation network. Type-2 compensation
eliminates R3 and C3.
Figure 22. Current-Limit System Schematic
Since the I_LIM voltage is set by a 10 µA current source
into the R_ILIM resistor, the basic equation for setting the
reference voltage is:
Figure 23. Compensation Network
© 2007 Fairchild Semiconductor Corporation
FAN2103 • Rev. 1.0.8
11
www.fairchildsemi.com