FAN5038 Datasheet, PDF (11/15 Pages) Fairchild Semiconductor – Dual Voltage Controller for DSP Power

PRODUCT SPECIFICATION
FAN5038
Short Circuit Considerations
For the Switch-Mode Regulator
The FAN5038 uses a current sensing scheme to limit the
load current if an output fault condition occurs. The current
sense resistor carries the peak current of the inductor, which
is greater than the maximum load current due to ripple cur-
rents flowing in the inductor. The FAN5038 will begin to
limit the output current to the load by turning off the top-side
FET driver when the voltage across the current-sense resistor
exceeds the short circuit comparator threshold voltage (Vth).
When this happens the output voltage will temporarily go
out of regulation. As the voltage across the sense resistor
becomes larger, the top-side MOSFET will continue to turn
off until the current limit value is reached. At this point, the
FAN5038 will continuously deliver the limit current at a
reduced output voltage level. The short circuit comparator
threshold voltage is typically 90mV, with a variability of
+10/-20mV. The ripple current flowing through the inductor
is typically 0.5A. Refer to Application Note AM-53 for
detailed discussions. The sense resistor value can be approx-
imated as follows:
R_SENSE
=
V-----_{t-h---,-m---i--n-}
I_PK
×
(1
–
TF)
=
---------------V-----^t-^h--^,-^m----ⁱ-ⁿ----------------
0.5A + I_LOAD,MAX
×
(1
–
TF)
where TF = Tolerance Factor for the sense resistor and 0.5A
accounts for the inductor current ripple.
Since the value of the sense resistor is often less than 20mΩ,
care should be taken in the layout of the PCB. Trace resis-
tance can contribute significant errors. The traces to the
IFBH and IFBL pins of the FAN5038 should be Kelvin con-
nected to the pads of the current-sense resistor. To minimize
the influence of noise, the two traces should be run next to
each other.
For the Linear Regulator
The analysis for short circuit protection of the linear regula-
tor is much simpler than that of the switching regulator. The
formula for the inception point of short-circuit protection for
the linear regulator is:
R_SENSE
=
------V----^t--^h--^,-^m----ⁱ-ⁿ------
I_LOAD,MAX
×
(1
–
TF)
V_th = 50mV ± 10mV and I_LOAD,MAX = 500mA,
R_SENSE
=
4----0---m-----V---
0.5A
×
(1
–
29 % )
=
57mΩ for using an embedded
PC trace resistor
R_SENSE
=
4----0---m-----V---
0.5A
×
(
1
–
5%)
=
76mΩ for using a discrete
resistor
It should be noted that the presence of D2 in Figure 1
bypasses the short circuit protection of the linear regulator. If
D2 is used and short circuit protection is desired, D2 must be
rated to take the short circuit current of the switch-mode
regulator.
Schottky Diode
In Figure 1, MOSFET Q1 and flyback diode D1 are used as
complementary switches in order to maintain a constant cur-
rent through the output inductor L1. As a result, D1 will have
to carry the full current of the output load when the power
MOSFET is turned off. The power in the diode is a direct
function of the forward voltage at the rated load current dur-
ing the off time of the FET. The following equation can be
used to estimate the diode power:
P_DIODE = I_D × V_D × (1 – DutyCycle)
where ID is the forward current of the diode, VD is the for-
ward voltage of the diode, and DutyCycle is defined the
same as
Duty Cycle = ^{V--V---o--i--un---t}
For the Motorola MBRS835 Power Rectifier used in Figure
1,
P_DIODE = 4A × 0.35 × (1 – 36%) = 0.9W
Board Design Considerations
FAN5038 Placement
Preferably the PC layer directly underneath the FAN5038
should be the ground layer. This serves as extra isolation
from noisy power planes.
MOSFET Placement
Placement of the power MOSFET is critical in the design of
the switch-mode regulator. The FET should be placed in
such a way as to minimize the length of the gate drive path
from the FAN5038 SDRV pin. This trace should be kept
under 0.5" for optimal performance. Excessive lead length
on this trace causes high frequency noise resulting from the
parasitic inductance and capacitance of the trace. Since this
voltage can transition nearly 12V in around 100nsec, the
resultant ringing and noise will be very difficult to suppress.
This trace should be routed on one layer only and kept well
away from the “quiet” analog pins of the device: VREF,
CEXT, FBSW, IFBH, IFBL, and VFBL. Refer to Figure 3.
Inductor and Schottky Diode Placement
The inductor and fly-back Schottky diode must be placed
close to the source of the power MOSFET. The node con-
necting the inductor and the diode swing between the drain
voltage of the FET and the forward voltage of the Schottky
diode. It is recommended that this node be converted to a
plane if possible. This node is part of the high current path in
the design, and is best treated as a plane to minimize the par-
asitic resistance and inductance on that node.
Most PC board manufacturers utilize 1/2oz copper on the top
and bottom signal layers of the PCB; thus, it is not recom-
mended to use these layers to rout the high current portions
of the regulator design. Since it is more common to use 1 oz.
REV. 1.0.2 7/6/00
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