Linear Technology LTC4260, Datasheet, Page: 17 / 30

LTC4260
17
4260fc
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applicaTions inForMaTion
The SOA (safe operating area) curves of candidate FETs
must be evaluated to ensure that the heat capacity of the
package can stand 24W for 16ms. The SOA curves of the
Fairchild FDB3632 provide for 1A at 50V (50W) for 10ms,
satisfying the requirement.
The inrush current is set to 1A using C1:
C1 = C
L
I
GATE(UP)
I
INRUSH
= 0.33mF 18µA
1A
= 5.9nF
Default values of R5 = 10Ω and R6 = 100k are chosen as
discussed previously.
The power dissipated in the FET during overcurrent must
be limited. The active current limit uses a timer to prevent
excessive energy dissipation in the FET. The worst-case
power occurs when the voltage versus current profile of
the foldback current limit is at the maximum. This occurs
when the current is 5A and the voltage is 1/2 of the 48V
or 24V. See the Current Limit Sense Voltage vs FB Voltage
in the Typical Performance curves to view this profile. In
order to survive 120W, the FET SOA curve dictates the
maximum time at this power level. This particular FET
allows 300W at 1ms or less. Therefore, it is acceptable
to set the current limit timeout using C
T
to be 0.81ms:
C
T
=
0.81ms
12 ms/µF
[ ]
=
68nF
Note the minimum value for C
T
is 0.1nF.
Choose R1, R2, R3, R7 and R8 for the UV, OV and PG
threshold voltages:
V
OVRISING
= 71.2V, V
OVFALLING
= 69.44V (using V
OV(TH)
=
3.5V rising and 3.41V falling)
V
UVRISING
= 43V, V
UVFALLING
= 38.5V, (using V
UV(TH)
=
3.5V rising and 3.12V falling)
V
PGRISING
= 46.14V, V
PGFALLING
= 45V, (using V
FB
= 3.5V
rising and 3.411V falling)
In addition a 0.1µF ceramic bypass capacitor is placed on
the INTV
CC
pin. The complete circuit is shown in Figure 1.
Layout Considerations
To achieve accurate current sensing, a Kelvin connection
is recommended. The minimum trace width for 1oz cop-
per foil is 0.02” per amp to make sure the trace stays at
a reasonable temperature. Using 0.03” per amp or wider
is recommended. Note that 1oz copper exhibits a sheet
resistance of about 530µΩ/square. Small resistances add
up quickly in high current applications. To improve noise
immunity, put the resistive divider to the UV, OV and FB
pins close to the device and keep traces to V
DD
and GND
short. It is also important to put C3, the bypass capacitor
for the INTV
CC
pin, as close as possible between INTV
CC
and GND. A 0.1µF capacitor from the UV pin (and OV pin
through resistor R2) to GND also helps reject supply noise.
Figure 5 shows a layout that addresses these issues. Note
that a surge suppressor, Z1, is placed between supply and
ground using wide traces.
SENSE
LTC4260
V
DD
UV
R1
SENSE RESISTOR R
S
I
LOAD
V
IN
GND
I
LOAD
R2
R3
R
8
C3
4260 F05
C
F
OV
GND
INTV
CC
FB
Z1
Figure 5. Recommended Layout for
R1, R2, R3, R8, C
F
, C3, Z1 and R
S