Datasheet

LTC4218
14
4218ff
APPLICATIONS INFORMATION
Next, the power dissipated in the MOSFET during overcur-
rent must be limited. The active current limit uses a timer
to prevent excessive energy dissipation in the MOSFET. The
worst-case power occurs when the voltage versus current
profi le of the foldback current limit is at the maximum.
This occurs when the current is 6A and the voltage is one
half of 12V or (6V). See the Current Limit Sense Voltage vs
FB Voltage in the Typical Performance curves to view this
profi le. In order to survive 36W, the MOSFET SOA dictates
a maximum time at this power level. The Si7108DN allows
60W for 10ms or less. Therefore, it is acceptable to set
the current limit timeout using C
T
to be 1.2ms:
C
T
= 1.2ms/12[ms/μF] = 0.1μF
After the 1.2ms timeout the FLT pin needs to pull down on
the UV pin to restart the power-up sequence.
Since the default values for overvoltage, undervoltage and
power good thresholds for the 12V fi xed version match
the requirements, no external components are required
for the UV, OV and FB pins.
The fi nal schematic results in very few external com-
ponents. Resistor R1 (10Ω) prevents high frequency
oscillations in Q1 while R
GATE
of 1k isolates C
GATE
during
fast turn-off. The pull-up resistor, (R2), connects to the
PG pin while the 20k (R3) converts the I
MON
current to a
voltage at a ratio:
V
IMON
= 6.67
μA
mV
⎡
⎣
⎢
⎤
⎦
⎥
•2
mV
A
⎡
⎣
⎢
⎤
⎦
⎥
•20k•I
OUT
= 0.267
V
A
⎡
⎣
⎢
⎤
⎦
⎥
•I
OUT
In addition, there is a 0.1μF bypass (C1) on the INTV
CC
pin.
Layout Considerations
To achieve accurate current sensing, a Kelvin connection
for the sense resistor is recommended. The PCB layout
should be balanced and symmetrical to minimize wiring
errors. In addition, the PCB layout for the sense resistors
and the power MOSFETs should include good thermal
management techniques for optimal device power dissipa-
tion. A recommended PCB layout for the sense resistor
and power MOSFET is illustrated in Figure 6.
In Hot Swap applications where load currents can be 6A,
narrow PCB tracks exhibit more resistances than wider
tracks and operate at elevated temperatures. The minimum
trace width for 1oz copper 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 0.5mΩ/square. Small
resistances add up quickly in high current applications.
Figure 6. Recommended Layout
4218 F06
R1
R
S
C
Q1
LTC4218