Datasheet

LTC4219
15
4219fb
APPLICATIONS INFORMATION
The power good threshold using the internal resistive
divider on the FB pin matches the 10.5V requirement.
The fi nal schematic in Figure 5 results in very few external
components. The pull-up resistors, R1 and R3, connect
to the FLT and PG pins while the 20k (R2) converts the
I
MON
current to a voltage at a ratio:
V
IMON
= 20 µA/A
⎡
⎣
⎤
⎦
•20k•I
OUT
= 0.4 V/A
⎡
⎣
⎤
⎦
•I
OUT
In addition there is a 0.1µF bypass (C1) on the INTV
CC
pin.
Layout Considerations
In Hot Swap applications where load currents can be 5A,
narrow PCB tracks exhibit more resistance 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 0.5mW/square. Small
resistances add up quickly in high current applications.
There are two V
DD
pins on opposite sides of the package
that connect to the sense resistor and MOSFET. The PCB
layout should be balanced and symmetrical to each V
DD
pin to balance current in the MOSFET bond wires. Figure 6
shows a recommended layout for the LTC4219.
Although the MOSFET is self protected from overtem-
perature, it is recommended to solder the backside of the
package to a copper trace to provide a good heat sink.
Note that the backside is connected to the SENSE pin and
cannot be soldered to the ground plane. During normal
loads the power dissipated in the package is as high as
1.9W. A 10mm ¥ 10mm area of 1oz copper should be suf-
fi cient. This area of copper can be divided in many layers.
It is also important to put C1, the bypass capacitor for
the INTV
CC
pin as close as possible between the INTV
CC
and GND.
Figure 6. Recommended Layout
4217 F06
HEAT SINK
VIA TO
SINK
GND
C
OUTV
DD
C
T
0.1µF
5V
5V
R
MON
20k
4219 F07
R2
10k
5V
R1
10k
C
L
100µF
V
DD
EN1
EN2
OUT
PG
GND
I
MON
LTC4219DHC-5
INTV
CC
TIMER
FLT
+
V
OUT
5V
5A
ADC
Additional Applications
The LTC4219 has a wide operating range from 2.9V to 15V.
The PG threshold is set with an internal resistive divider.
All other functions are independent of supply voltage.
Figure 7 shows a 5V application with a PG threshold of
4.35V.
In addition to Hot Swap applications, the LTC4219 also
functions as a backplane resident switch for removable
load cards (see the Typical Application section).
Figure 7. 5V, 5A Card Resident Application