LT6300
W U U
APPLICATIO S I FOR ATIO
U
Heat Sinking Using PCB Metal
the input capacitance to form a pole that can cause
frequency peaking. In general, use feedback resistors of
1k or less.
Designing a thermal management system is often a trial
and error process as it is never certain how effective it is
until it is manufactured and evaluated. As a general rule,
the more copper area of a PCB used for spreading heat
away from the driver package, the more the operating
junction temperature of the driver will be reduced. The
limit to this approach however is the need for very com-
pact circuit layout to allow more ports to be implemented
on any given size PCB.
Compensation
The LT6300 is stable in a gain 10 or higher for any supply
andresistiveload.Itiseasilycompensatedforlowergains
with a single resistor or a resistor plus a capacitor.
Figure 8showsthatforinvertinggains,aresistorfromthe
inverting node to AC ground guarantees stability if the
parallel combination of RC and RG is less than or equal to
RF/9. For lowest distortion and DC output offset, a series
capacitor, CC, can be used to reduce the noise gain at
lower frequencies. The break frequency produced by RC
and CC should be less than 5MHz to minimize peaking.
To best extract heat from the GN16 package, a generous
areaoftoplayerPCBmetalshouldbeconnectedtothefour
corner pins (Pins 1, 8, 9 and 16). These pins are fused to
the leadframe where the LT6300 die is attached. It is
important to note that this heat spreading metal area is
electrically connected to the V– supply voltage.
Figure9showscompensationinthenoninvertingconfigu-
ration. The RC, CC network acts similarly to the inverting
case. The input impedance is not reduced because the
network is bootstrapped. This network can also be placed
between the inverting input and an AC ground.
Fortunately xDSL circuit boards use multiple layers of
metal for interconnection of components. Areas of metal
beneath the LT6300 connected together through several
small 13 mil vias can be effective in conducting heat away
from the driver package. The use of inner layer metal can
free up top and bottom layer PCB area for external compo-
nent placement.
R
F
V
V
–R
F
O
R
=
G
–
+
R
I
G
V
I
When PCB cards containing multiple ports are inserted
into a rack in an enclosed cabinet, it is often necessary to
provide airflow through the cabinet and over the cards.
This is also very effective in reducing the junction-to-
ambient thermal resistance of each line driver. To a limit,
this thermal resistance can be reduced approximately
5°C/W for every 100lfpm of laminar airflow.
(R || R ) ≤ R /9
R
V
C
G
F
C
O
C
C
1
< 5MHz
(OPTIONAL)
2πR C
C
C
6300 F08
Figure 8. Compensation for Inverting Gains
Layout and Passive Components
R
R
V
O
F
= 1 +
+
V
I
V
G
I
With a gain bandwidth product of 200MHz the LT6300
requires attention to detail in order to extract maximum
performance. Use a ground plane, short lead lengths and
acombinationofRF-qualitysupplybypasscapacitors(i.e.,
0.1µF). As the primary applications have high drive cur-
rent, uselowESRsupplybypasscapacitors(1µFto10µF).
R
C
V
(R || R ) ≤ R /9
C
O
C
G
F
–
C
1
(OPTIONAL)
< 5MHz
2πR C
C
C
R
F
R
G
6300 F09
The parallel combination of the feedback resistor and gain
setting resistor on the inverting input can combine with
Figure 9. Compensation for Noninverting Gains
11
Linear [ Linear ]
