LT1812 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1812' PDF : 16 Pages View PDF
LT1812
APPLICATIONS INFORMATION
junction temperature (TJ) is calculated from the ambient
temperature (TA) and power dissipation (PD) as follows:
TJ = TA + (PD ā¢ ĪøJA) (Note 9)
Power dissipation is composed of two parts. The ļ¬rst is due
to the quiescent supply current and the second is due to
on-chip dissipation caused by the load current. The worst-
case load induced power occurs when the output voltage
is at 1/2 of either supply voltage (or the maximum swing
if less than 1/2 supply voltage). Therefore PDMAX is:
PDMAX = (V + ā Vā)(ISMAX) + (V +/2)2/RL or
PDMAX = (V+ ā Vā)(ISMAX) + (V + ā VOMAX)(VOMAX/RL)
Example: LT1812CS5 at 70Ā°C, VS = Ā± 5V, RL = 100Ī©
PDMAX = (10V)(4.5mA) + (2.5V)2/100Ī© = 108mW
TJMAX = 70Ā°C + (108mW)(250Ā°C/W) = 97Ā°C
Circuit Operation
The LT1812 circuit topology is a true voltage feedback
ampliļ¬er that has the slewing behavior of a current feedback
ampliļ¬er. The operation of the circuit can be understood
by referring to the Simpliļ¬ed Schematic. The inputs are
buffered by complementary NPN and PNP emitter followers
that drive a 300Ī© resistor. The input voltage appears across
the resistor generating currents that are mirrored into the
high impedance node. Complementary followers form an
output stage that buffers the gain node from the load. The
bandwidth is set by the input resistor and the capacitance
on the high impedance node. The slew rate is determined by
the current available to charge the gain node capacitance.
This current is the differential input voltage divided by R1,
so the slew rate is proportional to the input. Highest slew
rates are therefore seen in the lowest gain conļ¬gurations.
The RC network across the output stage is bootstrapped
when the ampliļ¬er is driving a light or moderate load
and has no effect under normal operation. When driving
capacitive loads (or a low value resistive load) the network
is incompletely bootstrapped and adds to the compensation
at the high impedance node. The added capacitance slows
down the ampliļ¬er which improves the phase margin by
moving the unity-gain cross away from the pole formed
by the output impedance and the capacitive load. The zero
created by the RC combination adds phase to ensure that
the total phase lag does not exceed 180 degrees (zero
phase margin) and the ampliļ¬er remains stable. In this
way, the LT1812 is stable with up to 1000pF capacitive
loads in unity gain, and even higher capacitive loads in
higher closed-loop gain conļ¬gurations.
SIMPLIFIED SCHEMATIC
V+
RB
SHDN
Vā
āIN
BIAS
CONTROL
R1
300Ī©
12
+IN
RC
C
CC
OUT
1812 SS
1812fb
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