LT1010_15 Datasheet, PDF (7/20 Pages) Linear Technology – Fast 150mA Power Buffer

LT1010
APPLICATIONS INFORMATION
General
These notes briefly describe the LT1010 and how it is
used; a detailed explanation is given elsewhere.¹ Emphasis
here will be on practical suggestions that have resulted
from working extensively with the part over a wide range
of conditions. A number of applications are also outlined
that demonstrate the usefulness of the buffer beyond that
of driving a heavy load.
Design Concept
The schematic below describes the basic elements of the
buffer design. The op amp drives the output sink transistor,
Q3, such that the collector current of the output follower,
Q2, never drops below the quiescent value (determined by
I₁ and the area ratio of D1 and D2). As a result, the high
frequency response is essentially that of a simple follower
even when Q3 is supplying the load current. The internal
feedback loop is isolated from the effects of capacitive
loading by a small resistor in the output lead.
D1
A1
I₁
D2
Q2
V⁺
BIAS
I₂
Q1
R1
INPUT
OUTPUT
Q3
V^–
1010 AI01
The scheme is not perfect in that the rate of rise of sink
current is noticeably less than for source current. This
can be mitigated by connecting a resistor between the
bias terminal and V⁺, raising quiescent current. A feature
of the final design is that the output resistance is largely
independent of the follower quiescent current or the output
load current. The output will also swing to the negative rail,
which is particularly useful with single supply operation.
Equivalent Circuit
Below 1MHz, the LT1010 is quite accurately represented
by the equivalent circuit shown here for both small- and
large-signal operation. The internal element, A1, is an
idealized buffer with the unloaded gain specified for the
LT1010. Otherwise, it has zero offset voltage, bias current
and output resistance. Its output also saturates to the
internal supply terminals.²
I_B
V_OS
INPUT
+
V⁺
V_SOS+
R′
R_OUT
A1
OUTPUT
R′ R′ = R_SAT – R_OUT
V_SOS–
V^–
1010 AI02
Loaded voltage gain can be determined from the unloaded
gain, A_V , the output resistance, R_OUT, and the load resis-
tance, R_L, using:
A _VL
=
A _VR_L
R_OUT + R_L
Maximum positive output swing is given by:
V_OUT⁺
=
(V⁺ – V_SOS⁺ )R_L
R_SAT + R_L
The input swing required for this output is:
V_IN⁺
=
V_OUT⁺


1+
R_OUT
R_L


–
V_OS
+
∆V_OS
where ∆V_OS is the 100mV clipping specified for the satu-
ration measurements. Negative output swing and input
drive requirements are similarly determined.
Supply Bypass
The buffer is no more sensitive to supply bypassing than
slower op amps as far as stability is concerned. The
0.1µF disc ceramic capacitors usually recommended for
op amps are certainly adequate for low frequency work.
As always, keeping the capacitor leads short and using
1_{R. J. Widlar, “Unique IC Buffer Enhances Op Amp Designs; Tames Fast Amplifiers,”}
Linear Technology Corp. TP-1, April, 1984.
2_{See electrical characteristics section for guaranteed limits.}
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