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Performance enhancement techniques for three-terminal regulators
CHAPTER 1
11
conducts, forcing Q1’s base to 30V. This causes Q1’s emit-
ter to clamp 2 V
BE
s below V
Z
, well within the V
IN
–V
OUT
rating of the regulator. Under these conditions, Q1, a high
voltage device, sustains 90V V
CE
at whatever current the
transformer and the regulator’s current limit will support.
The transformer specified saturates at 130mA, keeping Q1
well within its safe area as it dissipates 12W. If Q1 and the
LT317A are thermally coupled, the regulator will soon go
into thermal shutdown and oscillation will commence. This
action will continue, protecting the load and the regulator
as long as the output remains shorted. The 500pF capaci-
tor and the 10Ω-0.02µF damper aid transient response and
the diodes provide safe discharge paths for the capacitors.
This approach to high voltage regulation is primarily
limited by the power dissipation capability of the device in
series with the regulator. Figure 1.11A uses a vacuum tube
(remember them?) to achieve very high short-circuit dis-
sipation capability. The tube allows high voltage operation
and is extremely tolerant of overloads. This circuit allows
the LT317A to control 600W at 2000V (V1’s plate limit is
300mA) with full short-circuit protection.
Power is not the only area in which regulator perfor-
mance can be augmented. Figure 1.12 shows a way to
increase the stability of a regulator’s output over time and
temperature. This is particularly useful in powering strain
gauge-based transducers. In this circuit the output voltage
is divided down and compared to the 2.5V reference by
A1, a precision amplifier. A1’s output is used to force the
LT317A’s adjustment pin to whatever voltage is required to
maintain the 10V output. A1 contributes negligible error.
The resistors specified will track within 5ppm/°C and the
reference contributes about 20ppm/°C. The regulator’s
internal circuitry protects against short circuits and ther-
mal overload.
Figure 1.13’s circuit allows a regulator to remotely sense
the feedback voltage, eliminating the effects of voltage
drop in the supply lines. This is a concern where high cur-
rents must be transmitted over relatively long supply rails
or PC traces. Figure 1.13’s circuit uses A1 to sense the
voltage at the point of load. A1’s output, summed with
the regulator’s output, modifies the adjustment pin volt-
age to compensate for the voltage lost across R
DROP
. The
feedback divider is returned through a separate lead from
the load, completing the remote sensing scheme. The 5µF
capacitor filters noise and the 1k value limits bypass capac-
itor discharge when power is turned off.
A final circuit allows voltage regulator-powered circuity
to run from 110VAC or 220VAC without having to switch
transformer windings. Regulator dissipation does not
increase for 220VAC inputs. In Figure 1.14, when T1 is
Figure 1.11A•Very High Voltage Regulator
Figure 1.12•Using a Regulator as a Protected Power Stage
for a Reference