APPLICATION NOTE
AN290/10/88
ENHANCE SUPPLY PERFORMANCE
By Paolo ANTONIAZZI and Arturo WOLFSGRUBER
VERY LOW DROP REGULATORS
Standard three-terminal voltage regulator ICs use
an NPN transistor as the series pass element, so the
input-output voltage drop is 1.5V-2V. Low dropout
regulators using lateral PNP pass transistors have
been available for several years, but a lateral PNP
transistor has low gain, so the base current is neces-
sarily high, and a low f
T
, so the settling time is poor.
Moreover, for stability a large output capacitor is
needed.
Applying a new bipolar technology (see APPEN-
DIX : Technology Is The Key) SGS-THOMSON has
developed two 5V low drop voltage regulators that
use a new vertical PNP transistor structure to obtain
low dropout and low quiescent current.
Type L4940 delivers up to 1.5A and offers an input-
output voltage drop of 700mV at the full 1.5A output
current. A 1A version, type L4941, has an input-out-
put drop of 450mV at 1A (100mV at 0.1A). Both ty-
pes have a quiescent current of 15mA at 1A (4mA
with no load). Consequently these devices dissipa-
tes less power, improving the efficiency of any sup-
ply system (figure 1).
Figure 1 : A voltage regulator using vertical isolated PNP transistor is more efficient than regulators with
NPN pass transistors because the drop out is lower. And it is more efficient than a regulator
having a lateral PNP pass transistor because the quiescent current is lower.
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