IEEE Solid-States Circuits Magazine - Spring 2019 - 12
Vout
M1
Vin
R1
+
X
A1
-
Zout
Transient
Currents
CM
Zout
R2
VREF
1
CMω
R
1
1+ 1
R2 A0gm
Load
ω0
(a)
ω
(b)
FIGURE 9: The use of a smoothing capacitor to absorb load transient currents.
bias current source (transistor 50)
are supplied by the output rather
than by the input. Since the output
exhibits much less fluctuation, this
method elegantly solves the op amp
supply rejection problem.
CG
Rop
M1
RL(R1 + R2)
R2
A
R2 + R1 1
CM
FIGURE 10: The open-loop LDO including
capacitances.
β
+
-
+
A1
-
Md
Mc
P
Ma
X
Vn,out
M1
Vin
RL
R1
VnDD
Another class of LDOs employs a
source follower as the pass transistor [Figure 12(a)]. One important
advantage of the follower-based
circuit is its superior PSRR. In fact,
A1
Vn,out
M1
Pass Transistor as Source Follower
Mb
VF
R1
VREF
R2
R2
(a)
RL
X
(b)
FIGURE 11: (a) The effect of op amp PSRR on the LDO PSRR and (b) the implementation
example illustrating input noise cancellation.
Vin
VF
M1
Vout
M1
R1
-
A1
+
X
VREF
R2
Vin
rO1
IL
Rout
Load
R2
A
R2 + R1 1
(a)
RL(R1 + R2)
(b)
FIGURE 12: (a) The LDO using a source follower and (b) the effect of transistor output resistance on the PSRR.
12
S P R I N G 2 0 19
IEEE SOLID-STATE CIRCUITS MAGAZINE
neglecting channel-length modulation in M 1 leads to 2Vout /2Vin " 0
because drain voltage changes have
no effect on the source voltage for
a transistor operating in saturation.
This point stands in contrast to (5)
for the previous LDO.
T h e fo l l o w e r i n Fig u r e 12(a )
incurs a substantially greater dropout voltage than does the current
source in Figure 5(a) if A 1 is supplied from Vin . This is because, at
most, VF can be equal to the supply
voltage of A 1 and Vout can be equal
to Vin -VGS1 . The key point is that
the dropout includes the threshold
voltage of M 1 in this case. As a remedy, a local CP can generate a higher
supply voltage for the op amp [5] so
that VF is high enough to place M 1 at
the edge of saturation.
We now repeat for this structure the
analyses carried out in the previous
section. Let us neglect the body effect
of M 1 and first compute 2Vout /2I L (i.e.,
the LDO output impedance). If R 1 + R 2
is sufficiently large and rO1 = 3, we
can simply divide the open-loop output impedance, 1/g m1, by one plus the
loop gain:
R out .
1+
1/g m1
.
R2 A
R1 + R2 1
(12)
The second term in the denominator is much greater than the first,
yielding
R out . c 1 + R 1 m 1 ,
R 2 g m1 A 1
(13)
the same as the output impedance
of the previous topology.
IEEE Solid-States Circuits Magazine - Spring 2019
Table of Contents for the Digital Edition of IEEE Solid-States Circuits Magazine - Spring 2019
Contents
IEEE Solid-States Circuits Magazine - Spring 2019 - Cover1
IEEE Solid-States Circuits Magazine - Spring 2019 - Cover2
IEEE Solid-States Circuits Magazine - Spring 2019 - Contents
IEEE Solid-States Circuits Magazine - Spring 2019 - 2
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