LM2640MTC-ADJ National Semiconductor, LM2640MTC-ADJ Datasheet - Page 14

• Manufacturer Part Number: LM2640MTC-ADJ
• Description: Dual Adjustable Step-Down Switching Power Supply Controller
• Manufacturer: National Semiconductor

www.national.com
Application Information
As can be seen in the approximation for Q
quency for f
lowest frequency for f
4.5V and V
As noted above, the location of the pole f
the range of about F
near the unity-gain crossover frequency, and it can signifi-
cantly reduce phase margin if left uncompensated. Fortu-
nately, the ESR of the output capacitor(s) forms a zero which
is usually very near the frequency of f
cancellation of the negative phase shift it would otherwise
cause. For this reason, the output capacitor must be care-
fully selected.
Most of the loop compensation for the LM2640 is set by an
R-C network from the output of the error amplifier to ground
(see Figure 4 ). Since this is a transconductance amplifier, it
has a very high output impedance (160 k ).
The components shown will add poles and zeros to the loop
gain as given by the following equations:
C10 adds a pole whose frequency is given by:
C12 adds a pole whose frequency is given by:
R11 adds a zero whose frequency is given by:
The output capacitor adds both a pole and a zero to the loop:
Where R
series resistance of the output capacitor(s).
The function of the compensation components will be ex-
plained in a qualitative discussion of a typical loop gain plot
for an LM2640 application, as illustrated in Figure 5 .
f
f
f
f
f
p
p
z
p
z
(R11) = 1 / [2 X R11 (C10 + C12) ]
(ESR) = 1 / [2 X ESR X C
(C10) = 1 / [2 X C10 (R11 + 160k) ]
(C12) = 1 / [2 X C12 (R11 || 160k) ]
(C
OUT
FIGURE 4. Typical Compensation Network
L
) = 1 / [2 X R
is the load resistance, and ESR is the equivalent
OUT
p
(HF) occurs at the maximum value of V
= 1.8V).
OSC
p
(HF) is about F
/10 to F
L
X C
OUT
OSC
OUT
]
/4. This pole will often be
]
p
OSC
(HF), and provides
(Continued)
p
(HF) is typically in
S
, the highest fre-
/10 (when V
DS100148-5
IN
. The
IN
=
14
C10 and R11 form a pole and a zero. Changing the value of
C10 moves the frequency of both the pole and the zero.
Changing R11 moves the zero without significantly affecting
the pole.
The C10 pole is typically referred to as the dominant pole,
and its primary function is to roll off loop gain and reduce the
bandwidth.
The R11 zero is required to add some positive phase shift to
offset some of the negative phase shift from the two
low-frequency poles. Without this zero, these two poles
would cause −180˚ of phase shift at the unity-gain crossover,
which is clearly unstable. Best results are typically obtained
if R11 is selected such that the frequency of f
range of f
quency.
The output capacitor (along with the load resistance R
forms a pole shown as f
this pole varies with R
ally which means the unity-gain crossover frequency stays
essentially constant regardless of R
C12 can be used to create an additional pole most often
used for bypassing high-frequency switching noise on the
COMP pin. In many applications, this capacitor is unneces-
sary.
If C12 is used, best results are obtained if the frequency of
the pole is set in the range F
bypassing for the high-frequency noise caused by switching
transitions, but add only a small amount of negative phase
shift at the unity-gain crossover frequency.
The ESR of C
the zero f
10 kHz and 50 kHz. This zero is very important, as it cancels
phase shift caused by the high-frequency pole f
important to select C
tance and ESR to place this zero near f
f
As an example, we will present an analysis of the loop gain
plot for the 3.3V output shown in the Typical Application Cir-
cuit. Values used for calculations are:
c
).
V
V
C
ESR = 60 m (each) = 30 m
F
f
p
OSC
IN
OUT
OUT
(HF)
= 12V
= 3.3V
= 200 kHz
= C14 + C16 = 200 µF
z
z
c
(ESR), which typically falls somewhere between
/4 to f
FIGURE 5. Typical Loop Gain Plot
40 kHz
OUT
@
c
(as well as the capacitance of C
4A
where f
OUT
L
, the loop gain also varies proportion-
p
(C
with the correct value of capaci-
c
OSC
OUT
is the unity-gain crossover fre-
). Although the frequency of
/2 to 2F
total
L
value.
c
OSC
(typical range f
. This will provide
DS100148-6
z
(R11) is in the
p
(HF). It is
OUT
) form
c
/2 to
L
)