LM2641MTC-ADJ Datasheet, PDF (17/31 Pages) Texas Instruments – LM2641 Dual Adjustable Step-Down Switching Power Supply Controller

LM2641
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SNVS040B – JANUARY 2000 – REVISED APRIL 2013
capacitors also have very low ESR over the full temperature range, but X5R/X7R dielectric types should be
used to assure sufficient capacitance will be provided (Z5U or Y5F types are not suitable).
– Some of the newer electrolytic types such as POSCAP, OSCON, and polymer electrolytic may also be
usable as input capacitors. However, care must be taken if the application will be used at low
temperatures as the ESR of these capacitors may increase significantly at temperatures below 0°C. Most
aluminum electrolytes are not usable with this IC at temperatures below this limit. Check the ESR
specifications of the selected capacitor carefully if low temperature operation will be required.
2. The input capacitors must be physically located not more than one centimeter away from the switching
FET's, as trace inductance in the switching current path can cause problems.
Loop Compensation
The LM2641 must be properly compensated to assure stable operation and good transient response. As with any
control loop, best performance is achieved when the compensation is optimized so that maximum bandwidth is
obtained while still maintaining sufficient phase margin for good stability.
Best performance for the LM2641 is typically obtained when the loop bandwidth (defined as the frequency where
the loop gain equals unity) is in the range of F_OSC/10 to F_OSC/5.
In the discussion of loop stability, it should be noted that there is a high-frequency pole f_p(HF), whose frequency
can be approximated by:
f_p(HF) ∼ F_OSC/2 X Q_S (Assumes Q_S < 0.5)
Where:
(4)
As can be seen in the approximation for Q_S, the highest frequency for f_p(HF) occurs at the maximum value of
V_IN. The lowest frequency for f_p(HF) is about F_OSC/10 (when V_IN = 4.5V and V_OUT = 1.8V).
As noted above, the location of the pole f_p(HF) is typically in the range of about F_OSC/10 to F_OSC/4. This pole will
often be near the unity-gain crossover frequency, and it can significantly reduce phase margin if left
uncompensated. Fortunately, the ESR of the output capacitor(s) forms a zero which is usually very near the
frequency of f_p(HF), and provides cancellation of the negative phase shift it would otherwise cause. For this
reason, the output capacitor must be carefully selected.
Most of the loop compensation for the LM2641 is set by an R-C network from the output of the error amplifier to
ground (see Figure 21). Since this is a transconductance amplifier, it has a very high output impedance (160 kΩ).
Figure 21. Typical Compensation Network
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:
f_p(C10) = 1 / [2π X C10 (R11 + 160k) ]
C12 adds a pole whose frequency is given by:
f_p(C12) = 1 / [2π X C12 (R11 || 160k) ]
R11 adds a zero whose frequency is given by:
f_z(R11) = 1 / [2π X R11 (C10 + C12) ]
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